LM61495RPHR_技术文档

LM61495, LM61480, LM62460

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LM62460, LM61480, and LM61495 Pin-Compatible 6-A/8-A/10-A Buck Converter

Optimized for Power Density and Low EMI

1 Features

3 Description

•

Functional Safety-Capable

– Documentation available to aid functional safety

system design

Input voltage range from 3 V to 36 V

RESET output with filter and delayed release

Designed for low EMI:

The LM6x4xx family of buck regulators are regulators

that provide either fixed or adjustable output voltage

that can be set from 1 V to 95% of expected input

voltage. These regulators operate under a wide input

voltage range or 3 V to 36 V and have transient

tolerance up to 42 V.

•

The family is designed for low EMI. The devices

incorporate pin-selectable spread spectrum and an

adjustable SW node rise time. Dual Random Spread

Spectrum (DRSS) frequency hopping is set to ±5.5%

(typical), drastically reducing peak emissions through

a combination of triangular and pseudo random

modulation. DRSS also includes advanced techniques

to reduce output voltage ripple caused by spread

spectrum modulation.

– CISPR 11/32 class B compliant EVM

– Pin-configurable spread spectrum

– Adjustable SW node rise time

– Above and below AM band operation: pin

configurable at fixed 400 kHz and 2.2 MHz or

adjustable from 200 kHz – 2.2 MHz

– Low EMI symmetrical pinout

– Light load mode is pin-configurable for constant

frequency or pulse frequency modulation (PFM)

High-efficiency solution

An open-drain RESET output with filtering and

delayed release gives a true indication of system

status. In auto mode, the device automatically

transitions between fixed-frequency pulse width

modulation (FPWM) and pulse frequency modulation

(PFM) modes of operation, allowing an unloaded

current consumption of only 5 µA (typical). Electrical

•

– 95% efficient for an 8-A load

– 5-µA input current while unloaded in auto mode

– <1-µA shutdown current (typical)

High power density

– Built-in compensation, soft start, current limit,

thermal shutdown, and UVLO

characteristics are specified over

temperature range of –40°C to +150°C.

a

junction

– 4.5-mm × 3.5-mm wettable flank QFN package

Device Information

2 Applications

PART NUMBER

LM61495

PACKAGE⁽¹⁾

BODY SIZE (NOM)

•

Industrial transport

Avionics

Industrial PC

PLC, DCS, and PAC

Semiconductor test equipment

LM61480

VQFN (16)

4.50 mm × 3.50 mm

LM62460

(1) For all available packages, see the orderable addendum at

the end of the data sheet.

3.0 V to 36 V input

VIN1

EN

VIN2

PGND2

PGND1

BIAS

SW

RESET

SPSP

CBOOT

RBOOT

FB

MODE/SYNC

VCC

RT

AGND

EVM Efficiency with ϴ_JA= 21.6°C/W:

V_OUT= 5 V, F_SW= 0.4 MHz

Simplified Schematic

An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,

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intellectual property matters and other important disclaimers. ADVANCE INFORMATION for preproduction products; subject to change

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Table of Contents

1 Features............................................................................1

2 Applications.....................................................................1

3 Description.......................................................................1

4 Revision History.............................................................. 2

5 Device Comparison Table...............................................3

6 Pin Configuration and Functions...................................3

7 Specifications.................................................................. 5

7.1 Absolute Maximum Ratings ....................................... 5

7.2 ESD Ratings .............................................................. 5

7.3 Recommended Operating Conditions ........................5

7.4 Thermal Information ...................................................6

7.5 Electrical Characteristics ............................................6

7.6 Timing Characteristics ................................................8

7.7 Switching Characteristics ...........................................9

7.8 System Characteristics .............................................. 9

8 Detailed Description......................................................11

8.1 Overview................................................................... 11

8.2 Functional Block Diagram.........................................12

8.3 Feature Description...................................................13

8.4 Device Functional Modes..........................................24

9 Application and Implementation..................................30

9.1 Application Information............................................. 30

9.2 Typical Application.................................................... 30

10 Power Supply Recommendations..............................42

11 Layout...........................................................................43

11.1 Layout Guidelines................................................... 43

11.2 Layout Example...................................................... 45

12 Device and Documentation Support..........................46

12.1 Device Support....................................................... 46

12.2 Receiving Notification of Documentation Updates..46

12.3 Support Resources................................................. 46

12.4 Trademarks.............................................................46

12.5 Glossary..................................................................46

12.6 Electrostatic Discharge Caution..............................46

13 Mechanical, Packaging, and Orderable

Information.................................................................... 47

4 Revision History

NOTE: Page numbers for previous revisions may differ from page numbers in the current version.

DATE

REVISION

NOTES

February 2021

*

Initial Release

2

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5 Device Comparison Table

DEVICE

VARIANT

LIGHT LOAD

Pin Selectable

SPREAD

SPECTRUM

OUTPUT

VOLTAGE

TYPICAL

FREQUENCY

CURRENT

10 A

LM61495

(10-A Rating)

LM61495RPHR

LM61480RPHR

LM62460RPHR

Pin Selectable

Adjustable

0.4 MHz

2.2 MHz

LM61480

(8-A Rating)

8 A

LM62460

6 A

(6-A Rating)

6 Pin Configuration and Functions

3.5 mm

16

PGND2

15

14

1

2

PGND1

VIN2

VIN1

13

12

3

4

RBOOT

CBOOT

BIAS

EN

SYNC/

MODE

SPSP 11

5

6

VCC

RESET

10

7

8

9

Figure 6-1. RPH Package 16-Pin VQFN (Top View)

Table 6-1. Pin Functions

I/O

DESCRIPTION

NAME

NO.

Power ground to internal low-side MOSFET. Connect to system ground. Low impedance

connection must be provided to PGND1. Connect a high-quality bypass capacitor or

capacitors from this pin to VIN2.

PGND2

1

G

Input supply to the regulator. Connect a high-quality bypass capacitor or capacitors from this

pin to PGND2. Provide a low-impedance connection to VIN1.

VIN2

2

3

4

P

Connect to CBOOT through a resistor. A resistance, typically between 0 Ω and 100 Ω, is

used to adjust the slew rate of the SW node rise time. See Figure 8-10.

RBOOT

CBOOT

High-side driver upper supply rail. Connect a 100-nF capacitor between the SW pin and

CBOOT. An internal diode charges the capacitor while SW node is low.

Input to internal voltage regulator. Connect the pin to an output voltage point or an external

bias supply from 3.3 V to 12 V. Connect an optional high-quality 0.1-µF capacitor from this

pin to GND for the best performance. If output voltage is above 12 V and no external supply

is used, tie the pin to ground.

BIAS

5

P

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Table 6-1. Pin Functions (continued)

I/O

DESCRIPTION

NAME

NO.

Internal regulator output. Used as supply to internal control circuits. Do not connect this pin

to any external loads. Connect a high-quality 1-µF capacitor from this pin to AGND.

VCC

6

O

Feedback input to regulator. Connect this pin to an output voltage sense point for fixed 3.3-V

and 5-V output. Connect this pin to a feedback divider tap point for adjustable output

options. Do not float or ground.

FB

7

8

I

Analog ground for regulator and system. All electrical parameters are measured with respect

to this pin. Connect this pin to PGND1 and PGND2 on PCB.

AGND

RT

G

Connect this pin to ground through a resistor with a value between 7 kΩ and 80 kΩ to set

switching frequency between 200 kHz and 2200 kHz. Connect to VCC for 400 kHz. Connect

to GND for 2.2 MHz. Do not float.

9

I/O

O

Open-drain RESET output. Connect to suitable voltage supply through a current limiting

resistor. High = power OK, low = fault. RESET goes low when EN = low.

RESET

10

Connect to VCC or through a resistor to ground to enable spread spectrum. Connect to GND

to disable spread spectrum. If using spread spectrum, a VCC connection turns off the spread

spectrum tone correction while a resistor to ground adjusts the tone correction to lower the

output voltage ripple. Do not float this pin. See Section 8.3.10.

SPSP

11

12

I

This pin controls the mode of operation of the LM62460, LM61480, and LM61495. Modes

include auto mode (automatic PFM/PWM operation), forced pulse width modulation

(FPWM), and synchronized to an external clock. The clock triggers on the rising edge of an

applied external clock. Pull low to enable PFM operation, pull high to enable FPWM, or

connect to a clock to synchronize to an external frequency in FPWM mode. Do not float this

pin.

SYNC/MODE

Precision enable input to regulator. High = on, low = off. Can be connected to VIN. Precision

enable allows the pin to be used as an adjustable UVLO. Do not float. See Section 8.3.2.

EN

13

14

I

Input supply to the regulator. Connect a high-quality bypass capacitor or capacitors from this

pin to PGND1. Low impedance connection must be provided to VIN2.

VIN1

P

Power ground to internal low-side MOSFET. Connect to system ground. Low impedance

connection must be provided to PGND2. Connect a high-quality bypass capacitor or

capacitors from this pin to VIN1.

PGND1

SW

15

16

G

P

Switch node of the regulator. Connect to the output inductor.

4

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7 Specifications

7.1 Absolute Maximum Ratings

Over the recommended operating junction temperature range⁽¹⁾

PARAMETER

MIN

–0.3

–3.5

0

MAX

42

UNIT

V

Transient VIN to AGND, PGND⁽²⁾

Continuous VIN to AGND, PGND⁽²⁾

SW to AGND, PGND⁽³⁾

36

V

V_IN+ 0.3

5.5

42

V

RBOOT, CBOOT to SW

V

Transient EN or SYNC/EN to AGND, PGND⁽²⁾

Voltages

V

Continuous EN or SYNC/EN to AGND, PGND⁽²⁾

36

V

BIAS to AGND, PGND

16

V

FB to AGND, PGND: Fixed Versions

FB to AGND, PGND: Adjustable Versions

RESET to AGND, PGND

16

V

5.5

20

V

Current

Voltages

T_stg

RESET sink current⁽⁵⁾

10

mA

V

Transient SYNC/MODE to AGND, PGND⁽²⁾

Continuous SYNC/MODE to AGND, PGND⁽²⁾

RT to AGND, PGND

–0.3

-0.3

–0.3

–1

42

36

V

5.5

2

V

VCC to AGND, PGND

V

PGND to AGND⁽⁴⁾

V

Storage temperature

–65

150

°C

(1) Operation outside the Absolute Maximum Ratings may cause permanent device damage. Absolute Maximum Ratings do not imply

functional operation of the device at these or any other conditions beyond those listed under Recommended Operating Conditions. If

used outside the Recommended Operating Conditions but within the Absolute Maximum Ratings, the device may not be fully

functional, and this may affect device reliability, functionality, performance, and shorten the device lifetime.

(2) A maximum of 42 V can be sustained at this pin for duration of ≤ 100 ms at a duty cycle of ≤0.01%. 36 V can be sustained for the life of

this device.

(3) A voltage of 2 V below GND and 2 V above VIN can appear on this pin for ≤ 200 ns with a duty cycle of ≤ 0.01%.

(4) This specification applies to voltage durations of 100 ns or less. The maximum D.C. voltage should not exceed ± 0.3 V.

(5) Do not exceed the voltage rating of the pin.

7.2 ESD Ratings

VALUE

UNIT

Human-body model (HBM), per ANSI/ESDA/

JEDEC JS-001⁽¹⁾

±2000

V

V_(ESD)

Electrostatic discharge

Charged-device model (CDM), per JEDEC

specification JESD22-C101⁽²⁾

±750

V

(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.

(2) JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.

7.3 Recommended Operating Conditions

Over the recommended operating junction temperature range of –40°C to 150°C (unless otherwise noted) ⁽¹⁾

MIN

MAX

UNIT

Input voltage

Input Voltage Range⁽¹⁾

3

1

0

36

V

A

Output Adjustment Range for adjustable output versions ⁽²⁾

0.95 * VIN

Output current I_OUT, LM62460

Output current I_OUT, LM61480

Output current I_OUT, LM61495

6

8

10

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Over the recommended operating junction temperature range of –40°C to 150°C (unless otherwise noted) ⁽¹⁾

MIN

MAX

UNIT

Temperature

Operating junction temperature, T_J

–40

150

°C

(1) An extended input voltage range to 3.0 V is possible after start up; see input UVLO for start-up conditions.

(2) Under no conditions should the output voltage be allowed to fall below zero volts.

7.4 Thermal Information

LM6x4xx

THERMAL METRIC⁽¹⁾

RPH (VQFN)

UNIT

16 PINS

51.3

19.2

12.2

1.1

R_θJA

Junction-to-ambient thermal resistance⁽²⁾

Junction-to-case (top) thermal resistance

Junction-to-board thermal resistance

°C/W

R_θJC(top)

R_θJB

Ψ_JT

Junction-to-top characterization parameter

Junction-to-board characterization parameter

Junction-to-case (bottom) thermal resistance

Ψ_JB

12

R_θJC(bot)

-

(1) For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application

report.

(2) The value of R_ΘJAgiven in this table is only valid for comparison with other packages and can not be used for design purposes. These

values were calculated in accordance with JESD 51-7, and simulated on a 4-layer JEDEC board. They do not represent the

performance obtained in an actual application. For example, the EVM R_ΘJA= 21.6 °C/W. For design information please see the

Layout Information section.

7.5 Electrical Characteristics

Limits apply over the recommended operating junction temperature range of –40°C to +150°C, unless otherwise noted.

Minimum and Maximum limits are guaranteed through test, design or statistical correlation. Typical values represent the most

likely parametric norm at T_J= 25°C, and are provided for reference purposes only. Unless otherwise stated the following

conditions apply: V_IN= 13.5 V. VIN1 shorted to VIN2 = V_IN. VOUT is output set point.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX UNIT

SUPPLY VOLTAGE (VIN PIN)

Needed to start up

3.7

3

V

V_IN

Minimum operating input voltage

Minimum voltage hysteresis

Once operating

V_{IN_OP_H}

I_Q

I_SD

I_B

1

Non-switching input current; measured

at VIN pin ⁽¹⁾

V_IN= 13.5 V, V_FB= +5%, V_BIAS= 5 V

V_EN= 0 V, V_IN= 13.5V

0.662

10

7.5

26

µA

Shutdown quiescent current; measured

at VIN pin

0.662

18.5

V_IN= 13.5 V, V_FB= +5%, V_BIAS= 5 V,

auto mode enabled

Current into BIAS pin (not switching)

ENABLE (EN PIN)

V_EN

Enable input-threshold voltage - rising

V_ENrising

1.161

0.25

0.4

1.263

0.3

1.365

0.5

V

V_{EN_HYST}

V_{EN_WAKE}

I_EN

Enable threshold hysteresis

Enable wake-up threshold

Enable pin input current

V

V_IN= V_EN= 13.5 V

50

nA

INTERNAL LDO (VCC PIN)

V_IN= 13.5 V, V_BIAS= 0 V

3.3

3.1

V_CC

Internal VCC voltage

V

V_IN= 13.5 V, V_BIAS= 3.3 V, 20 mA

V_INvoltage at which internal VCC

undervoltage lockout is released

V_{CC_UVLO}

I_VCC= 0 A

3.7

V

Internal VCC undervoltage lockout

hysteresis

V_{CC_UVLO_HYST}

Hysteresis below V_{CC_UVLO}

1.2

6

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Limits apply over the recommended operating junction temperature range of –40°C to +150°C, unless otherwise noted.

Minimum and Maximum limits are guaranteed through test, design or statistical correlation. Typical values represent the most

likely parametric norm at T_J= 25°C, and are provided for reference purposes only. Unless otherwise stated the following

conditions apply: V_IN= 13.5 V. VIN1 shorted to VIN2 = V_IN. VOUT is output set point.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX UNIT

VOLTAGE REFERENCE (FB PIN)

Initial reference voltage accuracy for

3.3-V option

V_{FB_3.3V}

V_{FB_5V}

V_FB

V_IN= 5 V to 36 V, FPWM mode

V_IN= 6 V to 36 V, FPWM mode

V_IN= 3.0 V to 36 V, FPWM mode

3.25875

4.9375

0.99

3.34125

V

Initial reference voltage accuracy for 5-

V option

5.0625

1.01

Initial reference voltage accuracy for

adjustable (1 V FB) versions

1

5-V fixed option

1.8

2.2

R_FB

Resistance from FB to AGND

Input current from FB to AGND

MΩ

nA

3.3-V fixed option

I_FB

CURRENT LIMITS

Adjustable versions only, V_FB= 1 V

50

I_{SC_6}

Short circuit high-side current Limit

9

6

10.35

6.9

11.4

7.7

A

I_{LS-LIMIT_6}

I_{PEAK-MIN_6}

I_{L-NEG_6}

Low-side current limit

6-A variant, duty cycle approaches 0%

8-A variant, duty cycle approaches 0%

Minimum peak inductor current

Negative current limit

0.9

–3.6

12

1.2

1.5

–3

–2.4

15.2

10.2

2

I_{SC_8}

Short circuit high-side current limit

Low-side current limit

13.8

9.2

I_{LS-LIMIT_8}

I_{PEAK-MIN_8}

I_{L-NEG_8}

8

Minimum peak inductor current

Negative current limit

1.2

–4.8

14

1.6

–4

–3.2

20

I_{SC_10}

Short circuit high-side current Limit

Low-side current limit

17.3

11.5

2

I_{LS-LIMIT_10}

I_{PEAK-MIN_10}

I_{L-NEG_10}

9.8

12.9

10-A variant, duty cycle approaches 0%

Auto mode, static measurement

Minimum peak inductor current

Negative current limit

–6.6

10

–5.3

–4

200

Zero-cross current limit. Positive current

direction is out of SW pin.

I_L-ZC

mA

V

V_HICCUP

Hiccup threshold on FB pin

0.36

0.4

0.44

POWER GOOD (/RESET PIN)

V _RESET-OVRESET upper threshold - rising

V _RESET-UV

% of FB voltage

110

92

112

94

114

%

RESET lower threshold - falling

96.5

RESET UV threshold as percentage of

steady state output voltage with output

voltage and UV threshold, falling, read

at the same T_J, and V_IN.

V _{RESET_GUARD}

Falling

97

%

V _RESET-HYS-

RESET fallling threshold hysteresis

RESET rising threshold hysteresis

% of FB voltage

0.5

1.3

2.5

1.2

0.4

%

V

FALLING

V _RESET-HYS-

RISING

Minimum input voltage for proper

RESET function

Measured when V_RESET< 0.4 V with 10-

kΩ pullup to external 5 V

V _{RESET_VALID}

46.0-µA pullup to RESET pin, V_IN= 1.0

V, V_EN= 0 V

RESET low-level function output

voltage

1-mA pullup to RESET pin, V_IN= 13.5

V, V_EN= 0 V

VOL

V

2-mA pullup to RESET pin, V_IN= 13.5

V, V_EN= 3.3 V

R_RESET

RESET ON resistance

V_EN= 5 V, 1-mA pullup current

V_EN= 0 V, 1-mA pullup current

44

18

125

40

Ω

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Limits apply over the recommended operating junction temperature range of –40°C to +150°C, unless otherwise noted.

Minimum and Maximum limits are guaranteed through test, design or statistical correlation. Typical values represent the most

likely parametric norm at T_J= 25°C, and are provided for reference purposes only. Unless otherwise stated the following

conditions apply: V_IN= 13.5 V. VIN1 shorted to VIN2 = V_IN. VOUT is output set point.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX UNIT

OSCILLATOR (SYNC/MODE PIN)

V_SYNCDL

SYNC/MODE input voltage low

0.4

V

V_SYNCDH

SYNC/MODE input voltage high

1.7

1

V

V_{SYNCD_HYST}

SYNC/MODE input voltage hysteresis

0.185

Internal pulldown resistor to ensure

SYNC/MODE doesn't float

R_SYNC

100

1.9

kΩ

V

OSCILLATOR (SPSP PIN)

HIGH SIDE DRIVE (CBOOT PIN)

Voltage on CBOOT pin compared to

SW which turns off high-side switch

V_{CBOOT_UVLO}

MOSFETS

R_DS-ON-HS

R_DS-ON-LS

High-side MOSFET on-resistance

Low-side MOSFET on-resistance

Load = 1 A, C_BOOT-SW= 3.2 V

21

13

39

25

mΩ

(1) This is the current used by the device open loop. It does not represent the total input current of the system when in regulation.

7.6 Timing Characteristics

over operating free-air temperature range (unless otherwise noted)

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX UNIT

PWM LIMITS (SW PIN)

t_ON-MIN

Minimum HS switch on time

Minimum HS switch off time

Maximum switch on time

V_IN=18 V, V_SYNC/MODE= 5 V, I_OUT= 2 A

V_IN= 5 V

62

70

81

103

11

ns

µs

t_OFF-MIN

t_ON-MAX

HS timeout in dropout

6.9

8.9

START UP

V_IN= 13.5 V, Cvcc = 1 µF, time from EN

high to first SW pulse if output starts at

0 V

t_EN

Turnon delay

0.82

1.2

2.7

ms

Time from first SW pulse to Vref at 90%,

of set point

t_SS

t_W

1.7

2.2

40

ms

Short circuit wait time (≈hiccup" time)

POWER GOOD (/RESET PIN) and OVERVOLTAGE PROTECTION

t_{RESET_FILTER}

t_{RESET_ACT}

RESET edge deglitch delay

RESET active time

10

26

45

µs

Time FB must be valid before RESET is

released.

1.2

2.1

3.75

ms

OSCILLATOR (SYNC/MODE PIN)

High duration needed to be recognized

on SYNC/MODE pin

t_{PULSE_H}

t_{PULSE_L}

t_MSYNC

t_LOCK

100

7

ns

Low duration needed to be recognized

on SYNC/MODE pin

Time at one level needed to indicate

FPWM or auto mode

20

µs

ms

Time needed for clock to lock to a valid

synchronization signal

RT = 39.2 kΩ

4.3

8

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7.7 Switching Characteristics

over operating free-air temperature range (unless otherwise noted)

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX UNIT

OSCILLATOR (RT and SYNC PINS)

f_OSC

Internal oscillator frequency

RT = GND

RT = VCC

1.9

2.2

2.42

440

MHz

kHz

Internal oscillator frequency

350

400

Oscillator frequency measured using

f_{FIXED_2.2MHz}

maximum value of RT resistor to select RT = 6.81 kΩ

2.2 MHz

1.95

352

2.2

2.42

448

MHz

kHz

Oscillator frequency measured using

minimum value of RT resistor to select RT = 40.2 kΩ

0.4 MHz

f_{FIXED_0.4MHz}

400

700

f_ADJ

Center trim oscillator frequency

Synchronization frequency range

RT = 22.6 kΩ

630

200

770

kHz

f_SYNC

2200

SPREAD SPECTRUM

Frequency increase of internal oscillator

from spread spectrum

ΔFc+

2

4

6.5

-2

%

Frequency decrease of internal

oscillator from spread spectrum

ΔFc-

-7

-5

SWITCH NODE

While in frequency fold-back

fsw =1.85 MHz

98

D_MAX

Maximum switch duty cycle

%

87

7.8 System Characteristics

The following specifications apply only to the typical application circuit, with nominal component values. Specifications in the

typical (TYP) column apply to T_J= 25°C only. Specifications in the minimum (MIN) and maximum (MAX) columns apply to the

case of typical components over the temperature range of T_J= –40°C to 150°C. These specifications are not ensured by

production testing.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX UNIT

SUPPLY VOLTAGE (VIN PIN)

Input voltage for full functionality at

reduced load, after start-up.

V_{VIN_MIN1}

3

V

Input voltage for full functionality at

100% of maximum rated load, after

start-up.

V_{VIN_MIN2}

V_OUTset to 3.3 V

3.95

V_IN= 13.5 V, V_OUT= 3.3 V fixed, I_OUT

0 A, Auto mode

=

5

8

Input current to V_INnode of DC/DC for

fixed V_OUTversions

I_{Q_VIN}

µA

V_IN= 13.5 V, V_OUT= 5 V fixed, I_OUT= 0

A, Auto mode

VOLTAGE REFERENCE (FB PIN)

V_OUT= 5 V, V_IN= 6 V to 36 V, I_OUT= 1 A

V_{OUT_5V_ACC}

V_IN= 6 V to 36 V , PWM Operation

–1.5

1.5

2.5

1.5

2.5

%

to full load ⁽¹⁾

V_OUT= 5 V, V_IN= 6 V to 36 V, I_OUT= 0 A V_IN= 6 V to 36 V, PFM and PWM

to full load ⁽¹⁾

operation

V_OUT= 3.3 V, V_IN= 3.8 V to 36 V, I_OUT

1 A to full load ⁽¹⁾

=

V_{OUT_3r3V_ACC}

V_IN= 3.8 V to 36 V , PWM Operation

V_OUT= 3.3 V, V_IN= 3.8 V to 36 V, I_OUT= V_IN= 3.8 V to 36 V, PFM and PWM

0 A to full load ⁽¹⁾

THERMAL SHUTDOWN

T_{SD_R}Thermal shutdown tripping threshold

T_{SD_F}Thermal shutdown recovery threshold

operation

158

150

168

159

180

ºC

OTHER PARAMATERS

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The following specifications apply only to the typical application circuit, with nominal component values. Specifications in the

typical (TYP) column apply to T_J= 25°C only. Specifications in the minimum (MIN) and maximum (MAX) columns apply to the

case of typical components over the temperature range of T_J= –40°C to 150°C. These specifications are not ensured by

production testing.

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX UNIT

Input to output voltage differential to

maintain regulation accuracy, without

inductor DCR drop

V_DROP1

0.45

V

Input to output voltage differential to

maintain f_SW≥ 1.85 MHz, without

inductor DCR drop

V_DROP2

1.2

V

V_IN=13.5 V, V_OUT= 5.0 V, I_OUT= 5 A,

R_RBOOT= 0 Ω

Typical 2.2MHz Efficiency

Typical 400 kHz Efficiency

Typical 250 kHz Efficiency

92.6

95.1

93.7

V_IN= 13.5 V, Vout = 5.0 V, I_OUT= 8 A,

R_RBOOT= 0 Ω

ƞ

%

V_IN= 13.5 V, Vout = 5.0 V, I_OUT= 10 A,

R_RBOOT= 0 Ω

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8 Detailed Description

8.1 Overview

The LM6x4xx is a wide input and output voltage range, low quiescent current, high-performance regulator that

operates over a wide range of frequencies and conversion ratios. If the minimum on-time or minimum off-time

does not support the desired conversion ratio, the frequency is reduced. This action automatically allows

regulation to be maintained during load dump and with very low dropout during cranking.

This device is designed to minimize end-product cost and size while operating in high-performance industrial

environments. The LM6x4xx can be set to operate at fixed 400 kHz, fixed 2.2 MHz, or is adjustable from 200 kHz

to 2.2 MHz using the RT pin. Internal compensation and an accurate current limit scheme minimizes BOM cost

and component count. In addition, the RESET output feature with built-in delayed release and low current light-

load mode let you eliminate a backup LDO and reset chip in many applications.

The LM6x4xx has been designed for low EMI. The device includes the following:

•

Adjustable switch node slew rate

Pin-configurable spread spectrum

Low input inductance package

Operation over a frequency range above and below AM radio band operation

Together, these features can eliminate shielding and other expensive EMI mitigation measures.

To use the device in reliability-conscious environments, the LM6x4xx has a package with enlarged corner

terminals for improved BLR and wettable flanks, allowing optical inspection.

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8.2 Functional Block Diagram

SPSP

RT

VCC

Clock

VCC

MODE/

SYNC

Detect

MODE

/SYNC

Sync

Oscillator

BIAS

VCC UVLO

OTP

Slope

compensation

LDO

VIN

Over

Temperature

detect

FPWM/Auto

Frequency Foldback

RBOOT

CBOOT

VIN1

System enable

Enable

EN

HS Current

sense

Error

amplifier

+

œ

VIN

VIN2

Comp Node

œ

Clock

+

High and

low limiting

circuit

+

Output

low

HS

Current

Limit

œ

SW

System enable

OTP

FB

Drivers and

logic

Soft start

circuit and

bandgap

Hiccup active

VCC UVLO

LS

Current

Limit

œ

AGND

+

Voltage Reference

œ

PGND1

PGND2

+

LS

Current

Min

FPWM/Auto

Vout OV

PGOOD

Logic with

filter and

LS Current

sense

release delay

System enable

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8.3 Feature Description

8.3.1 Output Voltage Selection

A voltage divider between output voltage and the FB pin is used to adjust output voltage. See Figure 8-1.

V_OUT

R_FBT

FB

R_FBB

AGND

Figure 8-1. Setting Output Voltage of Adjustable Versions

The LM6x4xx uses a 1-V reference for control to derive Equation 1. This equation can be used to determine

R_FBBfor a desired output voltage and a given R_FBT. Usually, R_FBTis limited to a maximum value of 100 kΩ to

prevent shifting due to PCB leakage under harsh conditions.

R_FBT

R_FBB

=

V_OUTÅ 1

(1)

In addition, a feedforward capacitor C_FFcan be used to optimize the transient response.

8.3.2 Enable EN Pin and Use as V_INUVLO

Apply a voltage less than 0.4 V to the EN pin to put the device into shutdown mode. In shutdown mode, the

quiescent current drops to 0.66 µA (typical). Above this voltage but below the LM6x4xx lower EN threshold, VCC

is active but the SW node remains inactive. Once EN is above V_EN, the chip operates normally as long as input

voltage is above the minimum operating voltage.

The EN terminal cannot be left floating. The simplest way to enable the operation is to connect the EN pin to

VIN. This action allows the self-start-up of the device when VIN drives the internal VCC above its UVLO level.

However, many applications benefit from employing an enable divider string, which establishes a precision input

undervoltage lockout (UVLO). The precision UVLO can be used for the following:

•

Sequencing

Preventing the device from retriggering when used with long input cables

Reducing the occurrence of deep discharge of a battery power source

Note that EN thresholds are accurate. The rising enable threshold has 8.1% tolerance. Hysteresis is enough to

prevent retriggering upon shutdown of the load (approximately 25%). The external logic output of another IC can

also be used to drive the EN terminal, allowing system power sequencing.

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V_IN

R_ENT

EN

R_ENB

AGND

Figure 8-2. VIN UVLO Using the EN Pin

Resistor values can be calculated using Equation 2:

V_EN

R

=

‡

R_ENB

ENT

V_ONÅ V_EN

(2)

Here, V_ONis the desired typical start-up voltage for the circuit being designed.

8.3.3 SYNC/MODE Uses for Synchronization

The LM6x4xx SYNC/MODE pin can be used to synchronize the internal oscillator to an external clock. The

internal oscillator can be synchronized by coupling a positive edge into the SYNC/MODE pin. The coupled edge

voltage at the SYNC/MODE pin must exceed the SYNC amplitude threshold of V_SYNCDHto trip the internal

synchronization pulse detector. The minimum SYNC rising pulse and falling pulse durations must be longer than

t_PULSE. The LM6x4xx switching action can be synchronized to an external clock from 200 kHz to 2.2 MHz.

SYNC/MODE

Clock

Range

AGND

Figure 8-3. Typical Implementation Allowing Synchronization Using the SYNC/MODE Pin

t_{PULSE_L}

V_SYNCDH

V_SYNCDL

t

t_{PULSE_H}

This image shows the conditions needed for detection of a synchronization signal.

Figure 8-4. Typical SYNC/MODE Waveform

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8.3.4 Clock Locking

Once a valid synchronization signal is detected, a clock locking procedure is initiated. After approximately 2048

pulses, the clock frequency abruptly changes to the frequency of the synchronization signal. While the frequency

adjusts suddenly, phase is maintained so the clock cycle lying between operation at the default and

synchronization frequencies is of intermediate length. There are no very long or very short pulses. Once

frequency is adjusted, phase is adjusted over a few tens of cycles so that rising synchronization edges

correspond to rising the SW node pulses. See Figure 8-5.

Pulse

~2048

Pulse

~2049

Pulse

~2050

Pulse

~2051

Pulse 1

Pulse 2

Pulse 3

Pulse 4

V_SYNCDH

V_SYNCDL

Synchronization

signal

Spread Spectrum is on between pulse 1 and pulse 2048,

there is no change to operating frequency. At pulse 4,

the device transitions from Auto Mode to FPWM.

Also clock frequency matches the

synchronization signal and phase

locking begins

Phase lock achieved, Rising edges

align to within approximately 45 ns,

no spread spectrum

On approximately pulse 2048, spread

spectrum turns off

SW Node

VIN

GND

At pulse 4, the synchronization signal is detected. After approximately pulse 2048, it is ready to synchronize and the frequency is

adjusted using a glitch-free technique. Later, phase is locked.

Figure 8-5. Synchronization Process

8.3.5 Adjustable Switching Frequency

The RT pin is configurable. This pin can be tied to VCC for 400-kHz operation, grounded for 2.2-MHz operation,

or a resistor to AGND can be used to set an adjustable operating frequency. See Figure 8-6 for resistor values.

Note that if a resistor value falls outside of the recommended range, it can cause the LM6x4xx to revert to 400

kHz or 2.2 MHz. Do not apply a pulsed signal to this pin to force synchronization. If synchronization is needed,

see the SYNC/MODE pin in Section 8.3.3.

R_T(kΩ) = (16.4 / f_{SW (MHz)}) - 0.633

(3)

For example, for f_SW= 400 kHz, R_T= (16.4 / 0.4) - 0.633 = 40.37, so a 40.2-k resistor is selected as the closest

choice.

Figure 8-6. Setting Clock Frequency

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8.3.6 RESET Output Operation

While the RESET function of the LM6x4xx resembles a standard power-good function, the functionality is

designed to replace a discrete reset IC, reducing BOM cost. There are three major differences between the reset

function and the normal power-good function seen in most regulators:

•

A delay has been added for release of reset. See Table 8-1.

RESET output signals a fault (pulls its output to ground) while the part is disabled.

RESET continues to operate with input voltage as low as 1.2 V. Below this input voltage, RESET output can

be high impedance.

V_OUT

V_RESET-OV

V_{RESET-HYS-FALLING}

V_{RESET-HYS-RISING}

V_RESET-UV

RESET

High = Power Good

Low = Fault

Figure 8-7. RESET Static Voltage Thresholds

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VIN

V_{CC_UVLO_HYS}

V_{CC_UVLO}

V_RESET-VALID

t

Glitches do not cause false operation nor reset timer

V_OUT

V_{RESET-HYS-RISING}

V_RESET-UV

< t_dg

t

RESET

< 20 V

RESET

t

t_rise-delay

t_dg

t_rise-delay

Figure 8-8. RESET Timing Diagram (Excludes OV Events)

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Table 8-1. Conditions that Cause RESET to Signal a Fault (Pull Low)

FAULT CONDITION ENDS (AFTER WHICH t_{RESET_ACT}MUST PASS

BEFORE RESET OUTPUT IS RELEASED)

FAULT CONDITION INITIATED

FB below V_{RESET_UV}for longer than t_{RESET_FILTER}

FB above V_{RESET_OV}for longer than t_{RESET_FILTER}

Junction temperature exceeds T_{SD_R}

EN low

FB above V_{RESET_UV}+ V_{RESET_HYST}for longer than t_{RESET_FILTER}

FB below V_{RESET_OV}- V_{RESET_HYST}for longer than t_{RESET_FILTER}

(1)

Junction temperature falls below T_{SD_F}

t_ENpasses after EN becomes high⁽¹⁾

VIN falls low enough so that VCC falls below V_{CC_UVLO}

-

(1)

Voltage on VIN is high enough so that VCC pin exceed V_{CC_UVLO}

V_{CC_UVLO_HYST}. This value is called V_{IN_OPERATE}

.

(1) As an additional operational check, RESET remains low during soft start. It is defined as until the lesser of either full output voltage

reached or t_SS2has passed since initiation. This is true even if all other conditions in this table are met and t_{RESET_ACT}has passed.

Lockout during soft start does not require t_{RESET_ACT}to pass before RESET is released.

The threshold voltage for the RESET function is specified to take advantage of the availability of the LM6x4xx

internal feedback threshold to the RESET circuit. This allows a maximum threshold of 96.5% of selected output

voltage to be specified at the same time as 96% of actual operating point. The net result is a more accurate reset

function while expanding the system allowance for transient response. See the output voltage error stack-up

comparison in Figure 8-9.

In addition to signaling a fault upon overvoltage detection (FB above V_{RESET_OV}), the switch node is shut down

and a small, approximately 1-mA pulldown is applied to the SW node.

Typical SMPS upper specification

112%

Highest V_FB

101.25%

100%

1.25%

Selected output voltage

V_FBaccuracy

Typical V_FB- V_{RESET_UV}

-1.25%

Lowest V_FB

98.75%

98%

-5.75%

Typical V_FB- V_{RESET_UV}

Lowest V_OUT

-5.75%

-4%

V_GAURD

Highest V_{RESET_UV}

96.5%

95%

1%

Typical SMPS lower specification

Lowest V_OUT- V_GAURD

-1%

94%

93%

1%

-1%

Lowest V_{RESET_UV}

92%

High V_FBcase

Low V_FBcase

V_GAURDis available margin for

ripple and step loads

Figure 8-9. Reset Threshold Voltage Stack-up

8.3.7 Internal LDO, VCC UVLO, and BIAS Input

The LM6x4xx uses VCC as its internal power supply. VCC is, in turn, powered from VIN or BIAS. Once the

LM6x4xx is active, power comes from VIN if BIAS is less than approximately 3.1 V. Power comes from BIAS if

BIAS is more than 3.1 V. VCC is typically 3 V to 3.3 V under most conditions, but can be lower if VIN is very low.

To prevent unsafe operation, VCC has a UVLO that prevents switching if the internal voltage is too low. See

V_{CC_UVLO}and V_{CC_UVLO_HYST}in Section 7.5. During start-up, VCC momentarily exceeds its normal operating

voltage until V_{CC_UVLO}is exceeded, then drops to its normal operating voltage. These UVLO values, when

combined with the dropout of the LDO when only powering the LM6x4xx, are used to derive minimum

V_{IN_OPERATE}and V_{IN_OP_H}

.

8.3.8 Bootstrap Voltage and V_CBOOT-UVLO(CBOOT Pin)

The driver of the power switch (HS switch) requires bias higher than VIN when the HS switch is ON. The

capacitor connected between CBOOT and SW works as a charge pump to boost voltage on the CBOOT

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terminal to (SW + VCC). The boot diode is integrated on the LM6x4xx die to minimize the physical solution size.

TI recommends a 100-nF capacitor rated for 10 V with X7R or better dielectric for the CBOOT capacitor. The

boot (CBOOT) rail has a UVLO to protect the chip from operation with too little bias. This UVLO has a threshold

of V_{BOOT_UVLO}and is typically 2.1 V. If the CBOOT capacitor voltage drops below V_{BOOT_UVLO}, then the device

initiates a charging sequence using the low-side FET before attempting to turn on the high-side device.

8.3.9 Adjustable SW Node Slew Rate

To allow optimization of EMI with respect to efficiency, the LM6x4xx is designed to allow a resistor to select the

strength of the high-side FET driver during turnon. See Figure 8-10. The current drawn through the RBOOT pin

(the dotted loop) is magnified and drawn through from CBOOT (the dashed line). This current is used to turn on

the high-side power MOSEFT.

VIN

VCC

CBOOT

HS FET RBOOT

HS

Driver

SW

LS FET

Figure 8-10. Simplified Circuit Showing How RBOOT Functions

Rise time is rapid with RBOOT short circuited to CBOOT. SW node harmonics do not roll off until above 150

MHz. If CBOOT and RBOOT are connected through 700 Ω, slew time due to high-side turnon is limited to no

more than 13 ns. 10 ns is typical when converting 13.5 V to 5 V. This slow rise time allows energy in SW node

harmonics to roll off near 50 MHz under most conditions. Rolling off harmonics eliminates the need for shielding

and common mode chokes in many applications. Note that rise time increases with increasing input voltage.

Noise due to stored charge is also greatly reduced with higher RBOOT resistance. Switching with a slower slew

rate decreases efficiency. Take care to optimize the resistance to provide the best EMI while not generating too

much heat. If RBOOT is left open, rise time is set to its maximum value; see the Electrical Characteristics.

8.3.10 Spread Spectrum

Spread spectrum is configurable using the SPSP pin. Spread spectrum is eliminates peak emissions at specific

frequencies by spreading these peaks across a wider range of frequencies than a part with fixed-frequency

operation. The LM6x4xx implements a modulation pattern designed to reduce low frequency-conducted

emissions from the first few harmonics of the switching frequency. The pattern can also help reduce the higher

harmonics that are more difficult to filter, which can fall in the FM band. These harmonics often couple to the

environment through electric fields around the switch node and inductor. The LM6x4xx uses a ±5% spread of

frequencies which can spread energy smoothly across the FM and TV bands. The device implements Dual

Random Spread Spectrum (DRSS). DRSS is a combination of a triangular frequency spreading pattern and

pseudorandom frequency hopping. The combination allows the spread spectrum to be very effective at

spreading the energy at the following:

•

Fundamental switching harmonic with slow triangular pattern

High frequency harmonics with additional psuedorandom jumps at the switching frequency

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The advantage of DRSS is its equivalent harmonic attenuation in the upper frequencies with a smaller

fundamental frequency deviation. This reduces the amount of input current and output voltage ripple that is

introduced at the modulating frequency. Additionally, the LM6x4xx also allows you to further reduce the output

voltage ripple caused by the spread spectrum modulating pattern. With the SPSP pin grounded, the spread

spectrum is disabled. With the SPSP pin tied to VCC, the spread spectrum is on. With the SPSP pin tied through

a resistor to ground, the spread spectrum is on. Also, a modulating tone correction is applied to the switcher to

reduce the output voltage ripple caused by the frequency modulation. The resistor is usually around 20 kΩ, and

can be more precisely calculated using Equation 4.

V_IN

14.17 x

V_OUT

R_SPSP(kꢀ) =

V_IN- V_OUT

+ 1.22

I _RATEDx L x f_SW

(4)

The spread spectrum is only available while the clock of the LM6x4xx are free running at their natural frequency.

Any of the following conditions overrides spread spectrum, turning it off:

•

The clock is slowed due to operation at low input voltage. This is operation in dropout.

The clock is slowed under light load in auto mode. This is normally not seen above 750-mA load. Note that if

the device is operating in FPWM mode, spread spectrum is active, even if there is no load.

The clock is slowed due to high input-to-output voltage ratio. This mode of operation is expected if on-time

reaches minimum on-time. See the Timing Characteristics.

•

The clock is synchronized with an external clock.

8.3.11 Soft Start and Recovery From Dropout

When designing with the LM6x4xx, slowed rise in output voltage due to recovery from dropout and soft start

must be considered separate phenomena. Soft start is triggered by any of the following conditions:

•

EN is used to turn on the device.

Recovery from a hiccup waiting period; see Section 8.3.13.

Recovery from shutdown due to overtemperature protection

Power is applied to the VIN of the IC or the VCC UVLO is released.

Once soft start is triggered, the IC takes the following actions:

•

The reference used by the IC to regulate output voltage is slowly ramped from zero. The net result is that

output voltage, if previously 0 V, takes t_SSto reach 90% of its desired value.

Operating mode is set to auto, activating diode emulation. This allows start-up without pulling output low if

there is a voltage already present on the output.

Hiccup is disabled for the duration of soft start; see Section 8.3.13.

All of these actions together provide start-up with limited inrush currents. They also allow the use of output

capacitors and loading conditions that cause current to border on current limit during start-up without triggering

hiccup. In addition, if output voltage is already present, output is not pulled down. See Figure 8-11.

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If selected, FPWM

is enabled after

regulation but no

later than t_SS2

If selected, FPWM

Triggering event

is enabled after

regulation but no

later than t_SS2

t_EN

t_SS

t_EN

t_SS

V

V_EN

V_OUTSet

Point

V_OUTSet

Point

V_OUT

90% of

V_OUTSet

Point

90% of

V_OUTSet

Point

t

0 V

Time

t_SS2

The left curves show soft start from 0 V. The right curves show soft starting behavior from a pre-biased or non-zero voltage. In either

case, the output voltage reaches within 10% of the desired setpoint t_SStime after soft start is initiated. During soft start, FPWM and

hiccup are disabled. Both hiccup and FPWM are enabled once output reaches regulation or t_SS2, whichever happens first.

Figure 8-11. Soft-Start Operation

Any time output voltage is more than a few percent low for any reason, output voltage ramps up slowly. This

condition, called recovery from dropout, differs from soft start in three important ways:

•

Hiccup is allowed only if output voltage is less than 0.4 times its set point. Note that during dropout regulation

itself, hiccup is inhibited. See Hiccup.

•

FPWM mode is allowed during recovery from dropout. If output voltage were to suddenly be pulled up by an

external supply, the LM6x4xx can pull down on the output. Note that all the protections that are present

during normal operation are in place, protecting the device if output is shorted to a high voltage or ground.

The reference voltage is set to approximately 1% above that needed to achieve the current output voltage. It

is not started from zero.

•

Despite the name, recovery from dropout is active whenever output voltage is more than a few percent lower

than the setpoint for long enough that:

•

Duty factor is controlled by minimum on-time or

When the part is operating in current limit.

This primarily occurs under the following conditions:

•

Dropout: When there is insufficient input voltage for the desired output voltage to be generated. See Section

8.3.13.

•

Overcurrent that is not severe enough to trigger hiccup or if the duration is too short to trigger hiccup. See

Section 8.3.13.

V

Load

current

V_OUTSet

Point

and max

output

Slope

V_OUT

the same

as during

soft start

current

t

Time

Whether output voltage falls due to high load or low input voltage, once the condition that causes output to fall below its setpoint is

removed, output climbs at the same speed as during start-up. Even though hiccup does not trigger due to dropout, it can, in principal, be

triggered during recovery if output voltage is below 0.4 times output the setpoint for more than 128 clock cycles during recovery.

Figure 8-12. Recovery From Dropout

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8.3.12 Overcurrent and Short Circuit Protection

The LM6x4xx is protected from overcurrent conditions by cycle-by-cycle current limiting on both the high-side

and the low-side MOSFETs.

High-side MOSFET overcurrent protection is implemented by the nature of the peak current mode control. The

HS switch current is sensed when the HS is turned on after a short blanking time. The HS switch current is

compared to the minimum of a fixed current setpoint, or the output of the voltage regulation loop minus slope

compensation, every switching cycle. Since the voltage loop has a maximum value and slope compensation

increases with duty, the cycle HS current limit decreases with increased duty factor if duty factor is above 35%.

See Figure 8-13.

12

10

8

6

4

2

HS Maximum Current

Rated Maximum Output

0

0.2

0.4 0.6 0.8

Duty Cycle

1

FEAT

Figure 8-13. Maximum Current Allowed Through the HS FET - Function of Duty Factor for LM62460-Q1

When the LS switch is turned on, the current going through it is also sensed and monitored. Like the high-side

device, the low-side device turnoff is commanded by the voltage control loop. For a low-side device, turnoff is

prevented if current exceeds this value, even if the oscillator normally starts a new switching cycle. See Section

8.4.3.4. Also like the high-side device, there is a limit on how high the turnoff current is allowed to be. This is

called the low-side current limit; see the Electrical Characteristics for values. If the LS current limit is exceeded,

the LS MOSFET stays on and the HS switch is not turned on. The LS switch is turned off once the LS current

falls below its limit. The HS switch is turned on again as long as at least one clock period has passed since the

last time the HS device has turned on.

V_SW

V_IN

t_ON< t_{ON_MAX}

0

t

Typically, t_SW> Clock setting

i_L

I_L-HS

I_L-LS

I_OUT

t

0

Figure 8-14. Current Limit Waveforms

The net effect of the operation of high-side and low-side current limit is that the IC operates in hysteretic control.

Since the current waveform assumes values between I_L-HSand I_L-LS, output current is close to the average of

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these two values unless duty factor is very high. Once operating in current limit, hysteretic control is used and

current does not increase as output voltage approaches zero.

If duty factor is very high, current ripple must be very low to prevent instability; see Section 9.2.2.3. Since current

ripple is low, the part is able to deliver full current. The current delivered is very close to I_L-LS

.

V_OUT

I_L-LS

I_OUTrated

I_L-HS

V_OUTSetting

V_IN> 2 ‡ V_OUTSetting

V_IN~ V_OUTSetting

I_OUT

0

Output Current

Under most conditions, current is limited to the average of I_L-HSand I_L-LS, approximately 1.4 times the rated current. If input voltage is

low, current can be limited to approximately I_L-LS. Current does not exceed the average of I_L-HSand I_L-LSas output drops to 0.4 times the

output voltage setting. Below 0.4 times the output voltage setting, the peak current does not exceed the average of I_L-HSand I_L-LSand

the hiccup mode activates, preventing excessive heating.

Figure 8-15. Output Voltage versus Output Current

Once the overload is removed, the device recovers as though in soft start; see Section 8.3.11. Note that hiccup

can be triggered if output voltage drops below approximately 0.4 times the intended output voltage.

8.3.13 Hiccup

The LM6x4xx employs hiccup overcurrent protection when the following conditions are met 128 consecutive

switching cycles:

•

A time greater than t_SS2has passed since soft start has started; see Section 8.3.11.

Output voltage is below approximately 0.4 times output setpoint.

The part is not operating in dropout defined as having minimum off-time controlled by duty factor.

In hiccup mode, the device shuts itself down and attempts to soft start after t_W. Hiccup mode helps reduce the

device power dissipation under severe overcurrent conditions and short circuits.

Figure 8-16. Inductor Current Bursts During

Figure 8-17. Short-Circuit Transient and Recovery

Hiccup

8.3.14 Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the internal switches when the IC junction

temperature exceeds 165°C (typical). Thermal shutdown does not trigger below 158°C. After thermal shutdown

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occurs, hysteresis prevents the device from switching until the junction temperature drops to approximately

155°C. When the junction temperature falls below 155°C (typical), the LM6x4xx attempts to soft start.

While the LM6x4xx is shut down due to high junction temperature, power continues to be provided to VCC. To

prevent overheating from a short circuit applied to VCC, the LDO providing power to VCC has reduced current

limit while the part is disabled due to high junction temperature. The LDO only provides a few milliamperes

during thermal shutdown.

8.4 Device Functional Modes

8.4.1 Shutdown Mode

The EN pin provides electrical on and off control of the device. When the EN pin voltage is below 0.4 V, both the

regulator and the internal LDO have no output voltage and the part is in shutdown mode. In shutdown mode, the

quiescent current drops to typically 2 µA.

8.4.2 Standby Mode

The internal LDO has a lower EN threshold than the output of the regulator. The internal LDO regulates the VCC

voltage at 3.3 V, typically when:

•

The EN pin voltage is above 1.1 V (maximum) and

The EN pin voltage is below the precision enable threshold for the output voltage.

The precision enable circuitry is ON once VCC is above its UVLO. The internal power MOSFETs of the SW node

remain off unless the voltage on the EN terminal goes above its precision enable threshold. The LM6x4xx also

employs UVLO protection. If the VCC voltage is below its UVLO level, the output of the regulator is turned off.

The control scheme of the LM6x4xx allows the device to operate under a wide range of conditions with a low

number of external components. Peak current mode control allows a wide range of input voltages and output

capacitance values, while maintaining a constant switching frequency. Stable operation is maintained while

output capacitance is changed during operation. This allows use in systems that require high performance

during load transients and have load switches which remove loads as the system operating state changes. Short

minimum on-times and off-times ensure constant frequency regulation over a wide range of conversion ratios.

This architecture uses frequency foldback to achieve low dropout voltage, maintaining output regulation as input

voltage falls close to output voltage. The foldback is smooth and continuous, and activated as off-time

approaches its minimum. Under these conditions, the LM6x4xx operates much like a constant off-time converter,

allowing the maximum duty cycle to reach at least 98%.

8.4.3 Active Mode

The LM6x4xx is in active mode when the following occurs:

•

The EN pin is above V_EN

.

V_INis above V_EN

.

V_INis high enough to satisfy the V_INminimum operating input voltage.

No other fault conditions are present.

See Section 8.3 for protection features. The simplest way to enable the operation is to connect EN to VIN,

allowing self-start-up when the applied input voltage exceeds the minimum V_{IN_OPERATE}

.

In active mode, depending on the load current, input voltage, and output voltage, the LM6x4xx is in one of six

sub-modes:

•

Continuous conduction mode (CCM) with fixed switching frequency and peak current mode operation

Discontinuous conduction mode (DCM) while in auto mode when the load current is lower than half of the

inductor current ripple

•

Minimum on-time operation while the on-time of the device needed for full-frequency operation at the

requested low-duty cycle is not supported by T_{ON_MIN}

Forced pulse width modulation (FPWM) similar to CCM with fixed-switching frequency, but extends the fixed

frequency range of operation from full to no load

Dropout mode when switching frequency is reduced to minimize dropout

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•

Recovery from dropout similar to other modes of operation except the output voltage setpoint is gradually

moved up until the programmed setpoint is reached.

8.4.3.1 Peak Current Mode Operation

The following operating description of the LM6x4xx refers to Section 8.2 and the waveforms in Figure 8-18. Both

supply a regulated output voltage by turning on the internal high-side (HS) and low-side (LS) NMOS switches

with varying duty cycle (D). During the HS switch on-time, the SW terminal voltage, V_SW, swings up to

approximately V_IN, and the inductor current, i_L, increases with a linear slope. The HS switch is turned off by the

control logic. During the HS switch off-time, t_OFF, the LS switch is turned on. Inductor current discharges through

the LS switch, forcing V_SWto swing below ground by the voltage drop across the LS switch. The regulator loop

adjusts the duty cycle to maintain a constant output voltage. D is defined by the on-time of the HS switch over

the switching period: D = T_ON/ T_SW

.

In an ideal buck converter where losses are ignored, D is proportional to the output voltage and inversely

proportional to the input voltage: D = V_OUT/ V_IN.

t_ON

V_OUT

V_IN

V_SW

D =

≈

t_SW

V_IN

t_OFF

t_ON

0

t

- I_OUT‡R_DSLS

t_SW

i_L

I_LPK

I_OUT

I_ripple

t

0

Figure 8-18. SW Voltage and Inductor Current Waveforms in Continuous Conduction Mode (CCM)

To get accurate DC load regulation, a voltage feedback loop is used. Peak and valley inductor currents are

sensed for peak current mode control and current protection. The regulator operates with continuous conduction

mode with constant switching frequency when load level is above one half of the minimum peak inductor current.

The internally-compensated regulation network achieves fast and stable operation with small external

components and low-ESR capacitors.

8.4.3.2 Auto Mode Operation

The LM6x4xx can have two behaviors while lightly loaded. One behavior, called auto mode operation, allows a

seamless transition between normal current mode operation while heavily loaded and in highly-efficient light-load

operation. The other behavior, called FPWM mode, maintains full frequency even when unloaded. Which mode

the LM6x4xx operates in depends on the SYNC/MODE pin. When SYNC/MODE is high, the part is in FPWM.

When SYNC/MODE is low, the part is in PFM.

In auto mode, light-load operation is employed in the LM6x4xx at load lower than approximately 1/10th of the

rated maximum output current. Light-load operation employs two techniques to improve efficiency:

•

Diode emulation, which allows DCM operation

Frequency reduction

Note that while these two features operate together to create excellent light load behavior, they operate

independently of each other.

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8.4.3.2.1 Diode Emulation

Diode emulation prevents reverse current though the inductor, which requires a lower frequency needed to

regulate given a fixed peak inductor current. Diode emulation also limits ripple current as frequency is reduced.

With a fixed peak current, as output current is reduced to zero, frequency must be reduced to near zero to

maintain regulation.

t_ON

V_OUT

V_IN

D =

V_SW

<

t_SW

V_IN

t_OFF

t_ON

t_HIGHZ

0

t

t_SW

i_L

I_LPK

I_OUT

0

t

In auto mode, the low-side device is turned off once inductor current is near zero. As a result, once output current is less than half of

inductor ripple in CCM, the part operates in DCM. This is equivalent to saying that diode emulation is active.

Figure 8-19. PFM Operation

The LM6x4xx has a minimum peak inductor current setting in auto mode. That being said, when current is

reduced to a low value with fixed input voltage, on-time is constant. Regulation is then achieved by adjusting

frequency. This mode of operation is called PFM mode regulation.

8.4.3.2.2 Frequency Reduction

The LM6x4xx reduces frequency whenever the output voltage is above the setpoint. This function is enabled

whenever Comp, an internal signal, is low and an offset between the regulation setpoint of FB and the voltage

applied to FB. The net effect is that there is larger output impedance while lightly loaded in auto mode than in

normal operation. Output voltage must be approximately 1% high when the part is completely unloaded, as

shown in Figure 8-20.

V_OUT

Current

Limit

1% Above

Set point

V_OUTSet

Point

I_OUT

Output Current

0

In auto mode, once output current drops below approximately 1/10th of the rated current of the part, output resistance increases so that

output voltage is 1% high while the buck is completely unloaded.

Figure 8-20. Steady State Output Voltage versus Output Current in Auto Mode

In PFM operation, a small DC positive offset is required on the output voltage to activate the PFM detector. The

lower the frequency in PFM, the more DC offset is needed on VOUT. If the DC offset on VOUT is not acceptable,

a dummy load at VOUT or FPWM mode can be used to reduce or eliminate this offset.

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8.4.3.3 FPWM Mode Operation

Like auto mode operation, FPWM mode operation during light-load operation is selected using the SYNC/MODE

pin.

In FPWM Mode, frequency is maintained while lightly loaded. To maintain frequency, a limited reverse current is

allowed to flow through the inductor. Reverse current is limited by reverse current limit circuitry. See the

Electrical Characteristics for reverse current limit values.

V_SW

t_ON

V_OUT

V_IN

D =

≈

t_SW

V_IN

t_OFF

t_ON

0

t

t_SW

i_L

I_LPK

I_OUT

0

I_ripple

t

FPWM mode Continuous Conduction (CCM) is possible even if I_OUTis less than half of Iripple.

Figure 8-21. FPWM Mode Operation

In FPWM mode, frequency reduction is still available if output voltage is high enough to command minimum on-

time, even while lightly loaded. This allows good behavior during faults which involves the output being pulled

up.

8.4.3.4 Minimum On-time (High Input Voltage) Operation

The LM6x4xx continues to regulate output voltage. This is true even if the input-to-output voltage ratio requires

an on-time less than the minimum on-time of the chip with a given clock setting. This is accomplished using

valley current control. At all times, the compensation circuit dictates both a maximum peak inductor current and a

maximum valley inductor current. If, for any reason, valley current is exceeded, the clock cycle is extended until

valley current falls below that determined by the compensation circuit. If it is not operating in current limit, the

maximum valley current is set above the peak inductor current. This prevents valley control from being used

unless there is a failure to regulate using peak current only. If the input voltage-to-output voltage ratio is too high,

even though current exceeds the peak value dictated by compensation, the high-side device cannot be turned

off quickly enough to regulate output voltage. See t_{ON_MIN}in the Electrical Characteristics. As a result, the

compensation circuit reduces both peak and valley current. Once a low enough current is selected by the

compensation circuit, valley current matches that being commanded by the compensation circuit. Under these

conditions, the low-side device is kept on and the next clock cycle is prevented from starting until inductor

current drops below the desired valley current. Since on-time is fixed at its minimum value, this type of operation

resembles that of a device using a COT control scheme. See Figure 8-22.

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t_ON

V_OUT

V_IN

V_SW

D =

≈

t_SW

V_IN

t_ON= t_{ON_MIN}

t_OFF

0

t

- I_OUT‡R_DSLS

t_SW> Clock setting

i_L

I_OUT

I_ripple

I_LVLY

t

0

In valley control mode, the minimum inductor current is regulated, not peak inductor current.

Figure 8-22. Valley Current Mode Operation

8.4.3.5 Dropout

Dropout operation is defined as any input-to-output voltage ratio that requires frequency to drop to achieve the

needed duty factor. At a given clock frequency, duty factor is limited by minimum off-time. Once this limit is

reached, if clock frequency is maintained, output voltage falls. Instead of allowing the output voltage to drop, the

LM6x4xx extends on-time past the end of the clock cycle until the required peak inductor current is achieved.

The clock can start a new cycle once peak inductor current is achieved or once a pre-determined maximum on-

time, t_{ON_MAX}, of approximately 9 µs passes. As a result, once the needed duty factor cannot be achieved at the

selected clock frequency due to the existence of a minimum off-time, frequency drops to maintain regulation. If

input voltage is low enough that the output voltage cannot be regulated even with an on-time of t_{ON_MAX}, output

voltage drops to slightly below input voltage, V_DROP1. See Section 7.

V_DROP2if

frequency =

1.85 MHz

Input

Voltage

i_L

V_DROP1

Output

Voltage

Output

Setting

V_IN

0

Input Voltage

i_L

Frequency

Setting

I_OUT

~100kHz

0

V_IN

Input Voltage

Output voltage and frequency versus input voltage: If there is little difference between input voltage and output voltage setting, the IC

reduces frequency to maintain regulation. If input voltage is too low to provide the desired output voltage at approximately 110 kHz,

output voltage tracks input voltage.

Figure 8-23. Frequency and Output Voltage in Dropout

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t_ON

V_OUT

V_IN

V_SW

D =

≈

t_SW

t_OFF= t_{OFF_MIN}

V_IN

t_ON< t_{ON_MAX}

0

t

- I_OUT‡R_DSLS

t_SW> Clock setting

i_L

I_LPK

I_OUT

I_ripple

t

0

This image shows the switching waveforms while in dropout. Inductor current takes longer than a normal clock to reach the desired peak

value. As a result, frequency drops. This frequency drop is limited by t_{ON_MAX}

.

Figure 8-24. Dropout Waveforms

8.4.3.6 Recovery from Dropout

In some applications, input voltage can drop below the desired output voltage then recover to a higher value

suddenly. With most regulators, the sudden increase in input voltage results in output voltage rising at a rate

limited only by current limit until regulation is achieved. As input voltage reaches the desired output voltage,

there is overshoot due to wind up in the control loop. This overshoot can be large in applications that have small

output capacitors and light loads. Also, large inrush currents can cause large fluctuations on the input line once

the regulator starts regulating the output voltage. This typically requires less current than during this initial inrush.

The LM6x4xx greatly reduces inrush current and overshoot. This is done by engaging the soft-start circuit

whenever the input voltage suddenly rises, after dipping low enough to cause the output voltage to droop. To

prevent this feature from accidently engaging, output voltage must rise more than 1% to engage this feature.

Also, this feature engages only if operating in dropout or current limit, preventing interference with normal

transient response but allowing several percent overshoot while engaging. If output voltage is very close to its

desired level, overshoot is reduced by inductor current not having time to rise to a high level before regulation

starts.

V

V_IN

Slope

V_OUT

V_OUTSet

Point

the same

as during

soft start

t

Time

Figure 8-25. When Output Voltage Falls, It Recovers Slowly Preventing Overshoot and Large Inrush

Currents

8.4.3.7 Other Fault Modes

Fault modes and their description can be found in Section 8.3 of this data sheet.

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9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification, and TI

does not warrant its accuracy or completeness. TI’s customers are responsible for determining

suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Application Information

The LM6x4xx step-down DC-to-DC converter is typically used to convert a higher DC voltage to a lower DC

voltage with a maximum output current of 10 A. If run at 400 kHz, 10 A can be sustained continuously. If run at

2.2 MHz, continuous current must be limited to 6 A if ambient temperature is 105°C. The following design

procedure can be used to select components for the LM6x4xx.

9.2 Typical Application

Figure 9-1 shows a typical application circuit for the LM6x4xx. This device is designed to function over a wide

range of external components and system parameters. However, the internal compensation is optimized for a

certain range of external inductance and output capacitance. As a quick start guide, Table 9-2 provides typical

component values for some of the most common configurations. The values given in the table are typical. Other

values can be used to enhance certain performance criterion as required by the application. Note that for this

QFN package, the input capacitors are split and placed on either side of the package. See Section 9.2.2.5 for

more details.

6 V to 36 V input

C_IN1

C_{IN_HF1}

VIN1

VIN2

C_IN2

C_{IN_HF2}

PGND1

PGND2

L₁

Output

SW

EN

C_BOOT

R_FF

C_FF

C_OUT

SYNC/MODE

SPSP

CBOOT

RBOOT

R_FBT

RT

FB

VCC

BIAS

R_RESET

R_FBB

C_VCC

RESET

AGND

Figure 9-1. Example Application Circuit - 400-kHz Adjustable Output

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6 V to 36 V input

C_IN1

C_{IN_HF1}

VIN1

VIN2

C_IN2

C_{IN_HF2}

PGND1

PGND2

L₁

Output

SW

EN

C_BOOT

C_OUT

SYNC/MODE

SPSP

CBOOT

RBOOT

RT

FB

VCC

BIAS

R_RESET

C_VCC

RESET

AGND

Figure 9-2. Example Application Circuit - 400-kHz Fixed Output

9.2.1 Design Requirements

Table 9-1 provides the parameters for our detailed design procedure example:

Table 9-1. Detailed Design Parameters

DESIGN PARAMETER

Input voltage

EXAMPLE VALUE

13.5 V (6 V to 36 V)

5 V

Output voltage

Maximum output current

Switching frequency

10 A continuous

400 kHz

Table 9-2. Typical External Component Values

f_SW

(kHz)

R_FBT

(kΩ)

R_FBB

(kΩ)

C_IN

C_HF(µF)

+

C_BOOT

(µF)

R_BOOT

(Ω)

C_VCC

(µF)

R_FF

(kΩ)

V_OUT(V) L1 (µH)

C_OUT(RATED)

C_FF(pF)

10

3 × 47 µF ceramic or 2 x 22

µF + 15 mΩ 150 µF

2 x 10 +

2 x 0.47

400

5

3.0

2.4

100

24.9

43.2

0.1

0

1

4.99

3 × 47 µF ceramic or 2 x 22

µF + 15 mΩ 150 µF

2 x 10 +

2 x 0.47

3.3

10

9.2.2 Detailed Design Procedure

The following design procedure refers to Figure 9-1 and Table 9-1.

9.2.2.1 Choosing the Switching Frequency

The choice of switching frequency is a compromise between conversion efficiency and overall solution size.

Lower switching frequency implies reduced switching losses, usually resulting in less power dissipated in the IC.

Lower power dissipated in the IC results in higher system efficiency and a lower IC temperature. However,

higher switching frequency allows the use of smaller inductors and output capacitors, hence, a more compact

design. Many applications require that the AM band be avoided. These applications tend to operate at either 400

kHz below the AM band, or 2.2 MHz above the AM band. In this example, 400 kHz is chosen.

9.2.2.2 Setting the Output Voltage

The output voltage of the LM6x4xx is externally adjustable using a resistor divider network. Two recommended

output voltages are found in Table 9-2. The divider network is comprised of the top and bottom feedback

resistors, R_FBTand R_FBB, and closes the loop between the output voltage and the converter. The converter

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regulates the output voltage by holding the voltage on the FB pin equal to the internal reference voltage, V_REF

=

1 V. The total resistance of the divider is a compromise between excessive noise pickup and excessive loading

of the output. Lower resistance values reduce noise sensitivity but also reduce the light-load efficiency. The

recommended value for R_FBTis 100 kΩ with a maximum value of 1 MΩ. If 1 MΩ is selected for R_FBT, then a

feedforward capacitor C_FFmust be used across this resistor to provide adequate loop phase margin (see

Section 9.2.2.9). Once R_FBTis selected, Equation 1 is used to select R_FBB. V_REFis nominally 1 V. For this 5-V

example, R_FBT= 100 kΩ and R_FBB= 24.9 kΩ.

9.2.2.3 Inductor Selection

The main parameters for selecting the inductor are the inductance and saturation current. The inductance is

based on the desired peak-to-peak ripple current. It is normally chosen to be in the range of 20% to 40% of the

maximum output current. Experience shows that the best value for inductor ripple current is 30% of the

maximum load current for systems with a fixed input voltage. For systems with a variable input voltage such as

the 12-V battery in a car, 25% is the best value. This example uses V_IN= 13.5 V, which is closer to the nominal

voltage of a 12-V car battery. When selecting the ripple current for applications with much smaller maximum load

than the maximum available from the device, the maximum device current must still be used for this calculation.

Equation 5 can be used to determine the value of the inductance. The constant K is the percentage of peak-to-

peak inductor current ripple to rated output current. For this 10-A, 400-kHz, 5-V example, K = 0.25 is chosen and

an inductance of approximately 3.15 μH is found. The closest standard value of 3.0 μH was selected.

V_INÅ V_OUT

f_SW‡ K ‡ I_OUT(MAX)

V_OUT

V_IN

L=

‡

(5)

Ideally, the saturation current rating of the inductor must be at least as large as the high-side switch current limit,

I_SC. This ensures that the inductor does not saturate, even during a soft-short condition on the output. A hard

short causes the LM6x4xx to enter hiccup mode (see Section 8.3.13). A soft short can hold the output current at

current limit without triggering hiccup. When the inductor core material saturates, the inductance falls to a very

low value, causing the inductor current to rise very rapidly. Although the valley current limit, I_LS-LIMIT, is designed

to reduce the risk of current runaway, a saturated inductor can cause the current to rise to high values very

rapidly. This can lead to component damage, so it is crucial that the inductor does not saturate. Inductors with a

ferrite core material have very hard saturation characteristics, but usually have lower core losses than powdered

iron cores. Powered iron cores exhibit a soft saturation, allowing some relaxation in the current rating of the

inductor. However, they have more core losses at frequencies typically above 1 MHz. In any case, the inductor

saturation current must not be less than the device high-side current limit, I_SC. To avoid subharmonic oscillation,

the inductance value must not be less than that given in Equation 6. The maximum inductance is limited by the

minimum current ripple required for the current mode control to perform correctly. As a rule-of-thumb, the

minimum inductor ripple current must be no less than about 10% of the device maximum rated current under

nominal conditions.

V_OUT

L í 0.19ì

f_SW

(6)

9.2.2.4 Output Capacitor Selection

The output capacitor value and ESR determine the output voltage ripple and load transient performance. The

output capacitor is usually limited by the load transient requirements rather than the output voltage ripple. Table

9-3 can be used to find capacitor values for C_OUTand C_FFfor a few common applications. Note that 4.99-kΩ R_FF

must be used in series with C_FF. In this example, good transient performance is desired, giving 3 x 47-µF

ceramic as the output capacitor and 10 pF as C_FF.

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Table 9-3. Selected Output Capacitor and C_FFValues

SIZE

VERSUS

COST

3.3-V OUTPUT

5-V OUTPUT

TRANSIENT

PERFORMANCE

FREQUENCY

C_OUT

C_FF

C_OUT

C_FF

400 kHz

Small size

Minimum

3 x 47-µF ceramic

4 x 47-µF ceramic

10 pF

33 pF

4 x 33-µF ceramic

3 x 47-µF ceramic

10 pF

33 pF

Better Transient

3 x 22-µF ceramic + 1 x 150 µF,

15 mΩ electrolytic

2 x 22-µF ceramic + 1 x 150

µF, 15 mΩ electrolytic

400 kHz

Low cost

Minimum

10 pF

33 pF

10 pF

22 pF

3 x 22-µF ceramic + 1 x 220 µF,

15 mΩ electrolytic

2 x 22-µF ceramic + 1 x 220

µF, 15 mΩ electrolytic

Better Transient

9.2.2.5 Input Capacitor Selection

The ceramic input capacitors provide a low impedance source to the regulator in addition to supplying the ripple

current and isolating switching noise from other circuits. A minimum of 10-μF ceramic capacitance is required on

the input of the LM6x4xx. This must be rated for at least the maximum input voltage that the application requires.

It is preferable to have twice the maximum input voltage to reduce DC bias derating. This capacitance can be

increased to help reduce input voltage ripple and maintain the input voltage during load transients. In addition, a

small case size (0603 or 0402) ceramic capacitor must be used at each input/ground pin pair, VIN1/PGND1 and

VIN2/PGND2, immediately adjacent to the regulator. The capacitor must have a voltage rating of at least double

the maximum input voltage to minimize derating. The capacitor must also have an X7R or better dielectric.

Choose the highest capacitor value with these parameters. This provides a high frequency bypass to reduce

switch-node ring and electromagnetic interference emissions. The QFN (RJR) package provides two input

voltage pins and two power ground pins on opposite sides of the package. This allows the input capacitors to be

split and placed optimally with respect to the internal power MOSFETs, thus improving the effectiveness of the

input bypassing. This example places two 10-μF, 50-V, 1206, X7R ceramic capacitors and two 0.47-μF, 50-V,

0603, X7R ceramic capacitors at each VIN/PGND pin pair.

Often, it is desirable to use an electrolytic capacitor on the input in parallel with the ceramics. This is especially

true if long leads/traces are used to connect the input supply to the regulator. The moderate ESR of this

capacitor can help damp ringing on the input supply caused by the inductance of the long power leads. The use

of this additional capacitor also helps with momentary voltage dips caused by input supplies with unusually high

impedance.

Most of the input switching current passes through the ceramic input capacitors. The approximate worst case

RMS value of this current can be calculated with Equation 7. This value must be checked against the

manufacturers' maximum ratings.

I_OUT

I_RMS

≈

2

(7)

9.2.2.6 BOOT Capacitor

The LM6x4xx requires a bootstrap capacitor connected between the CBOOT pin and the SW pin. This capacitor

stores energy which is used to supply the gate drivers for the power MOSFETs. A high-quality 100-nF ceramic

capacitor with a rating of at least 10 V is required. The package provides space between the VIN2 and RBOOT

pins to route SW to the boot capacitor without needing long traces or multi-layer routing.

9.2.2.7 BOOT Resistor

A BOOT resistor can be connected between the CBOOT and RBOOT pins to slow the rise-time of the SW node.

If EMI performance is not critical, these two pins can be shorted. If EMI is critical, use a 0-Ω placeholder. The

value can be increased if additional EMI margin is required. Increase to 200 Ω as a first step. This slows the rise-

time of the SW node, reducing EMI by hundreds of MHz by a few dBµV. This is at the expense of about 0.3%

efficiency at 400 kHz at 10 A. Use 50 Ω for a similar efficiency drop at 2.2 MHz at 6 A. In this example, 0 Ω is

chosen to maximize efficiency.

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9.2.2.8 VCC

The VCC pin is the output of the internal LDO used as a supply to the internal control circuits of the regulator.

This output requires a 1-μF, 16-V, X7R or similar, 0603 or similar ceramic capacitor connected from VCC to

AGND for proper operation. Generally avoid loading this output with any external circuitry. However, this output

can be used to supply the pullup for the RESET (power-good) function (see Section 8.3.6). A pullup resistor with

value of 100 kΩ is a good choice in this case. The nominal output voltage on VCC is 3.3 V. Do not short this

output to ground or any other external voltage.

9.2.2.9 C_FFand R_FFSelection

A feedforward capacitor, C_FF, is used to improve phase margin and transient response of circuits, which have

output capacitors with low ESR. Since this capacitor can conduct noise from the output of the circuit directly to

the FB node of the IC, a 4.99-kΩ resistor, R_FF, must be placed in series with C_FF. If the ESR zero of the output

capacitor is below 200 kHz, no C_FFmust be used.

If output voltage is less than 2.5 V, C_FFhas little effect, so it can be omitted. If output voltage is greater than 14 V,

C_FFmust be used cautiously since it can easily introduce too much gain at higher frequencies.

If 1 MΩ is selected for R_FBT, then a feedforward capacitor C_FFmust be used.

9.2.2.10 R_SPSPSelection

The SPSP pin can be connected to GND to disable spread spectrum. The pin can be connected to VCC to

enable spread spectrum. The pin can also be connected to GND through a resistor to enable spread spectrum

with modulating tone correction. This actively reduces the output ripple associated with spread spectrum, which

arises from the inductor current ripple amplitude modulation with spread spectrum frequency modulation. The

value is typically approximately 20 kΩ and can be more precisely calculated using Equation 4.

9.2.2.11 R_TSelection

The R_Tresistor sets the switching frequency of the converter. See Section 8.3.5 for more details. A resistor value

of 40.2 kΩ corresponds to 400 kHz. The pin is also configured to set the switching frequency at 400 kHz when

the RT pin is connected to VCC. Connecting the RT pin to VCC allows you to save cost and space, but placing a

40.2-k resistor allows for more flexibility if a different frequency is desired at a later time.

9.2.2.12 R_MODESelection

The SYNC/MODE pin allows you to synchronize the converter to an external clock with an external clock signal

(SYNC). The pin also allows the selection between two modes (MODE). The following are the selectable modes:

•

Forced pulse width modulation (FPWM) operation, which operates at a fixed frequency at all times

Auto mode which automatically switches to pulse-frequency modulation (PFM) at light loads

Connect the SYNC/MODE pin to VCC for FPWM. Connect to GND for auto. You can also apply a clock signal to

synchronize the switching frequency to an external clock. See Section 8.3.3 for more information.

9.2.2.13 External UVLO

In some cases, you need an input undervoltage lockout (UVLO) level different than that provided internal to the

device. This can be accomplished by using the circuit shown in Figure 9-3. The input voltage at which the device

turns on is designated V_ONwhile the turnoff voltage is V_OFF. First, a value for R_ENBis chosen in the range of 10

kΩ to 100 kΩ, then Equation 8 is used to calculate R_ENTand V_OFF

.

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V_IN

R_ENT

EN

R_ENB

AGND

Figure 9-3. UVLO Using EN

V

V_EN

^ONÅ 1

‡ R_ENB

R_ENT

=

(1 Å V

)

V_OFF=V_ON

‡

EN-HYST

(8)

where

•

V_ON= V_INturnon voltage

V_OFF= V_INturnoff voltage

9.2.2.14 Maximum Ambient Temperature

As with any power conversion device, the LM6x4xx dissipates internal power while operating. The effect of this

power dissipation is to raise the internal temperature of the converter above ambient temperature. The internal

die temperature (T_J) is a function of the following:

•

Ambient temperature

Power loss

Effective thermal resistance, R_θJAof the device

PCB combination

The maximum internal die temperature for the LM6x4xx must be limited to 150°C. This establishes a limit on the

maximum device power dissipation and, therefore, the load current. Equation 9 shows the relationships between

the important parameters. Larger ambient temperatures (T_A) and larger values of R_θJAreduce the maximum

available output current. The converter efficiency can be estimated by using the curves provided in this data

sheet. If the desired operating conditions cannot be found in one of the curves, then interpolation can be used to

estimate the efficiency. Alternatively, the EVM can be adjusted to match the desired application requirements

and the efficiency can be measured directly. The correct value of R_θJAis more difficult to estimate. As stated in

the Semiconductor and IC Package Thermal Metrics Application Report, the value of R_θJAgiven in the Thermal

Information is not valid for design purposes and must not be used to estimate the thermal performance of the

application. The values reported in the Thermal Information table were measured under a specific set of

conditions that are rarely obtained in an actual application.

(

T_J- T_A

R_qJA

)

∂

h

1- h

1

I_OUT

=

∂

MAX

(

)

V_OUT

(9)

where

•

η = efficiency

T_A= ambient temperature

T_J= junction temperature

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•

R_θJA= the effective thermal resistance of the IC junction to the air, mainly through the PCB

The effective R_θJAis a critical parameter and depends on many factors (just to mention a few of the most critical

parameters:

•

Power dissipation

Air temperature

Airflow

PCB area

Copper heat-sink area

Number of thermal vias under or near the package

Adjacent component placement

Due to the ultra-miniature size of the VQFN (RNX) package, a DAP is not available, requiring most of the heat to

flow from the pins to the board. This means that this package exhibits a somewhat large R_θJAvalue when the

layout does not allow for heat to flow from the pins. A typical curve of maximum output current versus ambient

temperature is shown in Figure 9-4 and Figure 9-5 for a good thermal layout. This data was taken with a device

and PCB combination, giving an R_θJAof about 22°C/W. It must be remembered that the data given in these

graphs are for illustration purposes only, and the actual performance in any given application depends on all of

the previously mentioned factors.

V_IN= 13.5 V

V_OUT= 5 V

V_IN= 13.5 V

V_OUT= 5 V

ƒ_SW= 400 kHz

R_θJA= 22°C/W

ƒ_SW= 2.2 MHz

R_θJA= 22°C/W

Figure 9-4. Maximum Output Current versus

Ambient Temperature

Figure 9-5. Maximum Output Current versus

Ambient Temperature

Use the following resources as a guide to optimal thermal PCB design and estimating R_θJAfor a given

application environment:

•

Thermal Design by Insight not Hindsight

A Guide to Board Layout for Best Thermal Resistance for Exposed Pad Packages

Semiconductor and IC Package Thermal Metrics

Thermal Design Made Simple with LM43603 and LM43602

PowerPAD^™Thermally Enhanced Package

PowerPAD^™Made Easy

Using New Thermal Metrics

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9.2.3 Application Curves

Unless otherwise specified, the following conditions apply: V_IN= 13.5 V, T_A= 25°C. The circuit is shown in Figure

9-1, with the appropriate BOM from Table 9-4.

V_OUT= 5 V

V_OUT= 3.3 V

V_OUT= 5 V

F_SW= 400 kHz

Auto mode

V_OUT= 5 V

V_OUT= 3.3 V

V_OUT= 5 V

F_SW= 400 kHz

FPWM mode

Figure 9-6. Efficiency

Figure 9-7. Efficiency

F_SW= 400 kHz

FPWM mode

Figure 9-8. Efficiency

Figure 9-9. Efficiency

F_SW= 2.2 MHz

FPWM mode

Figure 9-10. Efficiency

Figure 9-11. Efficiency

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V_OUT= 3.3 V

F_SW= 2.2 MHz

Auto mode

V_OUT= 3.3 V

F_SW= 2.2 MHz

FPWM mode

Figure 9-12. Efficiency

Figure 9-13. Efficiency

V_OUT= 5 V

F_SW= 400 kHz

Auto mode

V_OUT= 5 V

F_SW= 400 kHz

FPWM mode

Figure 9-14. Load and Line Regulation

Figure 9-15. Load and Line Regulation

V_OUT= 3.3 V

F_SW= 400 kHz

Auto mode

V_OUT= 3.3 V

F_SW= 400 kHz

FPWM mode

Figure 9-16. Load and Line Regulation

Figure 9-17. Load and Line Regulation

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V_OUT= 5 V

F_SW= 2.2 MHz

Auto mode

V_OUT= 5 V

F_SW= 2.2 MHz

FPWM mode

Figure 9-18. Load and Line Regulation

Figure 9-19. Load and Line Regulation

V_OUT= 3.3 V

F_SW= 2.2 MHz

Auto mode

V_OUT= 3.3 V

F_SW= 2.2 MHz

FPWM mode

Figure 9-20. Load and Line Regulation

Figure 9-21. Load and Line Regulation

V_OUT= 5 V

F_SW= 400 kHz

Auto mode

V_OUT= 3.3 V

F_SW= 400 kHz

Auto mode

Figure 9-22. Dropout

Figure 9-23. Dropout

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V_OUT= 5 V

F_SW= 2.2 MHz

Auto mode

V_OUT= 3.3 V

F_SW= 2.2 MHz

Auto mode

Figure 9-24. Dropout

Figure 9-25. Dropout

V_OUT= 5 V

F_SW= 400 kHz

Auto mode

V_OUT= 3.3 V

F_SW= 400 kHz

Auto mode

Figure 9-26. Frequency Dropout

Figure 9-27. Frequency Dropout

V_OUT= 5 V

F_SW= 2.2 MHz

Auto mode

V_OUT= 3.3 V

F_SW= 2.2 MHz

Auto mode

Figure 9-28. Frequency Dropout

Figure 9-29. Frequency Dropout

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Table 9-4. BOM for Typical Application Curves

V_OUT

3.3 V

5 V

FREQUENCY

400 kHz

R_FBB

C_OUT

C_IN+ C_HF

L

C_FF

43.2 kΩ

24.9 kΩ

6 x 22 µF

4 x 22 µF

6 x 22 µF

4 x 22 µF

2 × 4.7 µF + 2 × 470 nF

2.4 µH

0.68 µH

2.4 µH

0.68 µH

22 pF

10 pF

22 pF

10 pF

2100 kHz

400 kHz

5 V

2100 kHz

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10 Power Supply Recommendations

The characteristics of the input supply must be capable of delivering the required input current to the loaded

regulator. The average input current can be estimated with Equation 10.

V_OUT‡ I_OUT

V_IN‡ ꢀ

I_IN=

(10)

where

•

η is the efficiency

If the regulator is connected to the input supply through long wires or PCB traces, special care is required to

achieve good performance. The parasitic inductance and resistance of the input cables can have an adverse

effect on the operation of the regulator. The parasitic inductance, in combination with the low-ESR ceramic input

capacitors, can form an underdamped resonant circuit. This can result in overvoltage transients at the input to

the regulator or tripping UVLO. Consider that the supply voltage can dip when a load transient is applied to the

output depending on the parasitic resistance and inductance of the harness and characteristics of the supply. If

the application is operating close to the minimum input voltage, this dip can cause the regulator to momentarily

shut down and reset. The best way to solve these kinds of issues is to reduce the distance from the input supply

to the regulator. Additionally, use an aluminum input capacitor in parallel with the ceramics. The moderate ESR

of this type of capacitor helps damp the input resonant circuit and reduce any overshoots or undershoots. A

value in the range of 20 µF to 100 µF is usually sufficient to provide input damping and help hold the input

voltage steady during large load transients.

In some cases, a transient voltage suppressor (TVS) is used on the input of regulators. One class of this device

has a snap-back characteristic (thyristor type). It is not recommended to use a device with this type of

characteristic. When the TVS fires, the clamping voltage falls to a very low value. If this voltage is less than the

output voltage of the regulator, the output capacitors discharge through the device back to the input. This

uncontrolled current flow can damage the device.

The input voltage must not be allowed to fall below the output voltage. In this scenario, such as a shorted input

test, the output capacitors discharge through the internal parasitic diode found between the VIN and SW pins of

the device. During this condition, the current can become uncontrolled, possibly causing damage to the device. If

this scenario is considered likely, then use a Schottky diode between the input supply and the output.

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11 Layout

11.1 Layout Guidelines

The PCB layout of any DC-DC converter is critical to the optimal performance of the design. Bad PCB layout can

disrupt the operation of an otherwise good schematic design. Even if the converter regulates correctly, bad PCB

layout can mean the difference between a robust design and one that cannot be mass produced. Furthermore,

the EMI performance of the regulator is dependent on the PCB layout to a great extent. In a buck converter, the

most EMI-critical PCB feature is the loop formed by the input capacitor or capacitors and power ground. This is

shown in Figure 11-1. This loop carries large transient currents that can cause large transient voltages when

reacting with the trace inductance. Excessive transient voltages can disrupt the proper operation of the

converter. Because of this, the traces in this loop must be wide and short while keeping the loop area as small as

possible to reduce the parasitic inductance. Figure 11-2 shows a recommended layout for the critical

components of the LM6x4xx circuit.

•

Place the input capacitor or capacitors as close as possible to the input pin pairs: VIN1 to PGND1 and

VIN2 to PGND2. Place the small capacitors closest. Each pair of pins are adjacent, simplifying the input

capacitor placement. With the QFN package, there are two VIN/PGND pairs on either side of the package.

This provides for a symmetrical layout and helps minimize switching noise and EMI generation. Use a wide

VIN plane on a lower layer to connect both of the VIN pairs together to the input supply.

•

Place the bypass capacitor for VCC close to the VCC pin and AGND pins: This capacitor must be routed

with short, wide traces to the VCC and AGND pins.

Place the CBOOT capacitor as close as possible to the device with short, wide traces to the CBOOT

and SW pins: It is important to route the SW connection under the device through the gap between VIN2 and

RBOOT pins, reducing exposed SW node area. If an RBOOT resistor is used, place it as close as possible to

the CBOOT and RBOOT pins. If high efficiency is desired, RBOOT and CBOOT pins can be shorted. This

short must be placed as close as possible to the RBOOT and CBOOT pins.

•

Place the feedback divider as close as possible to the FB pin of the device: Place R_FBB, R_FBT, C_FFif

used, and R_FFif used, physically close to the device. The connections to FB and AGND through R_FBBmust

be short and close to those pins on the device. The connection to V_OUTcan be somewhat longer. However,

this latter trace must not be routed near any noise source (such as the SW node) that can capacitively couple

into the feedback path of the regulator.

Layer 2 of the PCB must be a ground plane: This plane acts as a noise shield and as a heat dissipation

path. Using layer 2 reduces the inclosed area in the input circulating current in the input loop, reducing

inductance.

•

Provide wide paths for V_IN, V_OUT, and GND: These paths must be as wide and direct as possible to reduce

any voltage drops on the input or output paths of the converter to maximize efficiency.

Provide enough PCB area for proper heat sinking: Enough copper area must be used to ensure a low

R_θJA, considering maximum load current and ambient temperature. Make the top and bottom PCB layers with

two-ounce copper and no less than one ounce. If the PCB design uses multiple copper layers

(recommended), thermal vias can also be connected to the inner layer heat-spreading ground planes. Note

that the package of this device dissipates heat through all pins. Wide traces must be used for all pins except

where noise considerations dictate minimization of area.

•

Keep switch area small: Keep the copper area connecting the SW pin to the inductor as short and wide as

possible. At the same time, the total area of this node must be minimized to help reduce radiated EMI.

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VIN1

VIN2

HS

FET

C_{IN_HF1}

C_{IN_HF2}

SW

LS

FET

PGND1

PGND2

Figure 11-1. Input Current Loop

11.1.1 Ground and Thermal Considerations

As mentioned above, TI recommends using one of the middle layers as a solid ground plane. A ground plane

provides shielding for sensitive circuits and traces. It also provides a quiet reference potential for the control

circuitry. The AGND and PGND pins must be connected to the ground planes using vias next to the bypass

capacitors. PGND pins are connected directly to the source of the low-side MOSFET switch, and connect

directly to the grounds of the input and output capacitors. The PGND net contains noise at the switching

frequency and can bounce due to load variations. The PGND trace, as well as the VIN and SW traces, must be

constrained to one side of the ground plane. The other side of the ground plane contains much less noise and

must be used for sensitive traces.

TI recommends providing adequate device heat sinking by using vias near ground and V_INto connect to the

system ground plane or V_INstrap, both of which dissipate heat. Use as much copper as possible for the system

ground plane on the top and bottom layers for the best heat dissipation. Use a four-layer board with the copper

thickness for the four layers, starting from the top as: 2 oz / 1 oz / 1 oz / 2 oz. A four-layer board with enough

copper thickness and proper layout provides low current conduction impedance, proper shielding, and lower

thermal resistance.

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11.2 Layout Example

Figure 11-2. Layout Example

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12 Device and Documentation Support

12.1 Device Support

12.1.1 Third-Party Products Disclaimer

TI'S PUBLICATION OF INFORMATION REGARDING THIRD-PARTY PRODUCTS OR SERVICES DOES NOT

CONSTITUTE AN ENDORSEMENT REGARDING THE SUITABILITY OF SUCH PRODUCTS OR SERVICES

OR A WARRANTY, REPRESENTATION OR ENDORSEMENT OF SUCH PRODUCTS OR SERVICES, EITHER

ALONE OR IN COMBINATION WITH ANY TI PRODUCT OR SERVICE.

12.2 Receiving Notification of Documentation Updates

To receive notification of documentation updates, navigate to the device product folder on ti.com. Click on

Subscribe to updates to register and receive a weekly digest of any product information that has changed. For

change details, review the revision history included in any revised document.

12.3 Support Resources

TI E2E^™support forums are an engineer's go-to source for fast, verified answers and design help — straight

from the experts. Search existing answers or ask your own question to get the quick design help you need.

Linked content is provided "AS IS" by the respective contributors. They do not constitute TI specifications and do

not necessarily reflect TI's views; see TI's Terms of Use.

12.4 Trademarks

PowerPAD^™and TI E2E^™are trademarks of Texas Instruments.

All trademarks are the property of their respective owners.

12.5 Glossary

TI Glossary

This glossary lists and explains terms, acronyms, and definitions.

12.6 Electrostatic Discharge Caution

This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled

with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.

ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may

be more susceptible to damage because very small parametric changes could cause the device not to meet its published

specifications.

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13 Mechanical, Packaging, and Orderable Information

The following pages include mechanical, packaging, and orderable information. This information is the most

current data available for the designated devices. This data is subject to change without notice and revision of

this document. For browser-based versions of this data sheet, refer to the left-hand navigation.

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IMPORTANT NOTICE AND DISCLAIMER

TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE

DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”

AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY

IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD

PARTY INTELLECTUAL PROPERTY RIGHTS.

These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate

TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable

standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you

permission to use these resources only for development of an application that uses the TI products described in the resource. Other

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TI’s products are provided subject to TI’s Terms of Sale (https:www.ti.com/legal/termsofsale.html) or other applicable terms available either

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applicable warranties or warranty disclaimers for TI products.IMPORTANT NOTICE

Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265


型号：	LM61495RPHR
厂家：	TEXAS INSTRUMENTS
描述：	LM62460, LM61480, and LM61495 Pin-Compatible 6-A/8-A/10-A Buck Converter Optimized for Power Density and Low EMI
文件：	总50页 (文件大小：2297K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM61495RPHR [TI]

相关型号：

LM614AIN

LM614AMJ/883

LM614AMN

LM614BIN

LM614CN

LM614CWM

LM614CWMX

LM614IJ

LM614IN

LM614IWM

LM614IWM/NOPB

LM614IWM/NOPB