LMZ14202EXTTZ/NOPB [TI]

2A SIMPLE SWITCHER Power Module with 42V Maximum Input Voltage for Military and Rugged App 7-TO-PMOD -55 to 125;


型号：	LMZ14202EXTTZ/NOPB
厂家：	TEXAS INSTRUMENTS
描述：	2A SIMPLE SWITCHER Power Module with 42V Maximum Input Voltage for Military and Rugged App 7-TO-PMOD -55 to 125 电源电路军事
文件：	总21页 (文件大小：3949K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LMZ14202EXT

2A SIMPLE SWITCHER® Power Module with 42V Maximum Input

Voltage for Military and Rugged Applications

Easy to use 7 pin package

Performance Benefits

Low radiated emissions / High radiated immunity

●

Passes vibration standard

MIL-STD-883 Method 2007.2 Condition A

JESD22–B103B Condition 1

Passes drop standard

MIL-STD-883 Method 2002.3 Condition B

JESD22–B110 Condition B

●

30117786

System Performance

TO-PMOD 7 Pin Package

10.16 x 13.77 x 4.57 mm (0.4 x 0.542 x 0.18 in)

Efficiency V_IN= 24V V_OUT= 5.0V

_JA= 20°C/W, θ_JC= 1.9°C/W

RoHS Compliant

Electrical Specifications

12W maximum total output power

●

Up to 2A output current

Input voltage range 6V to 42V

Output voltage range 0.8V to 6V

Efficiency up to 90%

●

Key Features

– 55°C to 125°C junction temperature range

30117736

●

Thermal Derating Curve

V_IN= 24V, V_OUT= 5.0V,

Integrated shielded inductor

Simple PCB layout

Flexible startup sequencing using external soft-start and

precision enable

Protection against inrush currents and faults such as input

UVLO and output short circuit

●

Single exposed pad and standard pinout for easy

mounting and manufacturing

●

Fast transient response for powering FPGAs and ASICs

●

Low output voltage ripple

Pin-to-pin compatible family:

LMZ14203EXT/2EXT/1EXT (42V max 3A, 2A, 1A)

LMZ14203/2/1 (42V max 3A, 2A, 1A)

LMZ12003/2/1 (20V max 3A, 2A, 1A)

30117737

Fully enabled for Webench® Power Designer

●

Applications

Point of load conversions from 12V and 24V input rail

●

Time critical projects

Space constrained / high thermal requirement applications

Negative output voltage applications (See AN-2027)

PRODUCTION DATA information is current as of

publication date. Products conform to specifications per

the terms of the Texas Instruments standard warranty.

Production processing does not necessarily include

testing of all parameters.

301177 SNVS665D

LMZ14202EXT

Radiated Emissions (EN 55022 Class B)

from Evaluation Board

30117739

Simplified Application Schematic

30117701

Connection Diagram

30117702

Top View

7-Lead TO-PMOD

Ordering Information

Order Number

LMZ14202EXTTZ

LMZ14202EXTTZX

LMZ14202EXTTZE

Package Type

TO-PMOD-7

NSC Package Drawing

TZA07A

Supplied As

250 Units in Tape and Reel

500 Units in Tape and Reel

45 Units in a Rail

TZA07A

LMZ14202EXT

Pin Descriptions

Name Description

VIN Supply input — Nominal operating range is 6V to 42V . A small amount of internal capacitance is contained within the

package assembly. Additional external input capacitance is required between this pin and exposed pad.

RON On Time Resistor — An external resistor from V_INto this pin sets the on-time of the application. Typical values range

from 25k to 124k ohms.

Enable — Input to the precision enable comparator. Rising threshold is 1.18V nominal; 90 mV hysteresis nominal.

Maximum recommended input level is 6.5V.

GND Ground — Reference point for all stated voltages. Must be externally connected to EP.

Soft-Start — An internal 8 µA current source charges an external capacitor to produce the soft-start function. This node

is discharged at 200 µA during disable, over-current, thermal shutdown and internal UVLO conditions.

Feedback — Internally connected to the regulation, over-voltage, and short-circuit comparators. The regulation

reference point is 0.8V at this input pin. Connected the feedback resistor divider between the output and ground to set

the output voltage.

VOUT Output Voltage — Output from the internal inductor. Connect the output capacitor between this pin and exposed pad.

Exposed Pad — Internally connected to pin 4. Used to dissipate heat from the package during operation. Must be

electrically connected to pin 4 external to the package.

LMZ14202EXT

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for

availability and specifications.

VIN, RON to GND

EN, FB, SS to GND

Junction Temperature

Storage Temperature Range

ESD Susceptibility(Note 2)

-0.3V to 43.5V

-0.3V to 7V

150°C

-65°C to 150°C

± 2 kV

Peak Reflow Case Temperature

(30 sec)

245°C

For soldering specifications, refer to the following document:

www.ti.com/lit/snoa549c

Operating Ratings (Note 1)

V_IN

6V to 42V

0V to 6.5V

Operation Junction Temperature

−55°C to 125°C

Electrical Characteristics Limits in standard type are for T_J= 25°C only; limits in boldface type apply over the

junction temperature (T_J) range of -55°C to +125°C. 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= 24V, Vout = 3.3V

Min

(Note 3)

Typ

Max

Symbol

Parameter

Conditions

Units

(Note 4) (Note 3)

SYSTEM PARAMETERS

Enable Control

V_EN

V_EN-HYS

Soft-Start

I_SS

EN threshold trip point

V_ENrising

V_ENfalling

1.10

4.9

1.18

1.26

EN threshold hysteresis

SS source current

V_SS= 0V

µA

I_SS-DIS

SS discharge current

-200

Current Limit

I_CL

Current limit threshold

d.c. average

2.3

2.6

3.65

ON/OFF Timer

t_ON-MIN

ON timer minimum pulse width

OFF timer pulse width

150

260

t_OFF

Regulation and Over-Voltage Comparator

V_FB

In-regulation feedback voltage V_SS>+ 0.8V

T_J= -55°C to 125°C

0.775

0.795

0.802

0.92

0.815

I_O= 2A

V_SS>+ 0.8V

T_J= 25°C

0.786

0.818

I_O= 10 mA

V_FB-OV

Feedback over-voltage

protection threshold

I_FB

I_Q

Feedback input bias current

Non Switching Input Current

μA

V_FB= 0.86V

I_SD

Shut Down Quiescent Current V_EN= 0V

Thermal Characteristics

T_SD

Thermal Shutdown

Thermal shutdown hysteresis

Rising

Falling

165

°C

T_SD-HYST

LMZ14202EXT

Min

(Note 3)

Typ

Max

Symbol

Parameter

Conditions

Units

(Note 4) (Note 3)

Junction to Ambient

4 layer JEDEC Printed Circuit Board,

100 vias, No air flow

19.3

°C/W

θ_JA

2 layer JEDEC Printed Circuit Board, No

air flow

21.5

1.9

Junction to Case

No air flow

θ_JC

PERFORMANCE PARAMETERS

Output Voltage Ripple

Line Regulation

Load Regulation

Efficiency

mV _PP

ΔV_O

V_IN= 12V to 42V, I_O= 2A

V_IN= 24V

.01

1.5

ΔV_O/ΔV_IN

mV/A

ΔV_O/I_OUT

V_IN= 24V V_O= 3.3V I_O= 1A

V_IN= 24V V_O= 3.3V I_O= 2A

Efficiency

Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the

device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: The human body model is a 100pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test method is per JESD-22-114.

Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical

Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).

Note 4: Typical numbers are at 25°C and represent the most likely parametric norm.

Note 5: EN 55022:2006, +A1:2007, FCC Part 15 Subpart B: 2007. See AN-2024 and layout for information on device under test.

Note 6: Theta JA measured on a 1.705” x 3.0” four layer board, with one ounce copper, thirty five 12 mil thermal vias, no air flow, and 1W power dissipation. Refer

to PCB layout diagrams

LMZ14202EXT

Typical Performance Characteristics

Unless otherwise specified, the following conditions apply: V_IN= 24V; Cin = 10uF X7R Ceramic; C_O= 100uF X7R Ceramic; Tambient

= 25 C for efficiency curves and waveforms.

Efficiency 6V Input @ 25°C

Efficiency 12V Input @ 25°C

Efficiency 24V Input @ 25°C

Dissipation 6V Input @ 25°C

Dissipation 12V Input @ 25°C

Dissipation 24V Input @ 25°C

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30117732

30117704

30117703

30117726

30117727

LMZ14202EXT

Efficiency 36V Input @ 25°C

Efficiency 42V Input @ 25°C

Efficiency 6V Input @ 85°C

Dissipation 36V Input @ 25°C

Dissipation 42V Input @ 25°C

Dissipation 6V Input @ 85°C

30117729

30117730

30117750

30117751

30117734

30117733

LMZ14202EXT

Efficiency 8V Input 85°C

Efficiency 12V Input@ 85°C

Efficiency 24V Input @ 85°C

Dissipation 8V Input 85°C

Dissipation 12V Input @ 85°C

Dissipation 24V Input @ 85°C

30117741

30117740

30117742

30117744

30117743

30117745

LMZ14202EXT

Efficiency 36V Input @ 85°C

Dissipation 36V Input @ 85°C

Dissipation 42V Input @ 85°C

Line and Load Regulation @ 85°C

30117747

30117746

Efficiency 42V Input @ 85°C

30117752

30117753

Line and Load Regulation @ 25°C

30117748

30117769

LMZ14202EXT

Line and Load Regulation @ –55°C

Output Ripple

24V_IN3.3V_O2A, BW = 200 MHz

30117705

30117772

Transient Response

24V_IN3.3V_O0.6A to 2A Step

Thermal Derating V_OUT= 3.3V

30117706

30117770

Current Limit 1.8V_OUT@ 25°C

Current Limit 3.3V_OUT@ 25°C

30117765

30117754

LMZ14202EXT

Current Limit 3.3V_OUT@ 85°C

Current Limit 3.3V_OUT@ –55°C

30117768

30117771

Application Block Diagram

30117708

General Description

The LMZ14202EXT SIMPLE SWITCHER® power module is an easy-to-use step-down DC-DC solution capable of driving up to

2A load with exceptional power conversion efficiency, line and load regulation, and output accuracy. The LMZ14202EXT is available

in an innovative package that enhances thermal performance and allows for hand or machine soldering.

The LMZ14202EXT can accept an input voltage rail between 6V and 42V and deliver an adjustable and highly accurate output

voltage as low as 0.8V. The LMZ14202EXT only requires three external resistors and four external capacitors to complete the

power solution. The LMZ14202EXT is a reliable and robust design with the following protection features: thermal shutdown, input

under-voltage lockout, output over-voltage protection, short-circuit protection, output current limit, and allows startup into a pre-

biased output. A single resistor adjusts the switching frequency up to 1 MHz.

COT Control Circuit Overview

Constant On Time control is based on a comparator and an on-time one shot, with the output voltage feedback compared with an

internal 0.8V reference. If the feedback voltage is below the reference, the main MOSFET is turned on for a fixed on-time determined

by a programming resistor R_ON. R_ONis connected to V_INsuch that on-time is reduced with increasing input supply voltage. Following

this on-time, the main MOSFET remains off for a minimum of 260 ns. If the voltage on the feedback pin falls below the reference

level again the on-time cycle is repeated. Regulation is achieved in this manner.

LMZ14202EXT

Design Steps for the LMZ14202EXT Application

The LMZ14202EXT is fully supported by Webench® and offers the following: Component selection, electrical and thermal simu-

lations as well as the build-it board for a reduction in design time. The following list of steps can be used to manually design the

LMZ14202EXT application.

• Select minimum operating V_INwith enable divider resistors

• Program V_Owith divider resistor selection

• Program turn-on time with soft-start capacitor selection

• Select C_O

• Select C_IN

• Set operating frequency with R_ON

• Determine module dissipation

• Layout PCB for required thermal performance

ENABLE DIVIDER, R_ENTAND R_ENBSELECTION

The enable input provides a precise 1.18V band-gap rising threshold to allow direct logic drive or connection to a voltage divider

from a higher enable voltage such as V_IN. The enable input also incorporates 90 mV (typ) of hysteresis resulting in a falling threshold

of 1.09V. The maximum recommended voltage into the EN pin is 6.5V. For applications where the midpoint of the enable divider

exceeds 6.5V, a small zener can be added to limit this voltage.

The function of this resistive divider is to allow the designer to choose an input voltage below which the circuit will be disabled. This

implements the feature of programmable under voltage lockout. This is often used in battery powered systems to prevent deep

discharge of the system battery. It is also useful in system designs for sequencing of output rails or to prevent early turn-on of the

supply as the main input voltage rail rises at power-up. Applying the enable divider to the main input rail is often done in the case

of higher input voltage systems such as 24V AC/DC systems where a lower boundary of operation should be established. In the

case of sequencing supplies, the divider is connected to a rail that becomes active earlier in the power-up cycle than the

LMZ14202EXT output rail. The two resistors should be chosen based on the following ratio:

R_ENT/ R_ENB= (V_{IN UVLO}/ 1.18V) – 1 (1)

The LMZ14202EXT demonstration and evaluation boards use 11.8kΩ for R_ENBand 68.1kΩ for R_ENTresulting in a rising UVLO of

8V. This divider presents 6.25V to the EN input when the divider input is raised to 42V.

OUTPUT VOLTAGE SELECTION

Output voltage is determined by a divider of two resistors connected between V_Oand ground. The midpoint of the divider is con-

nected to the FB input. The voltage at FB is compared to a 0.8V internal reference. In normal operation an on-time cycle is initiated

when the voltage on the FB pin falls below 0.8V. The main MOSFET on-time cycle causes the output voltage to rise and the voltage

at the FB to exceed 0.8V. As long as the voltage at FB is above 0.8V, on-time cycles will not occur.

The regulated output voltage determined by the external divider resistors RFBT and RFBB is:

V_O= 0.8V * (1 + R_FBT/ R_FBB) (2)

Rearranging terms; the ratio of the feedback resistors for a desired output voltage is:

R_FBT/ R_FBB= (V_O/ 0.8V) - 1 (3)

These resistors should be chosen from values in the range of 1.0 kohm to 10.0 kohm.

For V_O= 0.8V the FB pin can be connected to the output directly so long as an output preload resistor remains that draws more

than 20uA. Converter operation requires this minimum load to create a small inductor ripple current and maintain proper regulation

when no load is present.

A feed-forward capacitor is placed in parallel with R_FBTto improve load step transient response. Its value is usually determined

experimentally by load stepping between DCM and CCM conduction modes and adjusting for best transient response and minimum

output ripple.

A table of values for R_FBT, R_FBB, C_FFand R_ONis included in the applications schematic.

SOFT-START CAPACITOR SELECTION

Programmable soft-start permits the regulator to slowly ramp to its steady state operating point after being enabled, thereby reducing

current inrush from the input supply and slowing the output voltage rise-time to prevent overshoot.

Upon turn-on, after all UVLO conditions have been passed, an internal 8uA current source begins charging the external soft-start

capacitor. The soft-start time duration to reach steady state operation is given by the formula:

t_SS= V_REF* C_SS/ Iss = 0.8V * C_SS/ 8uA (4)

This equation can be rearranged as follows:

C_SS= t_SS* 8 μA / 0.8V (5)

Use of a 0.022μF capacitor results in 2.2msec soft-start duration which is recommended as a minimum value.

As the soft-start input exceeds 0.8V the output of the power stage will be in regulation. The soft-start capacitor continues charging

until it reaches approximately 3.8V on the SS pin. Voltage levels between 0.8V and 3.8V have no effect on other circuit operation.

LMZ14202EXT

Note that the following conditions will reset the soft-start capacitor by discharging the SS input to ground with an internal 200 μA

current sink.

• The enable input being “pulled low”

• Thermal shutdown condition

• Over-current fault

• Internal Vcc UVLO (Approx 4V input to V_IN)

C_OSELECTION

None of the required C_Ooutput capacitance is contained within the module. At a minimum, the output capacitor must meet the

worst case minimum ripple current rating of 0.5 * I_{LR P-P}, as calculated in equation (19) below. Beyond that, additional capacitance

will reduce output ripple so long as the ESR is low enough to permit it. A minimum value of 10 μF is generally required. Experi-

mentation will be required if attempting to operate with a minimum value. Ceramic capacitors or other low ESR types are

recommended. See AN-2024 for more detail.

The following equation provides a good first pass approximation of C_Ofor load transient requirements:

C_O≥I_STEP*V_FB*L*V_IN/ (4*V_O*(V_IN—V_O)*V_OUT-TRAN)(6)

Solving:

C_O≥ 2A*0.8V*10μH*24V / (4*3.3V*( 24V — 3.3V)*33mV)

≥ 43μF (7)

The LMZ14202EXT demonstration and evaluation boards are populated with a 100 uF 6.3V X5R output capacitor. Locations for

extra output capacitors are provided. See AN-2024 for locations.

C_INSELECTION

The LMZ14202EXT module contains an internal 0.47 µF input ceramic capacitor. Additional input capacitance is required external

to the module to handle the input ripple current of the application. This input capacitance should be located in very close proximity

to the module. Input capacitor selection is generally directed to satisfy the input ripple current requirements rather than by capac-

itance value. Worst case input ripple current rating is dictated by the equation:

I(C_IN(RMS)) ≊ 1 /2 * I_O* √ (D / 1-D) (8)

where D ≊ V_O/ V_IN

(As a point of reference, the worst case ripple current will occur when the module is presented with full load current and when

V_IN= 2 * V_O).

Recommended minimum input capacitance is 10uF X7R ceramic with a voltage rating at least 25% higher than the maximum

applied input voltage for the application. It is also recommended that attention be paid to the voltage and temperature deratings of

the capacitor selected. It should be noted that ripple current rating of ceramic capacitors may be missing from the capacitor data

sheet and you may have to contact the capacitor manufacturer for this rating.

If the system design requires a certain minimum value of input ripple voltage ΔV_INbe maintained then the following equation may

be used.

C_IN≥ I_O* D * (1–D) / f_SW-CCM* ΔV_IN(9)

If ΔV_INis 1% of V_INfor a 24V input to 3.3V output application this equals 240 mV and f_SW= 400 kHz.

C_IN≥ 2A * 3.3V/24V * (1– 3.3V/24V) / (400000 * 0.240 V)

≥ 2.5μF

Additional bulk capacitance with higher ESR may be required to damp any resonant effects of the input capacitance and parasitic

inductance of the incoming supply lines.

R_ONRESISTOR SELECTION

Many designs will begin with a desired switching frequency in mind. For that purpose the following equation can be used.

f_SW(CCM)≊ V_O/ (1.3 * 10^-10* R_ON) (10)

This can be rearranged as

R_ON≊ V_O/ (1.3 * 10 ^-10* f_SW(CCM))(11)

The selection of RON and f_SW(CCM)must be confined by limitations in the on-time and off-time for the COT control section.

The on-time of the LMZ14202EXT timer is determined by the resistor R_ONand the input voltage V_IN. It is calculated as follows:

t_ON= (1.3 * 10^-10* R_ON) / V_IN(12)

The inverse relationship of t_ONand V_INgives a nearly constant switching frequency as V_INis varied. R_ONshould be selected such

that the on-time at maximum V_INis greater than 150 ns. The on-timer has a limiter to ensure a minimum of 150 ns for t_ON. This

limits the maximum operating frequency, which is governed by the following equation:

f_SW(MAX)= V_O/ (V_IN(MAX)* 150 nsec) (13)

This equation can be used to select R_ONif a certain operating frequency is desired so long as the minimum on-time of 150 ns is

observed. The limit for R_ONcan be calculated as follows:

R_ON≥ V_IN(MAX)* 150 nsec / (1.3 * 10 ^-10) (14)

LMZ14202EXT

If R_ONcalculated in (11) is less than the minimum value determined in (14) a lower frequency should be selected. Alternatively,

V_IN(MAX)can also be limited in order to keep the frequency unchanged.

Additionally note, the minimum off-time of 260 ns limits the maximum duty ratio. Larger R_ON(lower F_SW) should be selected in any

application requiring large duty ratio.

Discontinuous Conduction and Continuous Conduction Modes

At light load the regulator will operate in discontinuous conduction mode (DCM). With load currents above the critical conduction

point, it will operate in continuous conduction mode (CCM). When operating in DCM the switching cycle begins at zero amps

inductor current; increases up to a peak value, and then recedes back to zero before the end of the off-time. Note that during the

period of time that inductor current is zero, all load current is supplied by the output capacitor. The next on-time period starts when

the voltage on the at the FB pin falls below the internal reference. The switching frequency is lower in DCM and varies more with

load current as compared to CCM. Conversion efficiency in DCM is maintained since conduction and switching losses are reduced

with the smaller load and lower switching frequency. Operating frequency in DCM can be calculated as follows:

f_SW(DCM)≊V_O*(V_IN-1)*10μH*1.18*10²⁰*I_O/(V_IN–V_O)*R_ON²(15)

In CCM, current flows through the inductor through the entire switching cycle and never falls to zero during the off-time. The

switching frequency remains relatively constant with load current and line voltage variations. The CCM operating frequency can

be calculated using equation 7 above.

Following is a comparison pair of waveforms of the showing both CCM (upper) and DCM operating modes.

CCM and DCM Operating Modes

V_IN= 24V, V_O= 3.3V, I_O= 2A/0.32A 2 μsec/div

30117712

The approximate formula for determining the DCM/CCM boundary is as follows:

I_DCB≊V_O*(V_IN–V_O)/(2*10 μH*f_SW(CCM)*V_IN) (16)

Following is a typical waveform showing the boundary condition.

Transition Mode Operation

V_IN= 24V, V_O= 3.3V, I_O= 0.35A 2 μsec/div

30117714

The inductor internal to the module is 10 μH. This value was chosen as a good balance between low and high input voltage

applications. The main parameter affected by the inductor is the amplitude of the inductor ripple current (I_LR). I_LRcan be calculated

with:

I_{LR P-P}=V_O*(V_IN- V_O)/(10µH*f_SW*V_IN) (17)

Where V_INis the maximum input voltage and f_SWis determined from equation 10.

LMZ14202EXT

If the output current I_Ois determined by assuming that I_O= I_L, the higher and lower peak of I_LRcan be determined. Be aware that

the lower peak of I_LRmust be positive if CCM operation is required.

POWER DISSIPATION AND BOARD THERMAL REQUIREMENTS

For the design case of V_IN= 24V, V_O= 3.3V, I_O= 2A, T_AMB(MAX)= 85°C , and T_JUNCTION= 125°C, the device must see a thermal

resistance from case to ambient of:

_CA< (T_J-MAX— T_AMB(MAX)) / P_IC-LOSS- θ_JC(18)

Given the typical thermal resistance from junction to case to be 1.9 °C/W. Use the 85°C power dissipation curves in the Typical

Performance Characteristics section to estimate the P_IC-LOSSfor the application being designed. In this application it is 1.5W.

_CA= (125 — 85) / 1.5W — 1.9) = 24.8

To reach θ_CA= 24.8, the PCB is required to dissipate heat effectively. With no airflow and no external heat, a good estimate of the

required board area covered by 1 oz. copper on both the top and bottom metal layers is:

Board Area_cm²= 500°C x cm²/W / θ_JC(19)

As a result, approximately 20.2 square cm of 1 oz copper on top and bottom layers is required for the PCB design. The PCB copper

heat sink must be connected to the exposed pad. Approximately thirty six, 10mils (254 μm) thermal vias spaced 59mils (1.5 mm)

apart must connect the top copper to the bottom copper. For an example of a high thermal performance PCB layout, refer to the

Evaluation Board application note AN-2024.

PC BOARD LAYOUT GUIDELINES

PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a DC-DC

converter and surrounding circuitry by contributing to EMI, ground bounce and resistive voltage drop in the traces. These can send

erroneous signals to the DC-DC converter resulting in poor regulation or instability. Good layout can be implemented by following

a few simple design rules.

30117711

1. Minimize area of switched current loops.

From an EMI reduction standpoint, it is imperative to minimize the high di/dt paths during PC board layout. The high current loops

that do not overlap have high di/dt content that will cause observable high frequency noise on the output pin if the input capacitor

(Cin1) is placed at a distance away from the LMZ14202EXT. Therefore place C_IN1as close as possible to the LMZ14202EXT VIN

and GND exposed pad. This will minimize the high di/dt area and reduce radiated EMI. Additionally, grounding for both the input

and output capacitor should consist of a localized top side plane that connects to the GND exposed pad (EP).

2. Have a single point ground.

The ground connections for the feedback, soft-start, and enable components should be routed to the GND pin of the device. This

prevents any switched or load currents from flowing in the analog ground traces. If not properly handled, poor grounding can result

in degraded load regulation or erratic output voltage ripple behavior. Provide the single point ground connection from pin 4 to EP.

3. Minimize trace length to the FB pin.

Both feedback resistors, R_FBTand R_FBB, and the feed forward capacitor C_FF, should be located close to the FB pin. Since the FB

node is high impedance, maintain the copper area as small as possible. The trace are from R_FBT, R_FBB, and C_FFshould be routed

away from the body of the LMZ14202EXT to minimize noise.

4. Make input and output bus connections as wide as possible.

This reduces any voltage drops on the input or output of the converter and maximizes efficiency. To optimize voltage accuracy at

the load, ensure that a separate feedback voltage sense trace is made to the load. Doing so will correct for voltage drops and

provide optimum output accuracy.

5. Provide adequate device heat-sinking.

Use an array of heat-sinking vias to connect the exposed pad to the ground plane on the bottom PCB layer. If the PCB has a

plurality of copper layers, these thermal vias can also be employed to make connection to inner layer heat-spreading ground planes.

For best results use a 6 x 6 via array with minimum via diameter of 10mils (254 μm) thermal vias spaced 59mils (1.5 mm). Ensure

enough copper area is used for heat-sinking to keep the junction temperature below 125°C.

LMZ14202EXT

Additional Features

OUTPUT OVER-VOLTAGE COMPARATOR

The voltage at FB is compared to a 0.92V internal reference. If FB rises above 0.92V the on-time is immediately terminated. This

condition is known as over-voltage protection (OVP). It can occur if the input voltage is increased very suddenly or if the output

load is decreased very suddenly. Once OVP is activated, the top MOSFET on-times will be inhibited until the condition clears.

Additionally, the synchronous MOSFET will remain on until inductor current falls to zero.

CURRENT LIMIT

Current limit detection is carried out during the off-time by monitoring the current in the synchronous MOSFET. Referring to the

Functional Block Diagram, when the top MOSFET is turned off, the inductor current flows through the load, the PGND pin and the

internal synchronous MOSFET. If this current exceeds 2.6 A (typical) the current limit comparator disables the start of the next on-

time period. The next switching cycle will occur only if the FB input is less than 0.8V and the inductor current has decreased below

2.6 A. Inductor current is monitored during the period of time the synchronous MOSFET is conducting. So long as inductor current

exceeds 2.6A, further on-time intervals for the top MOSFET will not occur. Switching frequency is lower during current limit due to

the longer off-time. It should also be noted that current limit is dependent on both duty cycle and temperature as illustrated in the

graphs in the typical performance section.

THERMAL PROTECTION

The junction temperature of the LMZ14202EXT should not be allowed to exceed its maximum ratings. Thermal protection is im-

plemented by an internal Thermal Shutdown circuit which activates at 165 °C (typ) causing the device to enter a low power standby

state. In this state the main MOSFET remains off causing V_Oto fall, and additionally the CSS capacitor is discharged to ground.

Thermal protection helps prevent catastrophic failures for accidental device overheating. When the junction temperature falls back

below 145 °C (typ Hyst = 20 °C) the SS pin is released, V_Orises smoothly, and normal operation resumes.

Applications requiring maximum output current especially those at high input voltage may require application derating at elevated

temperatures.

ZERO COIL CURRENT DETECTION

The current of the lower (synchronous) MOSFET is monitored by a zero coil current detection circuit which inhibits the synchronous

MOSFET when its current reaches zero until the next on-time. This circuit enables the DCM operating mode, which improves

efficiency at light loads.

PRE-BIASED STARTUP

The LMZ14202EXT will properly start up into a pre-biased output. This startup situation is common in multiple rail logic applications

where current paths may exist between different power rails during the startup sequence. The following scope capture shows proper

behavior during this event.

Pre-Biased Startup

30117725

LMZ14202EXT

Evaluation Board Schematic Diagram

30117707

Ref Des

Description

SIMPLE SWITCHER ®

1 µF, 50V, X7R

Case Size

TO-PMOD-7

1206

Case Size

National Semiconductor

Taiyo Yuden

Vishay Dale

TDK

Manufacturer P/N

LMZ14202EXTTZ-ADJ

UMK316B7105KL-T

UMK325BJ106MM-T

UMK316B7105KL-T

JMK325BJ107MM-T

CRCW06033K32FKEA

CRCW06031K07FKEA

CRCW060361k9FKEA

CRCW060368k1FKEA

CRCW060311k8FKEA

C1608X7R1H223K

C_in1

C_in2

10 µF, 50V, X7R

1 µF, 50V, X7R

1210

C_O1

1206

C_O2

100 µF, 6.3V, X7R

1210

R_FBT

R_FBB

R_ON

R_ENT

R_ENB

C_FF

0603

3.32 kΩ

1.07 kΩ

0603

61.9 kΩ

0603

68.1 kΩ

0603

11.8 kΩ

22 nF, ±10%, X7R, 16V

0603

C_SS

22 nF, ±10%, X7R, 16V

0603

TDK

C1608X7R1H223K

LMZ14202EXT

30117716

30117717

LMZ14202EXT

Physical Dimensions inches (millimeters) unless otherwise noted

7-Lead TZA Package

NS Package Number TZA07A

Notes

Incorporated

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LMZ14202EXTTZ/NOPB [TI]

相关型号：

LMZ14202EXTTZE

LMZ14202EXTTZE-ADJ

LMZ14202EXTTZE-ADJ

LMZ14202EXTTZE/NOPB

LMZ14202EXTTZX

LMZ14202EXTTZX-ADJ

LMZ14202EXTTZX/NOPB

LMZ14202EXT_13

LMZ14202H

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LMZ14202HTZ/NOPB