MIC2179BSM-TR [MICROCHIP]

Switching Regulator, Current-mode, 5.5A, 240kHz Switching Freq-Max, PDSO20;


型号：	MIC2179BSM-TR
厂家：	MICROCHIP
描述：	Switching Regulator, Current-mode, 5.5A, 240kHz Switching Freq-Max, PDSO20 光电二极管
文件：	总13页 (文件大小：353K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

Micrel, Inc.

MIC2179

1.5A Synchronous Buck Regulator

General Description

Features

The Micrel MIC2179 is a 200kHz synchronous buck (step-

down) switching regulator designed for high-efﬁciency, bat-

tery-powered applications.

• 4.5V to 16.5V input voltage range

• Dual-mode operation for high efﬁciency (up to 96%)

PWM mode for > 150mA load current

Skip mode for <150mA load current

• 150mΩ internal power MOSFETs at 12V input

• 200kHz preset switching frequency

• Low quiescent current

TheMIC2179operatesfroma4.5Vto16.5Vinputandfeatures

internal power MOSFETs that can supply up to 1.5A output

current. It can operate with a maximum duty cycle of 100%

for use in low-dropout conditions. It also features a shutdown

mode that reduces quiescent current to less than 5µA.

1.0mA in PWM mode

600µA in skip mode

< 5µA in shutdown mode

The MIC2179 achieves high efﬁciency over a wide output

current range by operating in either PWM or skip mode. The

operating mode is externally selected, typically by an intel-

ligentsystem,whichchoosestheappropriatemodeaccording

to operating conditions, efﬁciency, and noise requirements.

The switching frequency is preset to 200kHz and can be

synchronized to an external clock signal of up to 300kHz.

• Current-mode control

Simpliﬁed loop compensation

Superior line regulation

• 100% duty cycle for low dropout operation

• Current limit

• Thermal shutdown

TheMIC2179usescurrent-modecontrolwithinternalcurrent

sensing. Current-mode control provides superior line regula-

tionandmakestheregulatorcontrolloopeasytocompensate.

The output is protected with pulse-by-pulse current limiting

andthermalshutdown. Undervoltagelockoutturnstheoutput

off when the input voltage is less than 4.5V.

• Undervoltage lockout

Applications

• High-efﬁciency, battery-powered supplies

• Buck (step-down) dc-to-dc converters

• Cellular telephones

• Laptop computers

• Hand-held instruments

• Battery Charger

The MIC2179 and is packaged in a 20-lead SSOP package

with an operating temperature range of –40°C to +85°C.

Typical Application

V_IN

5.4V to 16.5V

10µF

16,17

20V

VIN

20k

22µH

3,4

V_OUT

PGND

3.3V/600mA

Output Good

Output Low

MBRM120

PWRGD

1,2,

MIC

2179-3.3

19,20

100µF

6.3V

Skip Mode

PWM Mode

PWM

SYNC

COMP

SGND

9–12

BIAS

0.01µF

4.02k

Pins 4 and 18 are not connected.

6.8nF

Pins 3 and 4 can be connected

together for a low-impedance

connection.

Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com

June 2009

M9999-063009

Micrel, Inc.

MIC2179

Ordering Information

Part Number

Voltage

Temperature Range

Package

Standard*

Pb-Free

MIC2179BSM

MIC2179YSM

Adj.

3.3V

5.0V

-40°C to +85°C

20-Lead SSOP

MIC2179-3.3BSM MIC2179-3.3YSM

MIC2179-5.0BSM MIC2179-5.0YSM

* Standard product will be supported as Pb-Free IAW PCCN #040004 effective 1-1-2005 pending residual depletion.

Pin Conﬁguration

PGND 1

20 PGND

PGND

VIN

PWM

PWRGD

BIAS

SYNC

SGND

COMP

SGND

20-Lead Wide SSOP

Pin Description

Pin Number

Pin Name

PGND

Pin Function

1, 2, 19, 20

Power Ground: Connect all pins to central ground point.

Switch (Output): Internal power MOSFET output switches.

PWM

PWM/Skip-Mode Control (Input): Logic-level input. Controls regulator

operating mode. Logic low enables PWM mode. Logic high enables skip

mode. Do not allow pin to ﬂoat.

PWRGD

Error Flag (Output): Open-drain output. Active low when FB input is 10%

below the reference voltage (V_REF).

Feedback (Input): Connect to output voltage divider resistors.

COMP

Compensation: Output of internal error ampliﬁer. Connect capacitor or

series RC network to compensate the regulator control loop.

9–12

SGND

SYNC

Signal Ground: Connect all pins to ground, PGND.

Frequency Synchronization (Input): Optional. Connect an external clock

signal to synchronize the oscillator. Leading edge of signal above 1.7V

terminates switching cycle. Connect to SGND if not used.

BIAS

Internal 3.3V Bias Supply: Decouple with 0.01µF bypass capacitor to

SGND. Do not apply any external load.

Enable (Input): Logic high enables operation. Logic low shuts down

regulator. Do not allow pin to ﬂoat.

16, 17

4, 18

VIN

Supply Voltage (Input): Requires bypass capacitor to PGND. Both pins

must be connected to V_IN.

not internally connected.

June 2009

M9999-063009

Micrel, Inc.

MIC2179

Absolute Maximum Ratings⁽¹⁾

Operating Ratings⁽²⁾

Supply Voltage [100ms transient] (V ) .........................18V

Supply Voltage (V ).......................................4.5V to 16.5V

...................................................

Output Switch Voltage (V

)

18V

Junction Temperature Range (T ) ............ –40°C to +125°C

Output Switch Current (I )......................................... 6.0A

Enable, PWM Control Voltage (V , V

)..................18V

PWM

Sync Voltage (V

)......................................................6V

SYNC

Electrical Characteristics⁽³⁾

V_IN= 7.0V; T_A= 25°C, bold indicates –40°C ≤ T_A≤ 85°C; unless noted.

Symbol

Parameter

Condition

Min

Typ

Max

Units

I_SS

Input Supply Current

PWM mode, output not switching,

4.5V ≤ V_IN≤ 16.5V

1.0

1.5

skip mode, output not switching,

600

750

µA

4.5V ≤ V_IN≤ 16.5V

V_EN= 0V, 4.5V ≤ V_IN≤ 16.5V

V_IN= 16.5V

3.4

µA

V_BIAS

V_FB

Bias Regulator Output Voltage

Feedback Voltage

3.10

3.30

1.245

3.3

MIC2179 [adj.]: V_OUT= 3.3V, I_LOAD= 0

1.22

1.27

V_OUT

Output Voltage

MIC2179 [adj.]: V_OUT= 3.3V,

5V ≤ V_IN≤ 16V, 10mA ≤ I_LOAD≤ 1A

3.20

3.14

3.40

3.46

MIC2179-5.0: I_LOAD= 0

4.85

5.0

5.15

MIC2179-5.0:

4.85

5.15

6V ≤ V_IN≤ 16V, 10mA ≤ I_LOAD≤ 1A

4.75

5.25

MIC2179-3.3: I_LOAD= 0

3.20

3.3

3.40

MIC2179-3.3:

5V ≤ V_IN≤ 16V, 10mA ≤ I_LOAD≤ 1A

3.20

3.14

3.40

3.46

V_TH

V_TL

I_FB

Undervoltage Lockout

Feedback Bias Current

upper threshold

lower threshold

MIC2179 [adj.]

4.25

4.15

4.35

3.90

150

µA

MIC2179-5.0, MIC2179-3.3

0.6V ≤ V_COMP≤ 0.8V

upper limit

A_VOL

Error Ampliﬁer Gain

Error Ampliﬁer Output Swing

0.9

1.5

0.05

lower limit

0.1

Error Ampliﬁer Output Current

Oscillator Frequency

Maximum Duty Cycle

Minimum On-Time

source and sink

µA

kHz

f_O

160

100

200

240

D_MAX

t_{ON min}

V_FB= 1.0V

V_FB= 1.5V

300

1.6

400

300

2.2

SYNC Frequency Range

SYNC Threshold

220

0.8

500

–1

kHz

SYNC Minimum Pulse Width

SYNC Leakage

I_SYNC

I_LIM

V_SYNC= 0V to 5.5V

PWM mode, V_IN= 12V

skip mode

0.01

4.3

600

160

140

µA

Current Limit

3.4

5.5

mΩ

µA

R_ON

I_SW

Switch On-Resistance

Output Switch Leakage

high-side switch, V_IN= 12V

low-side switch, V_IN= 12V

V_SW= 16.5V

350

June 2009

M9999-063009

Micrel, Inc.

MIC2179

Symbol

Parameter

Condition

Min

0.8

Typ

1.6

Max

2.2

Units

Enable Threshold

Enable Leakage

PWM Threshold

PWM Leakage

PWRGD Threshold

I_EN

V_EN= 0V to 5.5V

–1

0.01

1.1

µA

0.6

1.4

I_PWM

V_PWM= 0V to 5.5V

–1

0.01

1.13

4.54

3.00

0.25

0.01

µA

MIC2179 [adj.]: measured at FB pin

MIC2179-5.0: measured at FB pin

MIC2179-3.3: measured at FB pin

I_SINK= 1.0mA

1.09

4.33

2.87

1.17

4.75

3.13

0.4

PWRGD Output Low

PWRGD Off Leakage

V_PWRGD= 5.5V

µA

Notes:

1. Exceeding the absolute maximum rating may damage the device.

2. The device is not guaranteed to function outside its operating rating.

3. Speciﬁcation for packaged product only.

General. Devices are ESD sensitive. Handling precautions recommended.

June 2009

M9999-063009

Micrel, Inc.

MIC2179

Typical Characteristics

Oscillator Frequency

vs. Temperature

Reference Voltage

vs. Temperature

Reference Voltage

vs. Temperature

1.252

1.250

1.248

1.246

1.244

1.242

1.240

1.238

3.320

3.315

3.310

3.305

3.300

3.295

3.290

3.285

3.280

205

MIC2179 [adj.]

MIC2179-3.3

200

195

190

185

180

175

-60 -30

30 60 90 120 150

-60 -30

30 60 90 120 150

-60 -30

30 60 90 120 150

TEMPERATURE (°C)

Reference Voltage

vs. Temperature

Error-Amplifier Gain

vs. Temperature

Feedback Input Bias Current

vs. Temperature

5.030

5.020

5.010

5.000

4.990

4.980

4.970

19.0

18.5

18.0

17.5

17.0

16.5

16.0

120

MIC2179-5.0

100

-60 -30

30 60 90 120 150

-60 -30

30 60 90 120 150

-60 -30

30 60 90 120 150

TEMPERATURE (°C)

Current Limit

vs. Temperature

High-Side Switch

On-Resistance

Low-Side Switch

On-Resistance

350

300

250

200

150

100

400

350

300

250

200

150

100

5.5

5.3

5.1

4.9

4.7

4.5

4.3

4.1

3.9

3.7

3.5

125°C

85°C

25°C

0°C

85°C

25°C

0°C

10 12 14 16 18

-60 -30

30 60 90 120 150

INPUT VOLTAGE (V)

TEMPERATURE (°C)

PWM-Mode

Supply-Current

Skip- and PWM-Mode

Efficiency

5.4V

PWM

OUTPUT

SWITCHING

8.4V

Skip

8.4V

PWM

5.4V

Skip

10 12 14 16 18

100

600

INPUT VOLTAGE (V)

OUTPUT CURRENT (mA)

June 2009

M9999-063009

Micrel, Inc.

MIC2179

Block Diagram

V_IN

4.5V to 16.5V

100µF

16 17

VIN

UVLO,

Thermal

Shutdown

= 1.245

(

+ 1

)

V_OUT

1.245

OUT

110mΩ

P-channel

I_SENSE

Amp.

Output

Control

Logic

V_OUT

3.3V

Regulator

Enable

Shutdown

C_OUT

BIAS

110mΩ

PGND

N-channel

4.02k

0.01µF

internal

I_LIMIT

Comp.

supplyVoltage

* Connect

_GNDto P_GND

PWM

PWM/

Skip-Mode

Select

Skip Mode

PWM Mode

I_LIMIT

Thresh.

Voltage

Bold lines indicate

high current traces

Corrective

Ramp

SYNC

Stop

200kHz

Oscillator

Skip-Mode

Comp.

Reset

Pulse

V_IN

Power Good

Comp.

20k

PWM

Comp.

PWRGD

Output Good

R_C

COMP

C_C

1.13V

V_REF1.245V

MIC2179 [Adjustable]

10 11 12

SGND

June 2009

M9999-063009

Micrel, Inc.

MIC2179

connect an external load to the BIAS pin. It is not designed

to provide an external supply voltage.

Functional Description

Micrel’s MIC2179 is a synchronous buck regulator that oper-

ates from an input voltage of 4.5V to 16.5V and provides a

regulated output voltage of 1.25V to 16.5V. Its has internal

power MOSFETs that supply up to 1.5A load current and

operates with up to 100% duty cycle to allow low-dropout

operation. To optimize efﬁciency, the MIC2179 operates in

PWM and skip mode. Skip mode provides the best efﬁciency

when load current is less than 150mA, while PWM mode is

more efﬁcient at higher current. PWM or skip-mode opera-

tion is selected externally, allowing an intelligent system (i.e.

microprocessor controlled) to select the correct operating

mode for efﬁciency and noise requirements.

Frequency Synchronization

The MIC2179 operates at a preset switching frequency of

200kHz. It can be synchronized to a higher frequency by con-

necting an external clock to the SYNC pin. The SYNC pin is

a logic level input that synchronizes the oscillator to the rising

edge of an external clock signal. It has a frequency range of

220kHz to 300kHz, and can operate with a minimum pulse

width of 500ns. If synchronization is not required, connect

SYNC to ground.

Power Good Flag

The power good ﬂag (PWRGD) is an error ﬂag that alerts a

system when the output is not in regulation. When the output

voltage is 10% below its nominal value, PWRGD is logic low,

During PWM operation, the MIC2179 uses current-mode

control which provides superior line regulation and makes

the control loop easier to compensate. The PWM switching

frequencyissetinternallyto200kHzandcanbesynchronized

to an external clock frequency up to 300kHz. Other features

include a low-current shutdown mode, current limit, under-

voltage lockout, and thermal shutdown. See the following

sections for more detail.

signalingthatV

istolow. PWRGDisanopen-drainoutput

OUT

that can sink 1mA from a pull-up resistor connected to V .

Low-Dropout Operation

Output regulation is maintained in PWM or skip mode even

when the difference between V and V

decreases below

OUT

1V. As V – V

decreases, the duty cycle increases until

OUT

Switch Output

it reaches 100%. At this point, the P-channel is kept on for

several cycles at a time, and the output stays in regulation

The switch output (SW) is a half H-bridge consisting of a

high-sideP-channelandlow-sideN-channelpowerMOSFET.

TheseMOSFETshaveatypicalon-resistanceof150mΩwhen

the MIC2179 operates from a 12V supply. Antishoot-through

circuitry prevents the P-channel and N-channel from turning

on at the same time.

until V – V

falls below the dropout voltage (dropout

OUT

voltage = P-channel on-resistance × load current).

PWM-Mode Operation

Refer to “PWM Mode Functional Diagram” which is a simpli-

ﬁed block diagram of the MIC2179 operating in PWM mode

and its associated waveforms.

Current Limit

The MIC2179 uses pulse-by-pulse current limiting to protect

the output. During each switching period, a current limit com-

paratordetectsiftheP-Channelcurrentexceeds4.3A. When

it does, the P-channel is turned off until the next switching

period begins.

When operating in PWM mode, the output P-channel and N-

channel MOSFETs are alternately switched on at a constant

frequency and variable duty cycle. Aswitching period begins

whentheoscillator generates aresetpulse.Thispulse resets

the RS latch which turns on the P-channel and turns off the

Undervoltage Lockout

N-channel. During this time, inductor current (I ) increases

Undervoltage lockout (UVLO) turns off the output when the

and energy is stored in the inductor. The current sense ampli-

input voltage (V ) is to low to provide sufﬁcient gate drive

for the output MOSFETs. It prevents the output from turning

ﬁer (I

Amp) measures the P-channel drain-to-source

SENSE

voltage and outputs a voltage proportional to I . The output

on until V exceeds 4.3V. Once operating, the output will

of I

Amp is added to a sawtooth waveform (corrective

SENSE

not shut off until V drops below 4.2V.

ramp)generatedbytheoscillator,creatingacompositewave-

form labeled I on the timing diagram. When I is

Thermal Shutdown

SENSE

greater than the error ampliﬁer output, the PWM comparator

will set the RS latch which turns off the P-channel and turns

on the N-channel. Energy is then discharged from the induc-

Thermal shutdown turns off the output when the MIC2179

junction temperature exceeds the maximum value for safe

operation. After thermal shutdown occurs, the output will not

turn on until the junction temperature drops approximately

10°C.

tor and I decreases until the next switching cycle begins.

By varying the P-channel on-time (duty cycle), the average

inductor current is adjusted to whatever value is required to

regulate the output voltage.

Shutdown Mode

The MIC2179 has a low-current shutdown mode that is con-

trolled by the enable input (EN). When a logic 0 is applied

to EN, the MIC2179 is in shutdown mode, and its quiescent

current drops to less than 5µA.

The MIC2179 uses current-mode control to adjust the duty

cycle and regulate the output voltage. Current-mode control

has two signal loops that determine the duty cycle. One is an

outer loop that senses the output voltage, and the other is

a faster inner loop that senses the inductor current. Signals

from these two loops control the duty cycle in the following

Internal Bias Regulator

An internal 3.3V regulator provides power to the MIC2179

control circuits.Thisinternal supply isbrought outtotheBIAS

pin for bypassing by an external 0.01µF capacitor. Do not

way: V

is fed back to the error ampliﬁer which compares

OUT

the feedback voltage (V ) to an internal reference voltage

June 2009

M9999-063009

Micrel, Inc.

(V ). When V

MIC2179

is lower than its nominal value, the error

the P-channel. The output switch voltage (V ) then swings

REF

OUT

ampliﬁeroutputvoltageincreases.Thisvoltagethenintersects

the current sense waveform later in switching period which

increases the duty cycle and the average inductor current. If

from V to 0.4V below ground, and I ﬂows through the

Schottky diode. L1 discharges its energy to the output and

decreases to zero. When I = 0, V swings from –0.4V

ishigherthannominal,theerrorampliﬁeroutputvoltage

to V

, and this triggers a one-shot that resets the RS latch.

OUT

decreases, reducing the duty cycle.

ResettingtheRSlatchturnsontheP-channel,andthisbegins

another switching cycle.

The PWM control loop is stabilized in two ways. First, the

innersignalloopiscompensatedbyaddingacorrectiveramp

to the output of the current sense ampliﬁer. This allows the

regulator to remain stable when operating at greater than

50% duty cycle. Second, a series resistor-capacitor load

is connected to the error ampliﬁer output (COMP pin). This

places a pole-zero pair in the regulator control loop.

The skip-mode comparator regulates V

by controlling

OUT

when the MIC2179 skips cycles. It compares V to V

REF

and has 10mV of hysteresis to prevent oscillations in the

control loop. When V is less than V – 5mV, the com-

REF

parator output is logic 1, allowing the P-channel to turn on.

Conversely, when V is greater than V

+ 5mV, the P-

REF

channel is turned off.

One more important item is synchronous rectiﬁcation. As

mentioned earlier, the N-channel output MOSFET is turned

on after the P-channel turns off. When the N-channel turns

on, its on-resistance is low enough to create a short across

the output diode. As a result, inductor current ﬂows through

the N-channel and the voltage drop across it is signiﬁcantly

lower than a diode forward voltage. This reduces power dis-

sipation and improves efﬁciency to greater than 95% under

certain operating conditions.

Note that this is a self oscillating topology which explains

why the switching frequency and duty cycle are a function

of V , V

, and the value of L1. It has the unique feature

OUT

(for a pulse-skipping regulator) of supplying the same value

of maximum load current for any value of V , V , or L1.

This allows the MIC2179 to always supply up to 300mA of

load current when operating in skip mode.

OUT

Selecting PWM- or Skip-Mode Operation

To prevent shoot through current, the output stage employs

break-before-makecircuitrythatprovidesapproximately50ns

of delay from the time one MOSFET turns off and the other

turns on. As a result, inductor current brieﬂy ﬂows through

the output diode during this transition.

PWM or skip mode operation is selected by an external

logic signal applied to the PWM pin. A logic low places the

MIC2179 into PWM mode, and logic high places it into skip

mode.Skipmodeoperationprovidesthebestefﬁciencywhen

load current is less than 150mA, and PWM operation is more

efﬁcient at higher currents.

Skip-Mode Operation

Referto“SkipModeFunctionalDiagram”whichisasimpliﬁed

block diagram of the MIC2179 operating in skip mode and

its associated waveforms.

The MIC2179 was designed to be used in intelligent sys-

tems that determine when it should operate in PWM or skip

mode. This makes the MIC2179 ideal for applications where

a regulator must guarantee low noise operation when sup-

plying light load currents, such as cellular telephone, audio,

and multimedia circuits.

Skip-mode operation turns on the output P-channel at a

frequency and duty cycle that is a function of V , V

, and

IN OUT

the output inductor value. While in skip mode, the N-chan-

nel is kept off to optimize efﬁciency by reducing gate charge

There are two important items to be aware of when selecting

PWM or skip mode. First, the MIC2179 can start-up only in

PWM mode, and therefore requires a logic low at PWM dur-

ing start-up. Second, in skip mode, the MIC2179 will supply

a maximum load current of approximately 300mA, so the

output will drop out of regulation when load current exceeds

this limit. To prevent this from occurring, the MIC2179 should

change from skip to PWM mode when load current exceeds

200mA.

dissipation. V

is regulated by skipping switching cycles

OUT

that turn on the P-channel.

To begin analyzing MIC2179 skip mode operation, assume

the skip-mode comparator output is high and the latch out-

put has been reset to a logic 1. This turns on the P-channel

and causes I to increase linearly until it reaches a current

limit of 400mA. When I reaches this value, the current limit

comparator sets the RS latch output to logic 0, turning off

June 2009

M9999-063009

Micrel, Inc.

MIC2179

PWM-Mode Functional Diagram

V_IN

4.5V to 16.5V

C_IN

VIN

16 17

V_OUT= 1.245

(

+ 1)

110mΩ

P-channel

I_SENSE

Amp.

I_L1

C_OUT

V_OUT

110mΩ

PGND

N-channel

Corrective

Ramp

SYNC

Stop

200kHz

Oscillator

Reset

Pulse

PWM

Comp.

Error

Amp.

COMP

R_C

C_C

V_REF1.245V

MIC2179 [Adjustable] PWM-Mode Signal Path

SGND

10 11 12

V_SW

Reset

Pulse

I_LOAD

I_L1

∆I_L1

Error Amp.

Output

I_SENSE

June 2009

M9999-063009

Micrel, Inc.

MIC2179

Skip-Mode Functional Diagram

V_IN

4.5V to 16.5V

C_IN

VIN

16 17

Output Control Logic

V_OUT= 1.245

(

+ 1)

110mΩ

P-channel

One

Shot

I_SENSE

Amp.

I_L1

C_OUT

V_OUT

PGND

I_LIMIT

Comp.

I_LIMIT

Thresh.

Voltage

Skip-Mode

Comp.

V_REF1.245V

MIC2179 [Adjustable] Skip-Mode Signal Path

10 11 12

SGND

V_IN

V_OUT

V_SW

One-Shot

Pulse

I_LIM

I_L1

V_REF+ 5mV

V_FB

V_REF– 5mV

June 2009

M9999-063009

Micrel, Inc.

MIC2179

To maximize efﬁciency, the inductor’s resistance must

be less than the output switch on-resistance (preferably,

50mΩ or less).

Application Information

Feedback Resistor Selection (Adjustable Version)

The output voltage is programmed by connecting an external

resistive divider to the FB pin as shown in “MIC2179 Block

Diagram.” TheratioofR1toR2determinestheoutputvoltage.

Tooptimizeefﬁciencyduringlowoutputcurrentoperation, R2

should not be less than 20kΩ. However, to prevent feedback

error due to input bias current at the FB pin, R2 should not

be greater than 100kΩ. After selecting R2, calculate R1 with

the following formula:

Output Capacitor Selection

Select an output capacitor that has a low value of ESR.

This parameter determines a regulator’s output ripple volt-

age (V

) which is generated by ∆I × ESR. Therefore,

RIPPLE

ESR must be equal or less than a maximum value calculated

for a speciﬁed V

(typically less than 1% of the output

RIPPLE

voltage) and ∆I

L(max)

V_RIPPLE

V_OUT

ESR _MAX

R1 = R2 ((_1.245V) -1)

∆I_L(max)

Typically, capacitors in the range of 100 to 220µF have ESR

less than this maximum value. The output capacitor can be

a low ESR electrolytic or tantalum capacitor, but tantalum is

a better choice for compact layout and operation at tempera-

tures below 0°C. The voltage rating of a tantalum capacitor

Input Capacitor Selection

TheinputcapacitorisselectedforitsRMScurrentandvoltage

ratingandshouldbealowESR(equivalentseriesresistance)

electrolytic or tantalum capacitor. As a rule of thumb, the

voltage rating for a tantalum capacitor should be twice the

must be 2 × V

must be 1.4 × V

, and the voltage rating of an electrolytic

OUT

value of V , and the voltage rating for an electrolytic should

OUT

be 40% higher than V

The RMS current rating must be

IN.

Output Diode Selection

equal or greater than the maximum RMS input ripple cur-

rent. A simple, worst case formula for calculating this RMS

current is:

In PWM operation, inductor current ﬂows through the output

diode approximately 50ns during the dead time when one

output MOSFET turns off the other turns on. In skip mode,

the inductor current ﬂows through the diode during the entire

P-channel off time. The correct diode for both of these condi-

tions is a 1A diode with a reverse voltage rating greater than

I_LOAD(max)

I_RMS(max)

Tantalum capacitors are a better choice for applications that

require the most compact layout or operation below 0°C.

The input capacitor must be located very close to the VIN

pin (within 0.2in, 5mm). Also, place a 0.1µF ceramic bypass

capacitor as close as possible to VIN.

V . It must be a schottky or ultrafast-recovery diode

(t < 100ns) to minimize power dissipation from the diode’s

reverse-recovery charge.

Compensation

Inductor Selection

Compensation is provided by connecting a series RC load

to the COMP pin. This creates a pole-zero pair in the regu-

lator control loop, allowing the regulator to remain stable

with enough low frequency loop-gain for good load and line

regulation. At higher frequencies, the pole-zero reduces

loop-gain to a level referred to as the mid-band gain. The

mid-band gain is low enough so that the loop gain crosses

0db with sufﬁcient phase margin. Typical values for the RC

load are 4.7nF to 10nF for the capacitor and 5kΩ to 20kΩ

for the resistor.

The MIC2179 is a current-mode controller with internal slope

compensation. As a result, the inductor must be at least a

minimum value to prevent subharmonic oscillations. This

minimum value is calculated by the following formula:

L_MIN= V_OUTx 3.0 µH/V

In general, a value at least 20% greater than L

should

MIN

be selected because inductor values have a tolerance of

±20%.

Two other parameters to consider in selecting an inductor

are winding resistance and peak current rating. The inductor

must have a peak current rating equal or greater than the

peak inductor current. Otherwise, the inductor may satu-

rate, causing excessive current in the output switch. Also,

the inductor’s core loss may increase signiﬁcantly. Both of

these effects will degrade efﬁciency. The formula for peak

inductor current is:

Printed Circuit Board Layout

A well designed PC board will prevent switching noise and

ground bounce from interfering with the operation of the

MIC2179. Agooddesigntakesintoconsiderationcomponent

placement and routing of power traces.

The ﬁrst thing to consider is the locations of the input ca-

pacitor, inductor, output diode, and output capacitor. The

input capacitor must be placed very close to the VIN pin,

the inductor and output diode very close to the SW pin, and

the output capacitor near the inductor. These components

pass large high-frequency current pulses, so they must use

short, wide power traces. In addition, their ground pins and

PGND are connected to a ground plane that is nearest the

power supply ground bus.

∆I_L(max)

I_L(peak)= I_LOAD(max)

Where:

∆I_L(max)= V_OUT(1 -

V_OUT

) x

V_IN(max)

L • f

June 2009

M9999-063009

Micrel, Inc.

MIC2179

The feedback resistors, RC compensation network, and

BIAS pin bypass capacitor should be located close to their

respective pins. To prevent ground bounce, their ground

traces and SGND should not be in the path of switching

currents returning to the power supply ground bus. SGND

and PGND should be tied together by a ground plane that

extends under the MIC2179.

Suggested Manufacturers List

Inductors

Capacitors

Diodes

Transistors

Coilcraft

1102 Silver Lake Rd.

Cary, IL 60013

tel: (708) 639-2361

fax: (708) 639-1469

AVX Corp.

General Instruments (GI)

10 Melville Park Rd.

Melville, NY 11747

tel: (516) 847-3222

fax: (516) 847-3150

Siliconix

801 17th Ave. South

Myrtle Beach, SC 29577

tel: (803) 448-9411

fax: (803) 448-1943

2201 Laurelwood Rd.

Santa Clara, CA 96056

tel: (800) 554-5565

Coiltronics

Sanyo Video Components Corp.

2001 Sanyo Ave.

San Diego, CA 92173

tel: (619) 661-6835

International Rectiﬁer Corp.

233 Kansas St.

El Segundo, CA 90245

tel: (310) 322-3331

fax: (310) 322-3332

6000 Park of Commerce Blvd.

Boca Raton, FL 33487

tel: (407) 241-7876

fax: (407) 241-9339

fax: (619) 661-1055

Bi Technologies

4200 Bonita Place

Fullerton, CA

tel: (714) 447-2345

fax: (714) 447-2500

Sprague Electric

Lower Main St.

60005 Sanford, ME 04073

tel: (207) 324-4140

Motorola Inc.

MS 56-126

3102 North 56th St.

Phoenix, AZ 85018

tel: (602) 244-3576

fax: (602) 244-4015

June 2009

M9999-063009

Micrel, Inc.

MIC2179

Package Information

20-Pin SSOP (SM)

MICREL INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA

tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 web http://www.micrel.com

This information furnished by Micrel in this data sheet is believed to be accurate and reliable. However no responsibility is assumed by Micrel for its use.

Micrel reserves the right to change circuitry and speciﬁcations at any time without notiﬁcation to the customer.

Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can

reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into

the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a signiﬁcant injury to the user. A Purchaser's

use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser's own risk and Purchaser agrees to fully indemnify

Micrel for any damages resulting from such use or sale.

June 2009

M9999-063009

MIC2179BSM-TR [MICROCHIP]

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