IR3871MPBF 查看數據表（PDF） - International Rectifier

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IR3871MPBF

8A HIGHLY INTEGRATED WIDE-INPUT VOLTAGE, SYNCHRONOUS BUCK REGULATOR

International Rectifier 

IR3871MPBF

DESIGN EXAMPLE

Design Criteria:

Input Voltage, VIN, = 6V to 21V

Output Voltage, VOUT = 1.25V

Switching Frequency, FS = 400KHz

Inductor Ripple Current, 2ΔI = 3A

Maximum Output Current, IOUT = 6A

Over Current Trip, IOC = 9A

Overshoot Allowance, VOS = VOUT + 50mV

Undershoot Allowance, VDROP = 50mV

Find RFF :

RFF 

1.25V

 156 k

1V  20 pF  400kHz

Pick a standard value 158 kΩ, 1% resistor.

Find RSET :

RSET  1.4 10m  9A

20 A

 6.3 k

The RDSON of the lower MOSFET could be

expected to increase by a factor of 1.4 over

temperature. Therefore, pick a 6.49kΩ, 1%

standard resistor.

Find a resistive voltage divider for VOUT = 1.25V:

VFB  R2  VOUT  0.5V

R2  R1

R2 = 1.33kΩ, R1 = 1.96 kΩ, both 1% standard

resistors.

Choose the soft start capacitor:

Once the soft start time has chosen, such as

1000us to reach to the reference voltage, a 22nF

for CSS is used to meet 1000µs.

Choose an inductor to meet the design

specification:

  L  VOUT  VIN  VOUT

VIN  2ΔI  Fs

 1.25V  21V -1.25V 

21V 3A 400kHz

 1.0H

Choose an inductor with the lowest DCR and

AC power loss as possible to increase the

overall system efficiency. For instance, choose

an FDU0650-R82M manufactured by TOKO.

The inductance of this part is 820nH and has

4.2mΩ DCR. Ripple current needs to be

recalculated using the chosen inductor.

ΔI  1.25V 21V -1.25V  1.75A

2 21V  0.82H  400kHz

Choose an input capacitor:

IIN_RMS  6 A 

1.25V 

1



1





1.75

A

2





 1.5A

21V

3  6A 

A Panasonic 10µF (ECJ3YB1E106M)

accommodates 6 Arms of ripple current at

300KHz. Due to the chemistry of multilayer

ceramic capacitors, the capacitance varies over

temperature and operating voltage, both AC and

DC. One 10µF capacitor is recommended. In a

practical solution, one 1µF capacitor is required

along with 10µF. The purpose of the 1µF

capacitor is to suppress the switching noise and

deliver high frequency current.

Choose an output capacitor:

To meet the undershoot specification, select a

set of output capacitors which has an equivalent

ESR of 10mΩ (50mV/5A). To meet the

overshoot specification, equation 8 will be used

to calculate the minimum output capacitance.

As a result, 160µF will be needed for 5A load

remover. Combine those two requirements, one

can choose a set of output capacitors from

manufactures such as SP-Cap (Specialty

Polymer Capacitor) from Panasonic or

POSCAP from Sanyo. A 150µF

(EEFSL0D151R) from Panasonic is

recommended. This capacitor has 9mΩ ESR

which leaves margin for the voltage drop of the

ESL during load step up. The typical ESL for

this capacitor is around 2nH.

16

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