APW7176A Просмотр технического описания (PDF) - Anpec Electronics

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APW7176A

Dual 1.5MHz, 1A Synchronous Step-Down Converter

Anpec Electronics 

APW7176A

Application Information

Inductor Selection

Due to the high switching frequency as 1.5MHz, the in-

ductor value of the application of the APW7176A is usually

in the range from 1µH to 4.7µH. The criterion of selecting

a suitable inductor depends on its maximum current ripple.

The maximum current ripple defines as 40% of the fully

load current. In the APW7176A applications, the maximum

value of current ripple is 400mA, the 40% of 1A. Calculate

L by the equation (1):

( ) L = VIN − VOUT ⋅ VOUT ⋅ 1 ...............(1)

VIN

∆IL ⋅ fOSC

where fOSC is the switching frequency of APW7176A and

∆IL is the value of the maximum current ripple. It can be

any value of current ripple that smaller than the maximum

value you can accept. In order to perform high efficiency,

selecting a low DC resistance inductor is a helpful way.

Another important parameter is the DC current rating of

the inductor. The minimum value of DC current rating

equals the full load value of 1A, and then plus the half of

the current ripple. Choose inductors with suitable DC cur-

rent rating to ensure the inductors don’t operate in the

saturation.

Input Capacitor Selection

The input capacitor must be able to support the maxi-

mum input operating voltage and maximum RMS input

current. The Buck converter absorbs pulse current from

input power source.

I(Q1)

IIN

I(CIN)

Q1

L

VIN

CIN

I(L)

Q2

COUT

I(COUT)

IOUT

PWM

Figure-1

Figure-1 shows a schematic of a Buck converter. The

waveforms are shown as Figure-2.

IL

IOUT

IIN

0A

0A

IIN

I(CIN)

0A

I(COUT)

I(Q1)

IOUT

0A

D*TS

(1-D)*TS

0A

PWM

Figure-2

By observing the waveform of I(C ), the RMS value of I(C )

IN

IN

is

[ ] ( ) ( ) ( ) I CIN =

IOUT − IIN 2 ⋅

2

2

D + IIN ⋅ 1− D

.....(2)

Replace D and I by following relation:

IN

D = VOUT ..............................(3)

VIN

IIN = D ⋅IOUT .........................(4)

The RMS value of input capacitor current equal:

( ) I CIN = IOUT ⋅ D(1− D)............(5)

When D=0.5, the RMS current of input capacitor will be

maximum value. Use this value to choose the input ca-

pacitor with suitable current rating.

Copyright © ANPEC Electronics Corp.

9

Rev. A.1 - Oct., 2012

www.anpec.com.tw

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