APW7134 データシートの表示（PDF） - Kingbright

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APW7134

Dual 1.5MHz, 600mA Synchronous Step-Down Converter

Kingbright 

APW7134

Application Description (Cont.)

Input Capacitor Selection Cont.

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 structure. The

waveforms show as Figure-2.

Observe the waveform of I(CIN),the RMS value of I(CIN) is

[ ] ( ) ( ) ( ) I CIN =

IOUT −IIN 2 ⋅

2

D + IIN ⋅

2

1−D

(2)

Replace D and IIN by following relation:

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 capacitor with suitable current rating.

Output Capacitor Selection

IL

The output voltage ripple is a significant parameter to

IOUT

estimate the performance of a convertor. There are

two discrete components that affect the output

0A

IIN

voltage ripple bigger or smaller. It is recommended to

use the criterion has mentioned above to choose a

suitable inductor. Then based on this known inductor

0A

IIN

I(CIN)

current ripple condition, the value and properties of

output capacitor will affect the output voltage ripple

better or worse. The output voltage ripple consists of

two portions, one is the product of ESR and inductor

0A

current ripple, the other portion is a function of the

I(COUT)

inductor current ripple and the output capacitance.

Figure-3 shows the waveforms to explain the part

I(Q1)

decided by the output capacitance.

IOUT

0A

D*TS

(1-D)*TS

0A

PWM

Figure-2

Copyright © ANPEC Electronics Corp.

Rev. A.1 - Aug., 2006

0A

∆IL

I(COUT)

0.5TS

∆VOUT1

VOUT

Figure-3

11

www.anpec.com.tw

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