APW7061 データシートの表示（PDF） - Anpec Electronics

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APW7061

Synchronous Buck PWM Controller

Anpec Electronics 

APW7061

Application Information (Cont.)

Inductor Selection (Cont.)

The poles and zero of this transfer function are:

into lower output ripple voltage. The ripple current and

ripple voltage can be approximated by:

IRIPPLE =

VIN - VOUT x VOUT

Fs x L

VIN

∆VOUT = IRIPPLE x ESR

where Fs is the switching frequency of the regulator.

There is a tradeoff exists between the inductor’s ripple

current and the regulator load transient response time

A smaller inductor will give the regulator a faster load

trans ient res ponse at the expens e of higher ripple

current and vice versa. The maximum ripple current

occurs at the maximum input voltage. A good starting

point is to choose the ripple current to be approximately

30% of the maximum output current.

Once the induct ance value has been chosen, s elect

an inductor that is capable of carrying the required

peak current without going into s aturation. In s ome

ty pe of induc tors, es pec ially core that is make of

ferrite, the ripple current will increase abruptly when it

saturates. This will result in a larger output ripple

voltage.

Compensation

1

FLC =

2 × π × L × COUT

= FESR

1

2 × π × ESR× COUT

The FLC is the double poles of the LC filter, and FESR is

the zero introduced by the ESR of the output capacitor.

PHASE

L

Output

COUT

ESR

Gain

Figure 1. The Output LC Filter

F LC

-40dB/dec

FESR

-20dB/dec

The output LC filter introduces a double pole, which

contributes wit h – 40dB/decade gain slope and 180

degrees phase shift in the control loop. A compensation

network between COMP pin and ground should be

added. The simplest loop compensation network is

shown in Figure. 4.

The out put LC filter consist s of the output induc tor

and output capacitors. The transfer function of the LC

filter is given by:

GAINLC =

1+ s ×ESR × COUT

s2 ×L × COUT+ s× ESR +1

Frequency

Figure 2. The Output LC Filter Gain & Frequency

The PWM modulator is shown in Figure. 3. The input

is the output of the error amplifier and the output is the

P HA SE node. The trans fer func tion of t he PW M

modulator is given by:

VIN

GAINPWM = ∆VOSC

C opyright © ANPEC Electronics C orp.

11

Rev. A.7 - Nov., 2005

www.anpec.com.tw

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