APW1173 데이터 시트보기 (PDF) - Anpec Electronics

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APW1173

2A SWITCH STEP DOWN SWITCHING REGULATOR

Anpec Electronics 

APW1173

Application Description (Cont.)

Frequency Compensation (Cont.)

L

issue.

cause the phase decrease rapidly at the natural

frequency and lead the phase margin not enough to

maintain the stable status. The stable issue improved

by apply a zero in the frequency domain to increase

the phase margin.

FB PIN

VOUT

ESR

COUT

Ceramic

type

Loading

1.235V

EA

COMP PIN

RC1

CC1

CC2

Adding a resistor and a capacitor at the COMP pin is

the simplest way to generate a zero. The placement

of the components is the show of Figure-1. The

frequency of the zero is

f zero

=

1

2πRC1CC1

(10)

The relation of the zero and the natural frequency is

f zero = 0.8 ⋅ f natural

(11)

Locate the zero before the natural frequency to com-

pensate the phase. The another capacitor CC2 used to

bypass the noise. In general

CC 2

=

1

10

CC1

(12)

In the other applications, use the ceramic capacitor as

the output capacitor is very popular. Because the small

dimension of the ceramic capacitor save the PCB

(Printed Circuit Board) area, the low ESR(Equivalent

Series Resistance) of the ceramic one decrease the

power dissipation of the output capacitor.But the

serious drawbacks of the ceramic one is the stable

Consider the Figure-2, find the transfer function H(s)

as:

H (s)

=

S2

SCOUT (ESR) +1

LCOUT + SCOUT (ESR)

+1

pole1,2 = 2π

1

LCOUT

zero1

=

1

2π(ESR)COUT

Q= 1

L

(ESR) COUT

The pole1 and pole2 are the conjugate roots of the

denominator and the zero1 is the root of the numerator.

Find the Q factor from the quadratic function and the

description of Q factor as above.

The frequency response of the output stage show as

Figure-3.

0db

slope=-40db/

decade

0d

-90d

-135d

-180d

phase

f

Pole1,2 Zero1

Figure-3

Copyright © ANPEC Electronics Corp.

15

Rev. A.5 - Aug., 2005

www.anpec.com.tw

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