ACT4455 データシートの表示（PDF） - Active-Semi, Inc

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ACT4455

36V/5A Step Down DC/DC Converter

Active-Semi, Inc 

ACT4455

Rev 2, 21-Nov-12

STABILITY COMPENSATION

Figure 2:

Stability Compensation

If RCOMP is limited to 15kΩ, then the actual cross

over frequency is 6.36 / (VOUTCOUT). Therefore:

CCOMP = 6.67 ×10−6VOUTCOUT

(F)

(15)

c

c: CCOMP2 is needed only for high ESR output capacitor

The feedback loop of the IC is stabilized by the

components at the COMP pin, as shown in Figure

2. The DC loop gain of the system is determined by

the following equation:

AVDC

= 0 .808 V

I OUT

AVEA G COMP

(8)

The dominant pole P1 is due to CCOMP:

fP1

=

G EA

2 π AVEA C COMP

(9)

The second pole P2 is the output pole:

fP 2

=

I OUT

2πVOUT C OUT

(10)

The first zero Z1 is due to RCOMP and CCOMP:

fZ1

=

1

2πRCOMP CCOMP1

(11)

And finally, the third pole is due to RCOMP and

CCOMP2 (if CCOMP2 is used):

fP3

=

1

2πRCOMP CCOMP2

(12)

The following steps should be used to compensate

the IC:

STEP 1. Set the cross over frequency at 1/10 of the

switching frequency via RCOMP:

RCOMP

=

2πVOUT COUT fSW

10GEAGCOMP × 0.808V

= 0.48 × 10 8VOUT COUT (Ω)

(13)

STEP 2. Set the zero fZ1 at 1/4 of the cross over

frequency. If RCOMP is less than 15kΩ, the equation

for CCOMP is:

C COMP

= 3 .18 × 10 −5

R COMP

(F)

(14)

STEP 3. If the output capacitor’s ESR is high

enough to cause a zero at lower than 4 times the

cross over frequency, an additional compensation

capacitor CCOMP2 is required. The condition for using

CCOMP2 is:

RESRCOUT

≥

Min⎜⎜⎝⎛

1.1 ×10

COUT

−6

,0.012 ×VOUT

⎟⎟⎠⎞

(Ω) (16)

And the proper value for CCOMP2 is:

CCOMP 2

= C R OUT ESRCOUT

RCOMP

(17)

Though CCOMP2 is unnecessary when the output

capacitor has sufficiently low ESR, a small value

CCOMP2 such as 100pF may improve stability against

PCB layout parasitic effects.

Table 1 shows some calculated results based on

the compensation method above.

Table 1:

Typical Compensation for Different Output

Voltages and Output Capacitors

VOUT

2.5V

3.3V

5V

2.5V

3.3V

5V

COUT

RCOMP

47μF SP CAP 5.6kΩ

47μF SP CAP 7.5kΩ

47μF SP CAP 11kΩ

680μF/6.3V/30mΩ 15kΩ

680μF/6.3V/30mΩ 15kΩ

680μF/6.3V/30mΩ 15kΩ

CCOMP

5.6nF

4.7nF

3.3nF

3.3nF

3.3nF

4.7nF

CCOMP2c

None

None

None

220pF

220pF

220pF

c: CCOMP2 is needed for high ESR output capacitor.

Output Cable Resistance Compensation

To compensate for resistive voltage drop across the

charger's output cable, the ACT4455 integrates a

simple, user-programmable cable voltage drop

compensation using the impedance at the FB pin.

Use the curve in Figure 3 to choose the proper

feedback resistance values for cable compensation.

RFB1 is the high side resistor of voltage divider.

Innovative PowerTM

-8-

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

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