ACT6390 datasheet(7/11 Pages) ACTIVE-SEMI

7 / 11 page

ACT6390/ACT6391

Rev PrA, 01-Sep-07

Innovative Products. Active Solutions.

- 7 -

www.active-semi.com

(28)

(25)

(18)

(27)

(17)

(19)

(20)

the cross-over frequency (unity gain-frequency)

should be less than one-fifth of the right-half-plane

zero fZ(RHP), and lower than one-fifteenth of switch-

ing frequency fsw.

Choose

, then calculate CCOMP:

Select RCOMP to meet the transient-droop require-

ments.

Where:

is the transient droop percentage which can be

accepted, calculated by:

K: is defined in equation (4)

η:

is the typical efficiency.

VFB: is the feedback voltage, 1.24V

GM: is the trans-conductance of the error amplifier.

The output capacitor is chosen to set the output

pole for canceling the RCOMP, CCOMP zero.

CCOMP2 is optional and can be used when the output

capacitor has significant ESR. The ESR will form a

zero as follows:

If this zero occurs at a higher frequency than the

cross-over frequency, it can be ignored. Otherwise,

it should be canceled with the pole set by capacitor

CCOMP2,

If the value of CCOMP2 calculated by (23) is smaller

than 10pF, CCOMP2 can be omitted.

For example:

Choose CCOMP = 6.8nF

Assume that 200mV of transient droop can be

accepted:

Choose RCOMP = 180kΩ

COUT can be chosen to be either 22µF or 33µF,

choose 33µF to reduce droop.

If a ceramic capacitor is used with an assumed

ESR of 20mΩ,

fZ(ESR) > fC

Since the zero frequency is greater than the pole

frequency ,CCOMP2 can be omitted.

If a tantalum capacitor is used, whose ESR is about

0.5Ω,





OUT

LOAD

RHP









LOAD

OUT

LOAD

OUT

COMP





















 







OUT

COMP

OUT

COMP













 





OUT



LOAD

COMP

OUT









OUT

ESR





COMP

ESR

OUT

COMP

















kHz

250

RHP





















Choose





kHz

RHP



 

kHz

150

COMP







200



186

150

250

COMP















 





180

LOAD

COMP

OUT























(30)





kHz

241

ESR







(21)

(22)

(23)

(24)

(26)

(29)





RHP



(16)

233

COMP

OUT

LOAD

COMP











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