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ACT6390
(Rev.:2007)

1.7A/2.5A PWM Step-Up DC/DC Converters In MSOP

Active-Semi, Inc 

ACT6390/ACT6391

Rev PrA, 01-Sep-07

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.

fZ RHP 



VIN 2  RLOAD

2VOUT 2  π  L

(16)

Choose fC



1

5

fZ RHP  ,

then calculate CCOMP:

  CCOMP

 VFB

VOUT

 RLOAD

RCS



GM

2πfC

1D



VIN VFB

V2

OUT



RLOAD  GM

RCS  2πfC

(17)

Select RCOMP to meet the transient-droop require-

ments.

α

VFB

 GM

 RCOMP



RCS



VOUT  IOUT

VIN  η

1 



K

2





RCOMP



RCS

 VOUT

 IOUT

1 



K

2





α VFB  GM VIN  η

(18)

(19)

Where:

α is the transient droop percentage which can be

accepted, calculated by:

α  ΔVOUT

VOUT

(20)

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.

COUT

 RCOMP  CCOMP

RLOAD

(21)

CCOMP2 is optional and can be used when the output

capacitor has significant ESR. The ESR will form a

zero as follows:

fZ ESR 



2π

1

 RESR

 COUT

(22)

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,

CCOMP2



COUT  RESR

RCOMP

(23)

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

than 10pF, CCOMP2 can be omitted.

For example:

3.3V 2   12V 

fZ RHP 



2  12V 2

 250mA

π 10μH

 57.8kHz

(24)

Choose

fC



1

5

fZ RHP 

 11.56kHz

CCOMP



3.3V 1.24V

12V2



48Ω

0.45Ω



2π

150μS

11.56kHz



6.26nF

(25)

Choose CCOMP = 6.8nF

Assume that 200mV of transient droop can be

accepted:

α  200mV  1

12V 60

(26)

0.45Ω12V 250mA1 0.4 

RCOMP 

1

 2  186.3kΩ

1.24V 150μS 3.3V 0.85

(27)

60

Choose RCOMP = 180kΩ

COUT



RCOMP  CCOMP

RLOAD

 180kΩ 6.8nF

 12V 

 25.5μF

(28)

 0.25A 

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

choose 33µF to reduce droop.

RCOMP



RLOAD  COUT

CCOMP

 48Ω  33μF

6.8nF

 233kΩ

(29)

If a ceramic capacitor is used with an assumed

ESR of 20mΩ,

fZ ESR 



2π

1

 33 μF

 20mΩ



241kHz

(30)

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,

Innovative Products. Active Solutions.

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Copyright © 2007 Active-Semi, Inc.

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