ACT6390/ACT6391

Rev 1, 22-Aug-13

- 7 -

www.active-semi.com

Copyright © 2013 Active-Semi, Inc.

(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

Choose

ments.

Where:

α is the transient droop percentage which can be

accepted, calculated by:

K: is defined in equation (4)

η: is the typical efficiency.

The output capacitor is chosen to set 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

For example:

Assume that 200mV of transient droop can be

accepted:

choose 33µF to reduce droop.

If a ceramic capacitor is used with an assumed

ESR of 20mΩ,

Since the zero frequency is greater than the pole

If a tantalum capacitor is used, whose ESR is about

0.5Ω,

()

L

π

V

2

R

V

f

2

OUT

LOAD

2

IN

RHP

Z

×

×

×

=

()

C

CS

M

LOAD

2

OUT

FB

IN

C

M

CS

LOAD

OUT

FB

COMP

f

π

2

R

G

R

V

V

V

D

1

f

π

2

G

R

R

V

V

C

×

×

×

×

=

−

×

×

=

⎟

⎠

⎞

⎜

⎝

⎛ +

×

×

×

×

=

×

×

×

2

K

1

η

V

I

V

R

R

G

V

α

IN

OUT

OUT

CS

COMP

M

FB

η

V

G

V

α

2

K

1

I

V

R

R

IN

M

FB

OUT

OUT

CS

COMP

×

×

×

×

⎟

⎠

⎞

⎜

⎝

⎛ +

×

×

=

OUT

OUT

V

V

Δ

α

=

LOAD

COMP

COMP

OUT

R

C

R

C

×

=

()

OUT

ESR

ESR

Z

C

R

π

2

1

f

×

×

=

COMP

ESR

OUT

2

COMP

R

R

C

C

×

=

()

()

()

kHz

8

.

57

H

μ

10

π

V

12

2

mA

250

V

12

V

3

.

3

f

2

2

RHP

Z

≈

×

×

×

⎟

⎠

⎞

⎜

⎝

⎛

×

=

Choose

()

kHz

56

.

11

f

5

1

f

RHP

Z

C

=

=

()

nF

26

.

6

kHz

56

.

11

π

2

S

μ

150

Ω

45

.

0

Ω

48

V

12

V

24

.

1

V

3

.

3

C

2

COMP

=

×

×

×

×

=

60

1

V

12

mV

200

α

=

=

Ω

k

3

.

186

85

.

0

V

3

.

3

S

μ

150

V

24

.

1

60

1

2

4

.

0

1

mA

250

V

12

Ω

45

.

0

R

COMP

=

×

×

×

×

⎟

⎠

⎞

⎜

⎝

⎛ +

×

×

=

F

μ

5

.

25

A

25

.

0

V

12

nF

8

.

6

Ω

k

180

R

C

R

C

LOAD

COMP

COMP

OUT

=

⎟

⎠

⎞

⎜

⎝

⎛

×

=

×

=

(30)

()

kHz

241

Ω

m

20

F

μ

33

π

2

1

f

ESR

Z

=

×

×

=

(21)

(22)

(23)

(24)

(26)

(29)

()

RHP

Z

C

f

5

1

f

=

(16)

Ω

k

233

nF

8

.

6

F

μ

33

Ω

48

C

C

R

R

COMP

OUT

LOAD

COMP

=

×

=

×

=