12

KB3302

Kingbor Technology Co.,Ltd

TEL:(86)0755-26508846 FAX:(86)0755-26509052

APPLICATIONS INFORMATION

62

.

0

5

.

0

12

3

.

0

1

5

.

0

12

5

1

D

=

+

−

+

−

=

Therefore

()

()

KHz

68

.

4

Krads

4

.

29

F

10

8

.

6

2

1

2

p

=

=

µ

•

Ω

≤

ω

−

and

()

KHz

3

.

33

Krads

209

H

7

.

4

62

.

0

1

8

.

6

1

2

2

Z

=

=

µ

−

•

Ω

≥

ω

−

The spacing between p

2

and z

2

is the closest when the

converter is delivering the maximum output current from

the lowest V

IN

. This represents the worst-case compensation

condition. Ignoring C

5

and C

6

for the moment, C

4

forms a

low frequency pole with the equivalent output resistance

R

O

of the error amplifier:

Ω

=

Ω

µ

=

=

−

M

7

.

4

60

dB

49

ce

tan

c

Transcondu

Gain

Loop

Open

Amplifier

R

1

O

Hz

41

rads

260

pF

820

M

7

.

4

1

C

R

1

1

4

O

1

p

−

=

−

=

•

Ω

−

=

−

=

ω

−

C

4

and R

3

also forms a zero with angular frequency:

KHz

3

.

6

Krads

5

.

39

pF

820

K

9

.

30

1

C

R

1

1

4

3

1

Z

−

=

−

=

•

Ω

−

=

−

=

ω

−

The poles p

1

, p

2

and the RHP zero z

2

all increase phase

shift in the loop response. For stable operation, the overall

loop gain should cross 0dB with -20dB/decade slope. Due

to the presence of the RHP zero, the 0dB crossover frequency

should not be higher than

3

z

2

. Placing z

1

near p

2

nulls its

effect and maximizes loop bandwidth. Thus

)

MAX

(

OUT

2

OUT

4

3

I

2

C

V

C

R

≈

(15)

R

3

determines the mid-band loop gain of the converter.

Increasing R

3

increases the mid-band gain and the crossover

Figure 9. Suggested PCB Layout for the KB3302. Notice that there is no via

directly under the device. All vias are 12mil in diameter.

U1

VOUT

SHDN

R1

C5

C1

L1

D1

VIN

U1

C3

R4

C4

R3

C6

R2

GND

C2

U1

VOUT

SHDN

SHDN

R1

C5

C1

L1

D1

VIN

U1

C3

R4

C4

R3

C6

R2

GND

C2