BD9325FJ-LB

BD9326EFJ-LB

BD9327EFJ-LB

(a) Choosing Phase Compensation Resistor R3

Please decide the compensation resistor R3 by following the formula below.

Compensation Resister

Where

The larger value of R3, value of fc increases (response better and stability worse).

The smaller value of R3, value of fc decreases (response worse and stability better).

(b) Choosing Phase Compensation Capacitor C1

The phase delay which is from output LC filter, needs to be cancelled to stabilize the DC/DC converter, this is

done by inserting the phase lead.

The phase lead can be added by the zero introduced by the compensation resistor and capacitor.

LC Resonant Frequency

Zero C1 and R3

Compensation Capacitor

(c) The Condition of the Loop Compensation Stability

The stability of the DC/DC converter is important. To ensure the operating stability, please check the loop

compensation if it has enough phase-margin. For the condition of loop compensation stability, the phase-lag must

be less than 150 degrees when gain is 0 dB. Namely over 30 degrees phase-margin is needed.

Lastly, after the calculation above, find measures to adjust the phase-margin to more than 30 degrees.

Figure 20

Figure 21

(3) Design of Feedback Resistance Constant

Set the feedback resistance as shown below.

Reference Voltage

[V]

Figure 22

PHASE MARGIN

－180°

－90°

－180

－90

0

0

A

(a)

GBW(b)

F

F

Gain [dB]

PHASE

－

＋

R1

R2

C1

COMP

R3

FB

[ ]

Z

OUT

LC

H

LC

f

π

2

1

=

[ ]

Z

Z

H

R

C

f

3

1

2

1

π

=

[ ]

F

R

f

C

LC

3

2

3

1

π

=

×

+

=

2

2

1

R

R

R

V

OUT

Phase Margin

＋

－

R1

R2

ERR

Reference Voltage

FB

[ ]

Ω

×

×

×

=

OUT

OUT

V

fc

C

R

5800

3

10/19

09.Sep.2014 Rev.003

www.rohm.com

© 2012 ROHM Co., Ltd. All rights reserved.

TSZ22111･15･001

TSZ02201-0323AAJ00440-1-2