BD9329AEFJ-LB datasheet(10/21 Pages) ROHM

Part #	BD9329AEFJ-LB

Description	4.2V to 18V, 3A 1ch Synchronous Buck Converter integrated FET

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Manufacturer	ROHM [Rohm]

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Datasheet

10/18

TSZ02201-0323AAJ00320-1-2

27.Feb.2014 Rev.002

www.rohm.com

TSZ22111 • 15 • 001

BD9329AEFJ-LB

PHASE MARGIN

－180°

－90°

－180

－90

0

A

(a)

GBW(b)

F

Gain [dB]

PHASE

FCRS

( i ) Choosing phase compensation resistor RCMP

The compensation resistor RCMP can be on following formula.

RCMP =

2πx VOUT x FCRS x COUT

[Ω]

VFB x GMP x GMA

Where

VOUT ; Output voltage, FCRS ; Cross over frequency, COUT ; Output Capacitor,

VFB ; internal feedback voltage (0.9V(TYP)), GMP ; Current Sense Gain (7.8A/V(TYP)) ,

GMA ; Error Amplifier Trans-conductance (300µA/V(TYP))

Setting VOUT= 3.3V, FCRS= 38kHz, COUT= 20µF;

RCMP =

2πx 3.3 x 38k x 20u

=

7.48k ~= 7.5k

[Ω]

0.9 x 7.8 x 300u

( ii ) Choosing phase compensation capacitor CCMP

For the stability of DC/DC converter, canceling the phase delay that derives from output capacitor COUT and

resistive load ROUT by inserting the phase advance.

The phase advance can be added by the zero on compensation resistor RCMP and capacitor CCMP.

Making Fz= FCRS / 6 gives a first-order estimate of CCMP.

Compensation Capacitor

CCMP=

1

[F]

2π x RCMP x Fz

Setting Fz= FCRS/6 = 6.3kHz;

Compensation Capacitor

CCMP=

1

=

3.54n ~= 3.3n

[F]

2π x 7.5k x 6.3k

However, the best values of zero and FCRS are different between applications. After calculation above formula and

confirmation actual application, please decide values finally.

( iii ) The condition of the loop compensation stability

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

has the enough phase-margin. For the condition of loop compensation stability, the phase-delay must be less than

150 degree where Gain is 0 dB.

Feed forward capacitor CRUP boosts phase margin over a limited frequency range and is sometimes used to

improve loop response. CRUP will be more effective if RUP >> RUP||RDW

Figure 21

Figure 22

(3)

Design of Feedback Resistance constant

Set the feedback resistance as shown below.

Figure 23

VOUT =

R1 + R2

× 0.9

[V]

R2

－

＋

VOUT

RUP

CCMP

COM P

RCMP

FB

RDW

0.9V

CRUP

＋

－

VOUT

R1

R2

ERR

0.9V

FB

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