BD8966FVM-TR(2009) 查看數據表（PDF） - ROHM Semiconductor

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BD8966FVM-TR
(Rev.:2009)

Low Noise High Efficiency Step-down Switching Regulator with Built-in Power MOSFET

ROHM Semiconductor 

BD8966FVM

Technical Note

4. Determination of RITH, CITH that works as a phase compensator

As the Current Mode Control is designed to limit a inductor current, a pole (phase lag) appears in the low frequency area

due to a CR filter consisting of a output capacitor and a load resistance, while a zero (phase lead) appears in the high

frequency area due to the output capacitor and its ESR. So, the phases are easily compensated by adding a zero to the

power amplifier output with C and R as described below to cancel a pole at the power amplifier.

A

Gain

[dB] 0

Phase 0

[deg]

-90

fp(Min.)

fp(Max.)

IOUTMin.

IOUTMax.

fz(ESR)

fp=

1

2π×RO×CO

fz(ESR)=

1

2π×ESR×CO

Pole at power amplifier

When the output current decreases, the load resistance Ro

increases and the pole frequency lowers.

Fig.28 Open loop gain characteristics

fp(Min.)=

1

2π×ROMax.×CO

[Hz]←with lighter load

fp(Max.)=

1

2π×ROMin.×CO

[Hz] ←with heavier load

A

Gain

[dB]

0

0

Phase

[deg]

-90

fz(Amp.)

Zero at power amplifier

Increasing capacitance of the output capacitor lowers the pole

frequency while the zero frequency does not change. (This

is because when the capacitance is doubled, the capacitor

ESR reduces to half.)

fz(Amp.)=

1

2π×RITH×CITH

Fig.29 Error amp phase compensation characteristics

VCC

Cin

L

EN

VCC,PVCC

SW

VOUT

VOUT

VOUT

ESR

RO

ITH

GND,PGND

CO

RITH

CITH

Fig.30 Typical application

Stable feedback loop may be achieved by canceling the pole fp (Min.) produced by the output capacitor and the load

resistance with CR zero correction by the error amplifier.

fz(Amp.)= fp(Min.)

1

2π×RITH×CITH

=

1

2π×ROMax.×CO

5. Determination of output voltage

The output voltage VOUT is determined by the equation (7):

VOUT=(R2/R1+1)×VADJ・・・(7) VADJ: Voltage at ADJ terminal (0.8V Typ.)

With R1 and R2 adjusted, the output voltage may be determined as required.

(Adjustable output voltage range： 1.0V～1.8V)

Use 1 kΩ～100 kΩ resistor for R1. If a resistor of the resistance higher than

100 kΩ is used, check the assembled set carefully for ripple voltage etc.

6

SW

1

ADJ

4.7µH

10µF

Output

R2

R1

Fig.31 Determination of output voltage

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© 2009 ROHM Co., Ltd. All rights reserved.

11/14

2009.05 - Rev.A

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