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Technical Note

Phase compensation setting procedure

R

Feedback

C

FB

-

A

(1) Typical integrator (Low pass filter)

(2) Open loop characteristics of integrator

0

A

0

-90

-90˚

-180˚

-180

Phase

[˚]

Gain

[dB]

f

f

GBW(b)

-20dB/decade

Phase

margin

(a)

The following section describes the stability conditions of the negative feedback system.

Since the DC/DC converter application is sampled according to the switching frequency, GBW (frequency at 0-dB gain)

of the overall system should be set to 1/10 or less of the switching frequency. The following section summarizes the targeted

characteristics of this application.

At a 1 (0-dB) gain, the phase delay is 150˚ or less (i.e., the phase margin is 30˚ or more).

The GBW for this occasion is 1/10 or less of the switching frequency.

Responsiveness is determined with restrictions on the GBW. To improve responsiveness, higher switching frequency

should be provided.

Replace a secondary phase delay (-180˚) with a secondary phase lead by inserting two phase leads, to ensure the stability

through the phase compensation. Furthermore, the GBW (i.e., frequency at 0-dB gain) is determined according to phase

compensation capacitance provided for the error amplifier. Consequently, in order to reduce the GBW,

increase the capacitance value.

1.

Application stability conditions

Since the error amplifier is provided with (1) or (2) phase compensation, the low pass filter is applied. In the case of

the DC/DC converter application, the R becomes a parallel resistance of the feedback resistance.

50

100

150

200

250

300

100

50

200

300

400

500

600

700

800

OSCILATING FREQUENCY SETTING RESISTANCE : RT [kΩ]

800

50

100

150

200

250

300

350

400

450

500

100

0200

300

400

500

600

700

Fig.32 BD9001F reference layout pattern

Fig.33 BD9781HFP reference layout pattern

As shown above, design the GND pattern as large area as possible

within inner layer.

Gray zones indicate GND.

Co

L

C

Di

R1

C3

Cx2

R2

Co

L

C

Di

Cx1

2010.02 -

Rev. B

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BD9778F, BD9778HFP, BD9001F, BD9781HFP