BD9778F-E2 View Datasheet(PDF) - ROHM Semiconductor

Part Name

Description

Manufacturer

BD9778F-E2

Flexible Step-down Switching Regulators with Built-in Power MOSFET

ROHM Semiconductor 

BD9778F, BD9778HFP, BD9001F, BD9781HFP

Technical Note

Di

C

L

C3

Co

R1

Cx2

R2

C

Di

L

RT

CT Cx1

Co

Fig.32 BD9001F reference layout pattern

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

within inner layer.

Ţ Gray zones indicate GND.

500

450

400

350

300

250

200

150

100

50

0 100 200 300 400 500 600 700 800

OSCILATING FREQUENCY SETTING RESISTANCE : RT[kΩ]

Fig.34 RT vs fOSC (BD9781HFP/BD9778F/HFP)

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Phase compensation setting procedure

1. Application stability conditions

Fig.33 BD9781HFP reference layout pattern

300

250

200

150

100

50

50 100 200 300 400 500 600 700 800

OSCILATING FREQUENCY SETTING RESISTANCE : RT [kΩ]

Fig.35 RT vs fOSC &BD9001F'

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.

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

Ă 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) Typical integrator (Low pass filter)

(2) Open loop characteristics of integrator

Feedback R

FB

A

-

C

A

Gain

[dB]

0

0

Phase

[˚] -90

-180

(a)

-20dB/decade

GBW(b)

f

-90˚

Phase

margin

-180˚

f

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.

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11/16

2010.02 - Rev. B

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