BD8311NUV

Technical Note

11/15

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2009.04 - Rev.B

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(4) DC/DC converter frequency response adjustment system

Condition for stable application

The condition for feedback system stability under negative feedback is that the phase delay is 135 °or less when gain is 1 (0 dB).

Since DC/DC converter application is sampled according to the switching frequency, the bandwidth GBW of the whole

system (frequency at which gain is 0 dB) must be controlled to be equal to or lower than 1/10 of the switching frequency.

In summary, the conditions necessary for the DC/DC converter are:

- Phase delay must be 135°or lower when gain is 1 (0 dB).

- Bandwidth GBW (frequency when gain is 0 dB) must be equal to or lower than 1/10 of the switching frequency.

BD8311NUV incorporates phase compensation devices of

values decide the prim ary pole that determines the

bandwidth of DC/DC converter.

Primary pole point frequency

DC/DC converter DC Gain

pole point is as shown below.

Fig.34 Example of phase compensation setting

FB

VOUT

C2

R1

R2

Rs

R4

Cs

R3

Inside of IC

A: ERROR AMP Gain

About 100dB = 10

5

B: Oscillator amplification = 0.5

Input voltage

················(2)

2π A×(

fp=

1

DC Gain =A×

×

B

1

2πC2×(

1

×

×

B

1

················ (1)

················(3)