BD8312HFN

Technical Note

10/14

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2009.04- Rev.B

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(3) Output voltage setting

The internal standard voltage of the ERROR AMP is 1.0 V. Output voltage is acquired by Equation (4).

(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

(0dB).

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.

BD8313HFN incorporates phase compensation devices of

valuesdecide the primary pole that determines the bandwidth

of DC/DC converter.

Primary pole point frequency

DC/DC converter DC Gain

By Equations (1) and (2), the frequency fsw of point 0 dB under limitation of the bandwidth of the DC gain at the primary

pole point is as shown below.

×

×

It is recommended that fsw should be approx.10 kHz. When load response is difficult, it may be set at approx. 20 kHz.

enables easy setting.

Vo=

×

1.0 [V] ・・・ (4)

R2

(R1+R2)

VREF

1.0V

VOUT

ERROR AMP

R1

R2

INV

FB

VOUT

C2

R1

R2

Rs

R4

Cs

R3

Inside of IC

・・・・

(1)

・・・・

(2)

2π A×(

fp=

1

DC Gain =A×

×

B

1

1

・・・・

(3)

B

1

Fig.29 Setting of voltage feedback resistance

Fig.30 Example of phase compensation setting

A: Error AMP Gain

About 100dB = 10

5

B: Oscillator amplification = 0.5

Input voltage

Output voltage