BD8312HFN(2009) データシートの表示（PDF） - ROHM Semiconductor

部品番号

コンポーネント説明

メーカー

BD8312HFN
(Rev.:2009)

Output 1.5A or Less High-efficiency Step-down Switching Regulator with Built-in Power MOSFET

ROHM Semiconductor 

BD8312HFN

Technical Note

(3) Output voltage setting

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

VOUT

R1

ERROR AMP

INV

R2

(R1+R2)

Vo=

×1.0 [V] ・・・ (4)

R2

VREF

1.0 V

Fig.29 Setting of voltage feedback resistance

(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.

To satisfy those two points, R1, R2, R3, DS and RS in Fig. 30 should be set as follows.

[1] R1, R2, R3

BD8313HFN incorporates phase compensation devices of

R4=62kΩ and C2=200pF. These C2 and R1, R2, and R3

valuesdecide the primary pole that determines the bandwidth

of DC/DC converter.

Primary pole point frequency

1

fp=

2π

A×(

R1×R2

R1+R2

+R3)×C2

・・・・(1)

DC/DC converter DC Gain

DC Gain =A× 1 ×

B

VIN

VO

・・・・(2)

VOUT

Inside of IC

Cs

R1

R4 C2

Rs

FB

R2

R3

Fig.30 Example of phase compensation setting

A: Error AMP Gain

About 100dB = 105

B: Oscillator amplification = 0.5

VIN:

VOUT:

Input voltage

Output voltage

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.

1

fSW = fp×DC Gain =

2πC2×(

(R1・R2)

(R1+R2)

+R3 )

×

1

VIN

B × VO

・・・・(3)

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

By Equation (3), R1 and R2, which determine the voltage value, will be in the order of several hundred kΩ. If an

appropriate resistance value is not available since the resistance is so high and routing may cause noise, the use of R3

enables easy setting.

www.rohm.com

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

10/14

2009.04- Rev.B

Share Link: