BD8311NUV-E2(2009) View Datasheet(PDF) - ROHM Semiconductor

Part Name

Description

MFG CO.

BD8311NUV-E2
(Rev.:2009)

High-efficiency Step-up Switching Regulator with Built-in Power MOSFET

ROHM Semiconductor 

BD8311NUV

Technical Note

(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.

To satisfy above two items, R1, R2, R3, DS and RS in Fig. 34 should be set as follows.

[1] R1, R2, R3

BD8311NUV incorporates phase compensation devices of

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

values decide the prim ary pole that determines the

bandwidth of DC/DC converter.

Primary pole point frequency

VOUT

Inside of IC

Cs

R1

R4 C2

Rs

FB

R2

R3

1

fp=

2π

A×(

R1・R2

R1+R2

+R3)×C2

DC/DC converter DC Gain

DC Gain =A× 1 × VOUT

B

VOUT-VIN

················ (1)

················ (2)

Fig.34 Example of phase compensation setting

A: ERROR AMP Gain

About 100dB = 105

B: Oscillator amplification = 0.5

VIN: Input voltage

VOUT: 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.

fSW = fp×DC Gain =

1

2πC2×(

(R1・R2)

(R1+R2)

+R3 )

1

×

×

B

VOUT

VOUT-VIN

················ (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 this setting, R1 and R2, which determine the voltage value, will be in the order of several hundred kΩ. Therefore, if an

appropriate resistance value is not available and routing may cause noise, the use of R3 enables easy setting.

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

11/15

2009.04 - Rev.B

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