BD8311NUV-E2(2009) Просмотр технического описания (PDF) - ROHM Semiconductor

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BD8311NUV-E2
(Rev.:2009)

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

ROHM Semiconductor 

BD8311NUV

Technical Note

[2] Cs and Rs setting

In the step-up DC/DC converter, the secondary pole point is caused by the coil and capacitor as expressed by the

following equation.

fLC=

1－D

2π√(LC)

················ (4)

D: ON Duty = ( VOUT - VIN ) / VOUT

This secondary pole causes a phase rotation of 180°. To secure the stability of the system, put zero points in 2 places to

perform compensation.

Zero point by built-in CR

1

fZ1=

2πR4C2

= 13kHz

················ (5)

Zero point by Cs

fZ1=

1

2π(R1+R3)CS

················ (6)

Setting CS2 to be half to 2 times a frequency as large as fLC provides an appropriate phase margin.

It is desirable to set Rs at about 1/20 of (R1+R3) to cancel any phase boosting at high frequencies.

Those pole points are summarized in the figure below. The actual frequency property is different from the ideal

calculation because of part constants. If possible, check the phase margin with a frequency analyzer or network analyzer,

etc.. Otherwise, check for the presence or absence of ringing by load response waveform and also check for the

presence or absence of oscillation under a load of an adequate margin.

(5) (6)

(3)

(4)

Fig. 35 Example of DC/DC converter frequency property

(Measured with FRA5097 by NF Corporation)

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

12/15

2009.04 - Rev.B

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