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BD9486F

1ch Boost up type White LED Driver for large LCD

ROHM Semiconductor 

BD9486F

Datasheet

●3.4.Loop Compensation

A current mode DCDC converter has each one pole (phase lag) fp due to CR filter composed of the output capacitor and

the output resistance (= LED current) and zero (phase lead) fZ by the output capacitor and the ESR of the capacitor.

Moreover, a step-up DCDC converter has RHP zero (right-half plane zero point) fZRHP which is unique with the boost

converter. This zero may cause the unstable feedback. To avoid this by RHP zero, the loop compensation that the

cross-over frequency fc, set as follows, is suggested.

fc = fZRHP /5 (fZRHP: RHP zero frequency)

Considering the response speed, the calculated constant below is not always optimized completely. It needs to be

adequately verified with an actual device.

Figure 31. Output stage and error amplifier diagram

i.

Calculate the pole frequency fp and the RHP zero frequency fZRHP of DC/DC converter

fp



ILED

2  VOUT  COUT

　[Hz]

　

fZRHP



VOUT  (1  D)2

2  L  ILED

　[Hz]　　

Where ILED = the summation of LED current, D  VOUT  VIN (C　ontinuous Current Mode)

VOUT

ii.

Calculate the phase compensation of the error amp output (fc = fZRHP/5)

RFB1



fRHZP  RCS  ILED

5  fp  gm  VOUT  (1  D)

　[]

　

CFB1 

1

2π  RFB1 fc



2π



5

RFB1 

fZRHP

[F]

gm  4.0 104[S]

Above equation is described for lighting LED without the oscillation. The value may cause much error if the quick

response for the abrupt change of dimming signal is required.

To improve the transient response, RFB1 needs to be increased, and CFB1 needs to be decreased. It needs to be

adequately verified with an actual device in consideration of variation from parts to parts since phase margin is

decreased.

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