BD9486F_16 datasheet(22/38 Pages) ROHM

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Datasheet

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TSZ02201-0F1F0C100240-1-2

15.Feb.2016 Rev.006

www.rohm.com

TSZ22111・15・001

BD9486F

●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

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

Where ILED = the summation of LED current,

(Continuous Current Mode)

ii.

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

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.

]

[

)

(

LED

OUT

ZRHP











]

[

)

(

OUT

LED

RHZP









]

[

OUT

LED







OUT





]

[







]

[

ZRHP









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