Need help on HV9910B DC-DC Buck offtime mode

Thread Starter

Hitmanvxx

Joined May 16, 2019
4
Hi everyone. I make this topic because Im very need your help to understand HV9910B in offtime mode and because I tried so many many time with vary value of component but i can not to know what I was wrong.

Im doing DC-DC buck converter based HV9910B in offtime mode (mainly follow application note AN-H50 from supertex). Spec of my project:

Drive led string 12v typical ( real voltage forward at 7.5-10.5v)
Current needed : 5A
Inputvoltage: 12-24v from DC power supply ( real test from 12.5-24.8v)

Component:
Rosc (set in offtime mode, connect to Gate pin instead of GND): 68k for 3.6uS offtime.
Rsense: 0.01ohm ( low Pdissipation, low heat)
Voltage divider at LD pin ( connect with VDD 7.5v pin): upper 680k ohm , lower potentiometer 5k ==> get 50mV.
Bypass cap on VDD pin: 2.2uF
Input cap: 220uF, small 4.7uF
Mosfet: SM4028NSU
Diode: dual SS54 in parallel.
Inductor: 220uH (was tried from 50uH to 5mH)

And all of this, the result i got is VARY FLICKERING LED, with just 2.2A at 7.8v. Also, the current rise or fall depend on input voltage, its not stable.

I think mainly problem come from inductor and frequency. But i tried from 50uH to 5mH, 2.2uS to 4.3uS offtime set resistor and all same.

I need some help in formula or right calculating for component or please show me what i was wrong.

Thank very much
 
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