I realized this board from my schematic based on the AL8853, a boost LED controller. My circuit is calculated for a high power design (735 V LED output, 350 mA LED current). Before powering my board with a 600 VDC power supply, I tried to test the board with low voltage. J2 is powered with 12 V on pin 1 and 52 V on pin 2. The PWM controller's pin is at 3.85 V forced by R2 and R4 in order to not exceed 5 V. The FB controller's pin is at 150 mV. According the controller's datasheet, I don't understand why there is no oscillation on the GATE controller's pin and Vled is still equal to Vin (52 V).

Please, could you share your advices ? enter image description here

  • \$\begingroup\$ What causes 150 mV to appear on the FB pin when it is unconnected from any load in your diagram? \$\endgroup\$
    – Andy aka
    Jan 6, 2022 at 14:50
  • \$\begingroup\$ Datasheet says: "When the first rising edge is applied to the PWM pin, the AL8853 will power up immediately". Perhaps it needs a rising edge on that pin to start? You can try pulling the pin down trough your opto and letting it rise again shortly after power up. \$\endgroup\$ Jan 6, 2022 at 14:52
  • \$\begingroup\$ More from the datasheet: the OVP has lower thresholds in addition to the upper one (VOVP_OV). 1] 100 mV, see page 7, Startup; 2] 200 mV, see page 9, Vout Short Protection. When the LED supply is at 52 V, the max voltage at OVP pin (junction of R13/R14) will be about 135 mV. 1] depending on the tolerance (I couldn't find one) of the 100 mV, the IC might not start up. 2] if the IC starts up it will likely shut down again, thinkng that there is an output short. \$\endgroup\$
    – GeBJT
    Jan 6, 2022 at 15:55
  • \$\begingroup\$ 150mV on FB comes from R11, R12, R13, R14 divider. The datasheet says the controller needs FB pin above 100mV to work. \$\endgroup\$
    – Teddol
    Jan 6, 2022 at 17:51
  • \$\begingroup\$ I might not be understanding something here, but it looks to me that the R11, R12, R13 and R14 are connected to the OVP pin, not the FB pin, when no load is connected. \$\endgroup\$
    – Tyler
    Jan 6, 2022 at 23:37

1 Answer 1


Issue solved. I confirmed that OVP level may be higher than 200mV.


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