Other Parts Discussed in Thread: TPS5405
Hello,
I'm having some trouble getting the UCC28C40 running from high voltage DC in a flyback converter very similar to the one on page 24 of the datasheet. When I connect a DC voltage of 120V to the input capacitor, the resulting V_DD is only 2.84V, whereas I am looking to supply the chip with 12V. This is achieved using a startup resistor R_START = 360kOhm. I have measured the output on pin 6 to be a constant, but noisy 76mV (during writing it dropped from 100mV).
My hypothesis is that the output is being forced low which results in the auxiliary winding of the flyback transformer not receiving any voltage to power the controller. This is further supported by observations that lowering R_START will allow the controller to draw enough current to raise VDD above the UVLO ON threshold of 7V.
One of the output disable conditions is being met - The voltage on pin 1 (COMP) is 0 ("Enable and Disable" on page 19 of the datasheet). I have checked the connections on my PCB with the schematic and there are no incorrect connections. Furthermore, COMP is not being accidentally shorted to ground.
What could be forcing COMP low? Any suggestions for further troubleshooting?
Regards,
Nikola A. Žikić