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# UCC25640EVM-020: Functionality Questions

Part Number: UCC25640EVM-020
Other Parts Discussed in Thread: UCC256404, UCC256402, UCC256403, TL431

On the EVM, when HV pin is tied to PGND and VCC supplied by an external source, the EVM doesn't function at VCC = 24V; but it does when VCC is 26V.

Question 1:
From the above, It looks like VCCstartself should be within 25V~28V.
What is the allowable input voltage range for VCC, including when powered from external supply.
The ABS MAX is 30V, so is the procedure to apply max 28V on startup, then shift to 13V~26V range for normal operation?

Question 2:
It should be possible to disable the X-Capacitor Discharge function on the UCC256404 by replacing the 5kΩ resistor with a larger value resistor, (basically severely limiting the input current to HV) but is there a maximum resistance that is allowed for such a non-typical use-case?
This question comes from a desire to stick with UCC256404 for DC input, because UCC256402/3 devices Vblk voltage ABS MAX is 7.2V, and their input voltage range resistor divider will go above this voltage at their peak input voltage.

Regards,

Darren

• Hi Darren,

Yes, you would need to apply a voltage greater than VCCstartself to the VCC pin in order to fool the device into thinking HV startup is complete. You can then reduce VCC to 13V to 26V for normal operation

If you don't plan to utilize the HV startup, then you can make the HV pin resistance to be excessively big in order to make the HV pin hit the 9V threshold when the X cap test current is applied. With 400V on HV pin, a resistance of >700kOhm should be sufficient.

Regarding the choice of UCC256404 vs UCC256402/3, what may make sense to use a TL431 to drive the BLK pin or just use a zener to clamp the max voltage on the BLK pin. I would think using UCC256403 and using a zener to clamp the max BLK voltage would be preferable over trying to mess with fooling the HV startup function.

Best Regards,

Ben Lough