Please see question from customer below.
***Begin customer question.
I'm using the UC3825 with voltage feedforward, as shown on page 7 of the datasheet, see the figure "Forward Technique for Off-line Voltage Mode Application". The +330V is fed through a resistor, in my case, 400K, to a capacitor on Pin 7. During startup before the supply starts running there's voltage applied through this resistor to pin 7 but there are no other voltages on the IC pins. The voltage on pin 7 goes up to about 19V then drops when the supply starts running. The supply works fine, but this short-term high voltage seems to violate the max voltage spec for this pin. The source for this voltage is current-limited by the large resistance. Is there any damage being done to the IC?
Is the issue just voltage between pin 7 and ground, or also between pin 7 and the other pins, say, VREF or VCC?
Connecting the chip on the bench and applying voltage through a 10K resistor between pin 7 and ground, I see that the voltage gets clamped in a Zener-like fashion at about 19 volts. That explains what I see in-circuit.
This is a last-minute panic as we go into production. I would prefer to do nothing but only if I'm sure it won't hurt anything, the alternative is to kludge something in.
*** end customer question
Thanks for your help.
Regards,
John
