I have a customer seeing the following issue with the UCC27533:
My circuit is pretty simple. VDD and IN+ are tied together. IN- and GND are tied together. A decoupling capacitor of 100n is put right across VDD and GND. My application has this circuit excited with triangular pulses. During the voltage ramp up at the VDD, I see a small pulse at the output. It is happening before the voltage builds up to working voltage of UCC27533. I am simulated the same circuit in TINA using the model of the IC, I do not see this happening. I appreciate if you can help me on this. Is the startup operation glitch free? I saw another similar driver, which specifically mentions glitch free start up.
The current I have shown in earlier scope shot is the current that charges the decoupling capacitor. This current may have not gone through the pull-down resistor of IN+. I have lifted the IN+ and left it open; as per datasheet with any input open, the output should be low. I do see the glitch part still coming.
As per the functional block diagram, irrespective of the logic levels at input, as long as VDD is less than working voltage (8-9V), the output should be low. The glitch occurs at much below voltage.
It seems to me that there should not be a “glitch” in the start-up of this part. Can you look at this issue and let me know what you think is happening?
Thanks for your help with this!
Richard Elmquist