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So I have a problem with The UC382x Family of chips from TI. Here is the datasheet that I'm working off UC382x, I had some confusion already with datasheets seems there's a few floating around but TI themselves have confirmed that this is the data sheet I'm working off with the chips I'm using.

Now from that datasheet I made a circuit based off the UC3825ADW. I now want to get a 95% duty cycle output, so my understand is that I should just be able to pop a UC3823ADW down on the PCB instead and it should just work ¬¬.

Below is the circuit I'm using, trimmed down for this, the values are the ones I'm using the pots are both 10kΩ. 1 to set the frequency and 1 to set the error ref voltage. I have it set to a frequency of 15kHz and I can see the ramp and the clock. for me this is enough to say the chip is working. The other trimmer is set to have the full 5.1V from Vref going to NI and at the moment the feedback circuit is actually removed from the board so it's not needed here. The main thing is that EA out is at 5.1V which is going to drive maximum duty cycle out of Out A. I also know they should be schottkys, they are but the drawing is normal diodes, please forgive me.

Now I put a UC3825 down on the board I see the Ramp and I see the clock and I see the PWM switching the MOSFET gate at a good 12V with a 45% duty cycle. I put down a UC3823 on the same board. I see the ramp, I see the clock but Pin 11 Out A is a flat 800mV as is pin 14 Out B

I've been over the datasheet many times, I've been on to TI a few times and I'm stumped. Last ditch is to ask here to see if anyone might be able to offer a pair of fresh eyes or see something I'm missing. But from what I can see these 2 chips should be more or less interchangeable. I'd appreciate it if those fresh eye can confirm for me if they are interchangeable?

also the confirmation of using the right datasheets is coming from the ADW suffix there are UC3835 and UC3823 which are the same but different. as a double confirmation I've shorted pin 11 to pin 10 and probed pin 14 and still the same.

enter image description here

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  • \$\begingroup\$ Can you try connecting your ILIM pin to Vref? If you look at TI's open-loop test setup (Figure 14) they have it connected to Vref. The block diagram makes me believe your schematic is correct, but in my experience with TI chips, the block diagrams can sometimes be misleading. \$\endgroup\$
    – user187594
    Jun 22, 2020 at 14:31
  • \$\begingroup\$ I tried that and noticed that there's no A in the chip marking colleague ordered the wrong one. fml a few days wasted on that one. thanks for the suggestion. \$\endgroup\$
    – Jack
    Jun 22, 2020 at 15:46

1 Answer 1

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On the UC3823 (and I don't mean the UC3823A) pin 11 is an input: -

enter image description here

Your circuit shows this: -

enter image description here

Hence, either you have not actually used the UC3823 or, the circuit is incorrect because pin 11 is an input.

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  • \$\begingroup\$ I'm accepting your answer as it does answer my question. read over that datasheet more than a hand full of times and yet didn't notice that 1 line, thanks for keeping me sane. \$\endgroup\$
    – Jack
    Jun 22, 2020 at 15:48
  • \$\begingroup\$ No problem dude. \$\endgroup\$
    – Andy aka
    Jun 22, 2020 at 17:15

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