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I am working on a boost converter circuit that gets 12Volt as input (J1) and gives 18 Volts at the output (J4). I am having issues in the feedback circuit for this converter, especially with my AND gate SN74LVC1G08DBVR (marking "C14J", from DigiKey).

I am sensing the feedback of the converter (fb) and comparing this feedback with a reference voltage (pi_ref) using LT1213 (U2B). The output of U2B gives me the PI_Output.

For testing purpose, I have made sure that the feedback (fb) and reference voltage (pi_ref) are exactly same and I am getting pi_output as zero.

The AND gate receives this PI_output and a 100kHz 5V square wave as the two inputs. The output of the AND gate gives a very different truth table than what I am expecting.

I tried adding pullup resistors of 10k ohm at both inputs and the output, but still the same result.

I have attached here the oscilloscope screenshot for the AND gate input/output - Input 1 (100kHz 5V Square wave) as "Yellow", input 2 (pi_output) as " Green" and the output (Comp_out) as "Pink"

Can anyone please point out what I am doing wrong?

Schematic of boost converter

oscilloscope screenshot

Truth table

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  • \$\begingroup\$ What is the chip's marking? \$\endgroup\$
    – CL.
    Commented Dec 14, 2023 at 22:13
  • \$\begingroup\$ The marking on SN74LVC1G08DBVR says "C14J" \$\endgroup\$
    – Kinjal
    Commented Dec 14, 2023 at 22:28
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    \$\begingroup\$ That marking is consistent with a SN74LVC1G14, which is an inverter not an AND gate. \$\endgroup\$ Commented Dec 14, 2023 at 22:55
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    \$\begingroup\$ I ordered the chip from Digikey and the Digikey label on the packet does say manufacture part number - SN74LVC1G08DBVR. Thanks for pointing this out. I never imagined this can happen! \$\endgroup\$
    – Kinjal
    Commented Dec 14, 2023 at 23:12
  • \$\begingroup\$ It happens ... I also once received an incorrectly packaged part from Digikey a few years ago. "Luckily" for me though it was quite significantly different and I didn't waste time debugging like you've had to. \$\endgroup\$
    – brhans
    Commented Dec 15, 2023 at 0:22

2 Answers 2

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This AND gate behaves like an inverter. And a 5-pin chip with the marking "C14J" indeed is an inverter, as shown on https://www.ti.com/packaging/en/docs/partlookup.tsp?partmarking=C14J.

(This kind of error is quite unusual. In general, beginning to search for errors in your own schematic and board would have been the correct approach.)

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With problems like this it is always worth checking the part markings to ensure there hasn't been a mix up in supply or assembly, especially if you have multiple parts with the same form factor.

I had a production run of boards which just didn't behave right, and it turned out the pick and place machine had been loaded with an incorrectly tagged reel, and we just had the wrong parts...

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  • \$\begingroup\$ Just read comments and seen @kinjal has already correctly spotted this is the cause. \$\endgroup\$
    – colintd
    Commented Dec 15, 2023 at 8:39

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