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I would like to select a new op-amp replacing the old one (OPA171) for some reasons and then the TLV3701 was chosen for a new design. I simulated both in Pspice and found the output waveform of the TLV3701 becomes slow (H to L); it seems to have an RC constant.

Design information:
OPA171
TLV3701

My questions are:

  1. Which specification of an op-amp will cause this result?
  2. Can someone share op-amp design guidelines for reference?

OPA171 Pspice simulation: enter image description here

Result: enter image description here

TLV3701 Pspice simulation: enter image description here

Result (H to L timing becomes slow): enter image description here

10/13 update this application is for AC charging station from TI reference design schematic:TIDRJJ0A.PDF there enter image description here

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  • \$\begingroup\$ Generally, when replacing an op amp, you should replace it with another op amp, not a comparator like the TLV3701. \$\endgroup\$
    – Hearth
    Commented Oct 12, 2022 at 3:51
  • \$\begingroup\$ I still don't understand the OPA171 is designed as comparator for my application, why it can work under my expectation, but TLV3701 can't. @Hearth \$\endgroup\$
    – Ethan Ku
    Commented Oct 12, 2022 at 7:02
  • \$\begingroup\$ @EthanKu - you are exceeding it's voltage capability, all bets are off if you do that. It may be the simulation model. Have you measured it in real life? \$\endgroup\$ Commented Oct 12, 2022 at 22:52
  • \$\begingroup\$ not yet , a prototype will be ready at end of OCT. I update some information as above @Kevin White \$\endgroup\$
    – Ethan Ku
    Commented Oct 13, 2022 at 1:41

2 Answers 2

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The TLV3701 is not an opamp it is a comparator with extremely low supply current and output capability.

It also is limited to 16V total supply, you have 24V. You are exceeding the allowed voltage.

Since your PWM input is from 0v to 3.3v you do not need the negative supply. Just connect VEE to ground as the diode at the output prevents any negative voltage getting to the load anyway.

Neither of these would account for the symptoms that you are seeing but the TLV3701 does not seem to be a good choice.

What are the reasons for changing from the OPA171?

Minor issue: You are presenting a negative voltage to the MCU ADC input. Although it is attenuated enough to avoid damage the ADC will not be able to measure it. Again if you connect the opamp/comparator VEE to ground it avoids that problem.

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  • \$\begingroup\$ thanks, you pointed out supply Voltage of TLV3701 is wrong at my selection. we would like to cost down so searching new one . @Kevin White \$\endgroup\$
    – Ethan Ku
    Commented Oct 13, 2022 at 1:13
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In the datasheet for the TLV3701 comparator figure 10 shows a very slow negative-going fall time.

I converted your negative schematic to a normal positive but its parts were tiny because they were too far apart. I cropped and enlarged the positive schematic. opamp

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  • \$\begingroup\$ thanks you mention me good information at figure 10 \$\endgroup\$
    – Ethan Ku
    Commented Oct 13, 2022 at 2:05

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