Comparator TLV3201 – Troubleshooting Output Issues

comparator

I've put together the circuit from the TI document Comparator with Hysteresis Reference Design using the same reference voltage to the noninverting input (Vth 2.3V – 2.7V). The Inverting input is connected to a voltage divider that depends on a thermistor, but for testing just a potentiometer.

I can confirm that the initial threshold voltage is 2.3V, and that the Power(5V) and GND are correct at pins 5 & 2. When I sweep the inverting input past the threshold voltages, the output never changes from GND.

Ultimately this output is driving the base of an NPN transistor to power a relay, but again, I never get 5V on the output. I actually pulled the first TLV3201 off the board and replaced it, but same issue.

What stupidity have I done?

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EDIT – Reference circuit included for clarity.enter image description here

EDIT2 – There is a low impedance path from the U2.1 (comparator output) to GND. Even on a number of blank boards. The board was not designed as such, but I measure 0.1 Ohms between those two pins. See the board image below. The output net is not directly grounded, the only way to GND is supposed to be via R2 + R3 (100K + 576K).

enter image description here

Best Answer

Assertions:

  1. There is a false assumption in your BJT calculation for hFE.
    • The hFE applies in the linear region and not the saturated region as a switch. You should not choose a base current, Ib, less than 10% of this expected Ic/hFE min or use the rated Ic/Ib ratio for Vce(sat) which is 20:1 implied from the tables. It is not coincidental, that the B rank hFE= 200 minimum but you must choose a base current of 70 mA/20 = 3.5 mA and thus Rb max= 4.3V/3.5mA = 1.23k max.
  2. There is a short on the board or both IC's you have tested have been damaged as all IC's are 100% tested.

    • Exceeding the do not exceed by any method such as ESD handling damage or DO Not Exceed thresholds exceeded.

    • There should also be an observable voltage change from 0V output as the curves for this CMOS Op Amp indicate an RdsOn at room temp (25'C) of 35 Ohms. Thus you can expect and IR drop from the push-pull CMOS drain outputs.

    • Re-read all the datasheet parameters until understood.

Suggested:

  • check for a short on board.
  • Measure Resistance with power off between gnd. and Output. It must not be low resistance. The DMM current will not exceed the device tolerance of 10 mA for ESD diodes.
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