Comparator - fried due to excessive output current?

Discussion in 'Analog & Mixed-Signal Design' started by Col John Matrix, Sep 6, 2016.

  1. Col John Matrix

    Thread Starter Member

    Apr 16, 2009

    I'm using a TIA followed by a comparator in order to convert an optical signal received by a photodiode (a sequence of 0s and 1s) into a corresponding electrical signal that's 0 to 5 V. This is to allow an optical link to be used in place of the previous electrical link.

    The circuit seems to function perfectly well, converting the optical signal into an electrical signal. However, a couple of the comparators have failed - I observed it suddenly trying to draw a large current from the power supply (200 mA or more), followed by a drop in that current and a subsequent inability to output 0 to 5 V signals.

    My guess is that the way I had things set up meant the comparator output was delivering a much higher current than it was rated for, causing the part to heat up and eventually fail. The max. output current is given as +/- 50 mA, together with a note that at such a current "a heat sink may be required to keep the junction temperature below the absolute maximum rating". I had things set up so that the comparator output was 0 to 5 V, delivered to a 50 Ohm load of an oscilloscope - so would the comparator output current therefore be nominally far too high (e.g. up to a max 5V / 50 Ohm = 100 mA?) and fried the part?

    The part in question is the Linear Circuits LTC6752, in the MS8 package. The circuit is very similar to that shown in Figure 10 on the data sheet. The input voltages to the comparator inputs were about 1 V.

    I'm just looking to get confirmation (or not!) that I've identified the cause of failure, and some pointers on how to avoid it (I'm thinking - change the scope load to 1 MOhm, put on a heat sink maybe...)

  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    I would not expect a max 50mA output to put up with 100mA for very long.
    If your application requires this signal to feed a 50Ω load then you will need to use a higher current amplifier as well as/instead of the LTC6752.
    If your application requires this signal to feed a much higher load impedance then just don't use a 50Ω oscilloscope probe.
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Your suspicions confirmed. You could use the comparator output to drive the 50Ω load via a current-boosting emitter-follower stage.
  4. crutschow


    Mar 14, 2008
    Why are you using the 50Ω impedance setting for your oscilloscope. :confused:
    That's normally only used if you have a 50Ω source and 50Ω coax cable from the source to the oscilloscope to minimize high frequency reflections in the cable.

    For your measurements you should be using a 10:1 10MΩ probe with the oscilloscope set to 1MΩ.
  5. Col John Matrix

    Thread Starter Member

    Apr 16, 2009
    Simple carelessness! It was set to 50 Ohm to look at something else and I then connected the comparator input into it without bothering to think!

    I tried a fresh part. When connecting the comparator output to 1 MOhm scope, the current being drawn by the comparator was fairly low (< 10 mA). When switching the scope to 50 Ohm, the part attempted to draw a much larger current (prevented from doing so by the current limit set on the supply). That's consistent with an excessive current draw with a 50 Ohm output.

    Thanks for confirming my suspicions! I'll try to be more careful in future ;)
  6. Kjeldgaard


    Apr 7, 2016
    There may be a further problem:
    Absolute maximum supply for the output circuit (Vdd-Vee) is 3.6 Volt.
  7. Col John Matrix

    Thread Starter Member

    Apr 16, 2009
    Thanks for the heads up. Ultimately I'll be using 0 - 3.3 V, which wouldn't be an issue, but at the moment I'm using 0-5 V simply to avoid having to use an extra supply.