Thank you for taking some time to read this. I've recently come across a nice constant voltage/current source (http://www.mouser.com/ds/2/115/AP4306A_B_C-271586.pdf) called an AP4306. Upon reading its datasheet, I was interested in the fact that it could potentially (I think) be used as a voltage comparator with a built in voltage reference. I'm putting together a over-discharge circuit for a lithium polymer battery which will be done with a voltage reference, voltage divider, MOSFET, and an op amp.

My major hangup is this block diagram:

enter image description here

I'm not interested in actually using this device as much as I'm interested on why it can work when it's tying the outputs of those two op amps together, wouldn't that just make this device a magic smoke generator? (Obviously not, but right now I don't know why)

Let me know what your thoughts are if you'd like, thank you for your time!


The data sheet states that the outputs that can only sink current. This is very common for comparators to have open-drain(or collector) outputs.

Since they only sink current they do not have normal opamp push-pull outputs and it is acceptable to tie them together.

It also means that there must be an external current source feeding the output - normally this would be a resistor pull-up to the positive rail of the power supply.

  • \$\begingroup\$ DUH! Should have seen that, thanks! It would be something like this diagram then. \$\endgroup\$ – John Kraige Jul 3 '17 at 23:00
  • \$\begingroup\$ Yes. 1 megohm is rather a high value for a pull-up except in an unusual situation. \$\endgroup\$ – Kevin White Jul 4 '17 at 3:35

The datasheet doesn't explain it but a look at the load gives us a clue.

enter image description here

Figure 1. From AP4306 datasheet page 2.

  • Note that the load is the opto-LED which is powered from Vout+. The amplifiers (1) and (2) never have to source current. This means that open collector outputs can be used on the opamps. Open collectors can be paralleled to form an OR gate. In this case if either output 1 or 2 turns on the LED will turn on.
  • If amplifier 1 inverting input ever goes above 1.210 V its output will pull low turning on the LED and (presumeably) switch off the SMPS switcher as the voltage is now >= setpoint.
  • If the current exceeds the current limit threshold the voltage at the inverting input will exceed that on the non-inverting input and it will turn on the LED limiting the power delivered.

I can't see any DC feedback on the circuit so the opamps seem to be used more like comparitors.


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