I am planning on using an IC (https://datasheets.maximintegrated.com/en/ds/MAX38888.pdf) which has the following output pin:

enter image description here

It is an open-drain active-low backup indicator. The BKUPB is held low when the part is in backup mode. I would like an LED to light up when the IC is in backup mode.

I was wondering, is the following solution okay:

enter image description here

Do I need an additional resistor in series with the PMOS gate to limit current to the gate?

  • \$\begingroup\$ What kind of MOSFET are you planning to use there? You're calling it a "PMOS" but the symbol you've drawn is a N-Channel Depletion MOSFET (rather than a P-Channel Enhancement MOSFET which might work there). \$\endgroup\$ – brhans Jun 13 '20 at 0:47

Depending on BKUPB current capabilities you could just connect the LED cathode to the port pin and the anode to a current limiting resistor and then to 5V.

Updated schematic to show ~1.5mA draw with a 2.5kΩ resistor.


simulate this circuit – Schematic created using CircuitLab

Below is the other common way to do it using an P-Channel enhancement mode MOSFET , and then you don't have to worry about the current draw of your open collector pin.


simulate this circuit

  • 1
    \$\begingroup\$ +1 This is exactly how these pins are intended to be used. 100 ohms is a bit low though for 5V, will be very bright and likely more current than the IC open-drain output is intended to sink. \$\endgroup\$ – pericynthion Jun 13 '20 at 0:41
  • \$\begingroup\$ How do I find out if BKUPB has enough current drive capability? In the data sheet it has that when BKUPB is low with a 2mA current sink on it for a condition. \$\endgroup\$ – AlfroJang80 Jun 13 '20 at 2:18
  • \$\begingroup\$ Also, just for my own knowledge, if BKUPB did not have enough drive strength, would my MOSFET based setup be okay? \$\endgroup\$ – AlfroJang80 Jun 13 '20 at 2:22
  • \$\begingroup\$ @pericynthion good point, I updated the resistor value to be more realistic. \$\endgroup\$ – Aaron Jun 13 '20 at 5:02
  • \$\begingroup\$ @AlfroJang80 yes your PMOS circuit would have worked fine as drawn. \$\endgroup\$ – pericynthion Jun 13 '20 at 5:43

No need for an additional resistor. Most PMOS transistors would tolerate Vgs>5v. However you could check the datasheet of that PMOS to make sure.

By the way, you could easily remove the MOSFET and connect the LED directly to the backup pin with a resistor in series. But it's better to increase the resistor to about 1K. Obviously, the other side of LED should be connected to Vcc. If the intensity of light is ok with a 1K resistor, you can use my suggestion. But if its light is low, use your own circuit.


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