I want to use LTC4418 fuctionality of detecting which one of power supplies is on at this moment. Here is my approach to it:

enter image description here

PS_VALID1 is connected directly to VALID1 of LTC4418 (without any additional components), same for PS_VALID2. BLE_GPIO is a GPIO of 3.3V powered microcontroller.

What I got confused by is the documentation stating:

Output Currents
VALID1, VALID2, CAS..............................–2mA/+5mA

VALID1 and VALID2 are open-drain outputs, fine, they pull-down to GND or are Hi-Z, but why there is a +5mA output current stated in the docs? Is this circuit correct then?

  • 1
    \$\begingroup\$ Please explain why use transistors? Is it not possible to directly send a signal to the GPIO? \$\endgroup\$
    – AltAir
    Apr 5, 2019 at 10:00
  • \$\begingroup\$ Good point, actually I could use internal pull up resistors and connect it directly to GPIO without any external components. \$\endgroup\$
    – pbn
    Apr 5, 2019 at 12:08
  • \$\begingroup\$ Provided that you know the level of the GPIO is tri-state/low at power up. \$\endgroup\$
    – Huisman
    Apr 5, 2019 at 17:38
  • \$\begingroup\$ @Huisman I found a comment from manufacturer when starting up the GPIO pin will be in input mode, but input buffer is disconnected. So basically it's a floating pin with high impedance. origin \$\endgroup\$
    – pbn
    Apr 5, 2019 at 21:03

1 Answer 1


You're looking at the Absolute Maximum Ratings. Those are the values you should not exceed when forcefully sinking or sourcing that pin.
So, don't draw more than 5 mA from the pin, don't source it with more than 2mA.

Without this "Q?A" or "Q?B" transistor, you would be sourcing 3.3V/10k = 0.33 mA, so, the circuit is fine to me.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.