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ADCMP350 Vcc=0 V+>0

I have to supply rails: an adapter AC/DC and one USB. They are ORed together through diodes. I want to support 5V adapters (if it is connected the USB power is turned off) so if the voltage is greater than 4.5V on the adapter side I disable the USB power through a switch. ADCMP350 is powered from the USB side but the input is tied to the adapter side. However there might be a situation where the USB is not connected but the adapter is. In this case the device is not powered however the input is driven (high resistance - resistor divider).

I have the following question: what happens if ADCMP350 comparator is not supplied but there is some voltage through a resistor divider in its non-inverting input? Will be the output determined? Can I damage the device in this way?

According to the datasheet in normal conditions the input voltage can be much more greater than the supply but I don't know if it is true when the power is off.

  • Hi Somhegyi Tamas,

    As long as the VIN (non-inverting input) doesn't exceed the absolute maximum rating(-0.3V to 25V) the part would be alright as the max rating is not dependent to VCC(supply). When the ADCMP350 is not powered, the state of the output is unknown as we haven't tested the part on that condition. Basing on the description of you're application, the output will likely to be in high impedance state and the potential will be determined by the pull-up source.

    We suggest to connect the VCC of the ADCMP350 after the OR-ing so that it will always be powered and with known output state.

    Best regards,


  • Thank you.

    Unfortunately the adapter voltage can be greater than 5V (<26V) so it is not possible to connect there as Vcc,max is 5.5V. (I use high input range Switching PSU) The output of this comparator is connected (with pullup) to another IC's input (logic).

    I think I will put a FET to the upper side of the resistor divider and I turn it off when USB power is not available.