Regulator and Voltage Detector

Discussion in 'The Projects Forum' started by CraigHB, Dec 31, 2011.

  1. CraigHB

    Thread Starter Member

    Aug 12, 2011
    I'm using the following circuit to provide over-discharge protection for a 3.7V Li-Ion battery.

    The NCV551SN30T1G is a 3V regulator with enable that allows -.3V to Vin +.3 on the enable pin.

    The NCP300LSN33T1G is a 3.3V detector with a complimentary mosfet output on the Reset pin.

    I need the RC network (on the enable pin) to provide a delay so I don't get a reset from a dip in battery voltage due to over-loading. A µcontroller handles that situation within the delay provided by the RC network.

    My question is, do you think the diode is necessary to protect the regulator from enable voltge greater then Vin + .3V? That happens when the battery is removed and replaced or when there's a dip in battery voltage due to an over-load. I'm not sure if I need it or not, better without it. I figure the 33k resistor will limit current well enough to keep the chip from getting damaged, but I'm not sure.

  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Yes, according to the datasheet, the maximum voltage at the ENABLE pin is Vin+ 0.3V. If the battery needs to be replaced and you don't wnat to use the diode you could connect the enable pin to ground via a jumper.

    With the 33k resistor the ENABLE pin would probably be protected good enough, but you cannot be sure. For absolute certainty I'd use the diode.
  3. CraigHB

    Thread Starter Member

    Aug 12, 2011
    Thanks for the reply.

    The issue with the diode is that I want to avoid leakage. Normally, it's well within tolerance, but diodes being diodes, it goes up quickly with temperature. Since the point of the circuit is to protect the battery from over-discharge, any leakage is a big deal.

    It may just come down to setting up a little test bed to see for myself. Was hoping someone could save me the trouble. It's one of the last little design quandaries I have with my project.