Inverting the level of an Open-Drain output pin?

Hi.

I am developing a circuit using a DS3231 real time clock.

The RTC has an alarm output that can pull a voltage low when the alarm is active (open-drain output).

I need the "alarm" signal to instead apply a voltage (VBAT) to the enable pin of the TPS6109 boost convert IC.

The purpose of this circuit is to wake up every few minutes based on alarm output, do some processing, reset alarm, turn it's self back off.

I have included an image of the relevant part of the schematic and added three questionmarks where I am unsure what component to use.

Would using a PNP transistor be a good way to flip the logic?
Would it be a big constant drain on the battery?

Thank you.

megaBlocks:
Would using a PNP transistor be a good way to flip the logic?

I would say so. But Vbat may not be higher then the 5V you supply the DS. You may not lift the alarm pin above Vcc of the DS. Aka, the converter must be a step up type.

megaBlocks:
Would it be a big constant drain on the battery?

The PNP would be off is the alarm is off, so no.

Keep in mind most boost converters don't output 0V when they are switched off. They output the input voltage. This may keep things on / drawing power.

The attached would work - and takes no current until the input is pulled low...

But check the psu data to see if it allows volts when turned off

regards

Allan

invpnp.pdf (17.7 KB)