I am new to electronic circuit design. My current project is to monitor biasing voltage for under/ over voltage using IC from TI: TPS3700. If any of the under/over voltage occurs I will need it to drive a relay coil to cut off the main circuit supply.

I have studied the TPS3700 datasheet and found out its output pin is open-drain type and able to sink around 40mA of current. Could I connect NEC UA2 relay coil (pin8) to one of the TPS3700 output pin. This relay has coil current around 20mA and will have separate 5V supply connect to its pin1. My idea is to sink the relay coil current via the TPS3700 directly.

Another concern is will the relay switching noise coupled into the biasing voltage which is being monitored?

datasheet for TPS3700 and NEC UA2 relay http://www.ti.com/lit/ds/symlink/tps3700.pdf https://www.nec-tokin.com/english/product/pdf_dl/mini_data/relay_ua2_ub2_e.pdf


1 Answer 1


The 40mA is an absolute maximum spec and not a place where the device is intended to operate. Even 20mA is a stretch, and TI doesn't specify what voltage you will have across the drain-source of the internal FET at that level. You can see the operating specs specify the output voltage at currents in the range of .4 to 5mA.

These devices are not intended to drive relays directly. It's very simple to use a 2N7000 type FET or a 2N3904 BJT hooked to the supervisor part to drive the coil. Always use a diode across the coil to prevent the inductive kick during turn-off from killing your switch device. (The diode should be rated for something like 1.5X the coil current, and for a voltage greater than the coil supply voltage.)

There's a nice diagram of a relay driver in the first post here: http://forum.allaboutcircuits.com/showthread.php?t=29461

You can ignore the text unless you find it interesting.


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