Electronic – Does VSS (Logic Supply Voltage) on a L293D need to be connected for the IC to work

l293d

Here is my breadboard using an L293D breadboard. Pin 16 (VSS) is at the top right. I'm trying to have my project operate in sleep mode drawing as little current as possible. When I added the IC, my draw increased about 24mA. I was experimenting and pulled pin 16 which was connected to 5V and the draw immediately dropped to .54mA which is what I expected.

However the weird thing is that the IC still works. Pin 8 (VS) is connected to 12V which will power my motors.

Why is this circuit working?

Edit: It looks like 12V is connected to Row 57, but it is just hanging loose.

Best Answer

The reason it's drawing large amounts of current is that this is an older part that uses TTL input stages, instead of CMOS input buffers: here's what the inside looks like, at least from TI:

enter image description here

There does not appear to be an ESD protection diode to clamp the input to VCC1, so that's probably not the leakage path that is allowing this to work. I think it's still working because you have some leakage path elsewhere (please post a schematic) that is supplying VCC1 -- try measuring VCC1 in your case with a DMM and see what voltage it shows. I wouldn't leave VCC1 unconnected for the actual end product -- find a switch to turn it on/off when you need too.

As an aside, you do appear to have a bunch of floating inputs; while this isn't CMOS, I would still tie off your unused inputs to known values.