Hello. Part of my project is to power a device with rechargeable batteries, 5 Vdc. I am planning to use four (4) AA batteries to supply this power ~ 4.8 Vdc. The Pololu S7V8A voltage regulator will work perfectly to deliver a constant voltage.
I need help to find out how to use the SHDN (shutdown) feature of the S7V8A. I want to turn off the output power when the input voltage drops to 4.0 Vdc. At that point it’s time to recharge the batteries. Can anyone recommend the additional component(s) and wiring to make the low voltage shutoff (SHDN pin) work?
You can use a voltage reference, a resistive divider, and an analog comparator or opamp to make that circuit work.
However, the voltage reference, resistive divider, and comparator/opamp will be on the supply side, and will draw current even when the battery reaches low voltage – if you leave it hooked up, you will over-discharge the batteries.
What I’ve done in similar situations is to use a MOSFET power switch (P-channel) before anything else, and then put in the voltage comparing circuitry (I tend to like using an ATMega85 for this – everything you need in a single chip and have the comparing chip tell the MOSFET to power off.
I believe Pololu has a “soft switch” board with remote power off that you can use for this.
Yes, of course I will spend 20 minutes drawing up a schematic diagram for you.
The output will be high when the voltage is OK. Use this to drive, for example, a switching transistor (NPN or N-channel MOSFET as low-side switch.)
By flipping the + and - inputs to the comparator, the output will instead be high for undervoltage conditions, which you can then use to drive the gate of a high-side P-channel MOSFET.
R2 and R3 make up the voltage divider that sets the undervoltage threshold. It’s currently set for 4 V assuming a 1.25V reference (the component indicated is 1.225V which is close enough.)
and have the comparing chip tell the MOSFET to power off.