How to use "enable pin" in ldo  - Page 1
Forum section: electronics / projects

author: farsi
author_rank: Regular Contributor
timestamp: April 30, 2017, 10:20:14 am
content: Hello,I am playing with a small LDO design based on ST LD29300P2MTR.This device has an enable pin, but I am a bit unsure how to use that pin.Thinking either a pullup from VBAT to the ENable pin, or a voltage divider to switch the LDO on with VBAT.What options are there?

author: Rerouter
author_rank: Super Contributor
timestamp: April 30, 2017, 11:00:38 am
content: Below 0.8v is off, anything higher is on, so yes your pullup should work

author: farsi
author_rank: Regular Contributor
timestamp: April 30, 2017, 12:49:07 pm
content: thanks, that is what i will try. but somewhere else I saw RC circuits, or diode protections for that pin. also, maybe in the future i would try a switch on that pin. so, i thought i would ask ideas here.

author: ajb
author_rank: Super Contributor
timestamp: April 30, 2017, 10:03:03 pm
content: One typical application for enable pins on regulators is for undervoltage lockout (UVLO). The idea is that you use a resistive divider across the input voltage to this pin such that when the input voltage is too low to ensure proper operation, the enable input falls below the threshold and the regulator shuts down. However, for this to work properly, the enable input must have some hysteresis, and you will have to make sure that you don't violate any maximum voltage on this pin if one is specified (check the datasheet). Note that the voltage divider will consume some power from the input, so for battery applications the resistance should be suitably high to limit the current to acceptable levels.Another possible use is to shut down part of a device for power savings. For example, a sensor might have a fairly power-hungry analog frontend, so in order to save battery life you may want to shut the frontend down when you're not actually measuring anything.You can also use an enable input to implement a power-on delay by using a resistor and capacitor to ensure that the enable input doesn't cross its threshold until a certain amount of time after power has been applied. This can be useful for power sequencing, or in situations where the power supply takes some time to stabilize before it's safe to bring up the main circuitry. In this application, you may also see a diode across the resistor to ensure that the capacitor is discharged quickly at power down to avoid misbehavior during brief power interruptions.

author: Buriedcode
author_rank: Super Contributor
timestamp: May 02, 2017, 10:05:53 pm
content: Although I think it was implied with previous replies, 'enable' pins can also be used for push-button power switching.A pull down resistor on the enable line (assuming active high) keeps it off, and two diodes connected to it, one going to a push button to Vbatt, and one going to an MCU pin. Press the button long enough for the output voltage to stabilize and the MCU to power up, and the MCU can make its output pin 'high' holding the enable line high, keeping the LDO on. The MCU can then control when it powers off, either by other user input, or a timeout.Sounds trivial but its surprisingly handy.It can also be used as part of power sequencing where there's multiple power domains in the circuit - all controlled by logic level enables.