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[LP5900] Simple circuit to control LDO enable with a momentary switch w/ micro-controller disable.

John Condon
Prodigy 10 points
Other Parts Discussed in Thread: LP5900

Greetings,

I am designing a low-cost, battery powered consumer electronic product which will use a momentary switch to power the device on and off.  

To do this I'm hoping to make a simple circuit to control the enable pin of my LDO (LP5900).  When the momentary switch is pressed the LDO enable pin should latch high (with debouncing) turning on the LDO and powering the micro-controller.  When the momentary switch is pressed again, the LDO enable pin should pull low, unpowering the device.  Additionally the micro-controller should be able to pull the enable pin low when not in use for a period of time.

My current design is using a debouncing IC, but it is too expensive to make it into the final product.  Any ideas for a replacement?  I think there must be a way to take advantage of the regulator output to latch the enable after the LDO turn-on time, but I can't put my finger on implementation.

Thanks in advance!

  • 0
    TI__Intellectual 1955 points

    Hi John,

    You may use a large Resistor inseries with a cap (0.1uF to 1uF) Vdd to Resistor and ground to cap and hang a switch from the junction of the RC to ground.  You may use a momentary switch for a reset application.  Modern metalic "click/clap" switch many are virtually bounceless and the cap helps subdue it further.  You can use a latch or or FF configured in a divide by 2 to accomplish the latched toggle switch mechanism to serve the EN pin.   Use a scope to check what I mean by the clean action of many lightly-touch actuated micro switch of simple construction.    The uC can assert the reset pin of the FF or preset it, thus enables the system logic to control the LDO enable during waiting or power saving operations.

    You have to have some glue logic, OK, if 7474 is at the ceiling of the budget, then an OR gate 7402 (2 NORs) may also be used with the momentary switch above to implement.   One input of the OR goes to the MSW, the 2nd input goes to the LDO Vout, and the output of the OR controls the LDO EN.  Should you want to have a more snappy action to turn on, you might consider some C or RC delay..... only if you find you have to press the button long!    Hope that will help you to have a cheerful holidays.    Let me know for I will response within the next 32hours.

    Regards,

    Kern