Author Topic: Logic IC power-off low side switch & decoupling  (Read 694 times)

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Offline smoothVTerTopic starter

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Logic IC power-off low side switch & decoupling
« on: October 11, 2019, 03:32:48 pm »
I've designed a clock subsystem that runs off a clipped sine wave oscillator.  The oscillator itself has no enable pin;  enable/disable is done by turning on/off the supply to the oscillator via an LDO.   When the LDO is off, the oscillator is off, and the output of the oscillator is Hi-Z.  This leads to the inverter oscillating about its resonant frequency due to Rf/Cf, a situation which burns power when the clock subsystem is supposed to be off.

Thus I need to switch power to the inverter on or off.  The easiest way to do this is for me to switch ground, as I can use a low side NFET to switch ground to the inverter using the same logic as the EN to the LDO.

Question is:    where dooes the decoupling cap go between when using a low side NFET is this fashion?   Point #1 ( the GND pin of the inverter ) or #2 ( true ground point )?    Does it matter?   If it matters, why does it matter?

For reference schematic:

 

Online ledtester

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Re: Logic IC power-off low side switch & decoupling
« Reply #1 on: October 11, 2019, 10:42:23 pm »
Shooting from the hip here with an alternative idea... would a weak pull-down on the inverter input do the job?

Another idea - use a NAND gate instead with the other input fed from EN. Or somehow pull the inverter input to GND when EN is low.

 

Offline Ian.M

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Re: Logic IC power-off low side switch & decoupling
« Reply #2 on: October 12, 2019, 02:17:54 am »
If the inverter supply current is low enough, you can simply power it from the same I/O pin that drives the LDO EN signal. Reduce its decoupling to 1nF so you don't overload the pin for too long when switching it.

,
 
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Offline smoothVTerTopic starter

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Re: Logic IC power-off low side switch & decoupling
« Reply #3 on: October 24, 2019, 04:07:28 pm »
Thanks, this solution works great!
 
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