Electronic – arduino – How to use the Shutdown pin of a switching regulator

arduinobattery-operatedboostlow-powerswitch-mode-power-supply

What are the systematic/standard methods for incorporating the SHDN (shutdown) pin of a buck/boost IC?
Take MAX756 for example (datasheet).
Here are its I_q profile:
MAX756 I_q profile
Suppose I'm using an Arduino nano (ATmega328) as the MCU and nRF24l01+ as the transmitter. I'm reading some sensor values via ADC and transferring the data over the RF link. So the system has a sampling frequency (suppose 100Hz).
If I want to save battery, the system should sleep most of the time and wake up and transfer the data 100 times a second. All Vccs should come from the boost converter. Now:

1- I should make MCU and nRF sleep but how about the step-up IC? Should it also be shutdown and waked up 100 times a second? I guess I should take the IC's start-up delay into account? Here is its profile:
MAX756 start-up delay

Regarding this profile, I guess the IC wakes up in ~2ms… So the maximum sampling rate would be at best 500Hz?

2- What voltage would be on the OUT pin of MAX756 if it is shutdown? The same as Vin? or is it floating?

3- Is SHDN the same as Enable in different ICs?

4- If there is no way that the main MCU could control the SHDN pin, can I use like a 555 timer solution to control SHDN instead of an auxiliary tiny PIC MCU just for controlling the step-up IC? Because using a separate MCU just to control when an IC should shutdown seems to me rather an overdesign solution…Although if it is used in industry I have no problems then

Thank you very much

Best Answer

1- I should make MCU and nRF sleep but how about the step-up IC? Should it also be shutdown and waked up 100 times a second?

No. Startup times will make that impractical.

2- What voltage would be on the OUT pin of MAX756 if it is shutdown? The same as Vin? or is it floating?

0v (turned off)

3- Is SHDN the same as Enable in different ICs?

Yes, but the opposite label.

4- If there is no way that the main MCU could control the SHDN pin, can I use like a 555 timer solution to control SHDN instead of an auxiliary tiny PIC MCU just for controlling the step-up IC? Because using a separate MCU just to control when an IC should shutdown seems to me rather an overdesign solution...Although if it is used in industry I have no problems then

If you power your MCU from the regulator and the MCU controls the regulator, then when the MCU turns the regulator off it will be committing suicide. No way to turn itself back on, since there is no power to run the MCU.

It is more normal to power different sections of your circuit from different regulators and turn them on/off as you need them.

You need to take the startup time of the regulator into account (most have a "power good" pin to see when they have started up properly). If that takes longer than the sleeping time then switching off while sleeping is not possible.

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