How to use the Shutdown pin of a switching regulator?

Question

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:
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:

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

Answer 1

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.

Answer 2

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

For this specific converterr, the output will be 0V. But there are other components (just as an example I mention the TPS61291) with a bypass mode, where the output is connected directly to the input when the converter is shut-down. This could be useful if the MCU could still operate with Vin.

As a general comment, to answer questions like this (I'm mostly referring to your point #1) you have to make your calculations and see how much energy will you save if you do this or that. For example, you want to have 100 Hz sampling rate, that means max sleeping time 10 ms, which of course you will not have. Let's say it will be 5 ms. So if you shutdown the converter you will save around (200 uA - 20 uA) * 5 ms * (100*60*60) (times/h)= 0.09 mAh pro hour. Just very rough calculations here! If you think it is worth it, then do it, otherwise don't. Every design and every system has different specifications and needs, there is no general rule you can apply everywhere!

Also, something else. Have you considered the startup time from sleep mode of the MCU and how much time it will take you to sample, process and transmit your data? It may not be possible to go into sleep mode at all with this sampling rate you are talking about!