Connected Device Powerdown Suggestions

[JimTal001]

I plan to connect numerous 3.3V device/sensor to the pyboard. These device communicate with the pyboard using digital I/O. My plan is to go into low power mode (98% of time) then wake and read from the external device, log data, then go back to sleep. Battery life for this application will be critical. My question is: what would be the best way to power down these external device when the pyboard is sleeping? Does the pyboard has a 3.3V output that can be controlled via code?

Thank you
Jim

[dhylands]

The pyboard doesn't have a 3.3v output that can be controlled by the pyboard, so you'll need to create one.

You should be able to use a MOSFET, or find a voltage regulator that has an enable line.

[pythoncoder]

Bear in mind that, according to my own tests, if you issue pyb.standby() any Pyboard pins configured as outputs go high impedance, even if you enable internal pull up or pull down resistors. The solution I intend to use is to have a P-channel MOSFET switching the 3.3V supply, with a resistor between gate and 3.3V to ensure the device remains off when the pin goes high-z. I'm awaiting the delivery of a MOSFET with suitably low threshold voltage so I haven't actually tested this.

[JimTal001]

Hello pythoncoder,

Here is what I ordered: FDN337N . Based on your post I may have ordered the wrong chip.

[JimTal001]

pythoncoder,

What do you think about the FDN338P for this application?

[pythoncoder]

I'd have thought the FDN338P would be ideal.
EDIT
When I implemented this a thought occurred. To keep power consumption low when the peripheral power is on, the pullup resistor needs to be a fairly high value. I used 470K, consuming 7uA. To quickly discharge the capacitances of the FET (g-s and Miller capacitance) it's best to configure the pin driving it as push-pull. To turn the FET off I drive the pin high, then delay 10uS before issuing the standby. This gives the capacitances time to discharge through the pin, with the resistor maintaining the FET in the off state when standby drives the pin high impedance.

It works fine, by the way

[JimTal001]

Could you verify my understanding of the circuit in the image below?


I've never used a p-Channel before and it confuses me with the voltages in the datasheet listed as negative, e.g.
Vgs = -2.5

Thanks

[dhylands]

Vgs = -2.5 is just saying that the gate is 2.5v below the source.

[JimTal001]

Understood, Thanks.