Powering WiPy in deepsleep from a capacitor?

[robert-hh]

@guyadams99 Vin can be as low as 3V. 3.3V works well.

[Eric24]

@guyadams99 OK. What you're using and the TI parts are miles apart. I now understand the problem. :) TI has (almost too many) low-IQ buck, boost, and buck/boost regulators--here's just one type (for 12V inputs): http://www.ti.com/lsds/ti/power-management/buck-converter-integrated-switch-products.page#p238max=12;42&p236typ=0.0003;100

The one I referenced is suitable for a pycom OEM module. If you want to use a XxPy, you will need more voltage, but there are other variants of that same part that are 5V or adjustable (the same data sheet talks about all of them). I have (and currently) do use that exact part for an L01 module, and it works very well. The bad news is that you won't likely find it on a prefab module anywhere, so you will ultimately need to build a PCB. But in the mean time, TI offers "eval modules" for most/all of their parts (pricey, but OK if you only need one or two).

If you need a prefab module long-term, you might check out Pololu's range of buck modules (many of them are based on TI or MAXIM low-IQ chips): https://www.pololu.com/category/131/step-down-voltage-regulators

[guyadams99]

@robert-hh Good to know, thanks! I presume if Vin <3.3 V the 3V3 voltage will drop commensurately as well?

[Eric24]

@robert-hh Just a note on the Vin requirements--I can only comment on my personal experience, and I have not tried to power a XxPy on anything other than 5V when using the module's on-board LDO or 3V3 when bypassing the LDO (and 3V3 when powering an L01). So lower voltages may certainly work, I just haven't tried.

@guyadams99 And definitely, if you power the module with less than the LDO's output rating, you are essentially passing through whatever the input is (less some small drop).

[robert-hh]

@guyadams99 The regulator at Vin of xxPy switches to bypass mode if Vin is less than 3.4 V with a voltage drop of <60mV. L01 devices have no internal regulator.

[robert-hh]

@Eric24 I gave it a try with a LoPy, which worked fine down to 3V, at which point I stopped. The ESP32 data sheet gives a range of 2.3V - 3.6V for VDD.

[robert-hh]

@Eric24 The Pololu devices are nice and I like to use them. But there is a special aspect regarding it /SHDN input. It is connected by a 100k resistor to Vin. At 12V, when shut down, this resistor alone will cause a current of 120 µA. For this special application, that may not be an issue. But if you are fighting for every µA, that may be a surprise.
(Sorry @guyadams99 for taking away the discussion a little bit)

[Eric24]

@robert-hh Agreed. I only see the Pololu modules as prototyping parts (I have dozens of them for that purpose), not as anything that would go to production, so I'm less concerned about those details. However, I realize that some projects are small enough that using these kinds of modules in production is appropriate, so that's where the details--like you mention--start to matter.

[guyadams99]

@Eric24 I have discovered an interesting artefact of my shoddy design. If the cap is discharged, there is no way to turn the whole system on - the buck is only turned on by the wipy, but this can't be powered up because the buck is off and the cap is empty. I ended up adding a momentary button from the battery +ve to the buck enable which you have to hold for a second or two to prime the capacitor to a high enough voltage to bring the wipy on to hold the buck enable pin high - but not really a good design. Perhaps I will look more at these low power bucks and try and find one in a form factor I can use (i.e. no surface mount soldering!)

[robert-hh]

@guyadams99 About the form factor: The Pololu devices @Eric24 mentioned come in on a pretty small PCB, about to220 package sized, with part only on one side.

[guyadams99]

@robert-hh - Thanks - I'll get a couple and take a look. Since I only do this stuff for my own use, my volume scale is usually 1!