Single Cell 1S Lipo as a reservoir

[Experimentalist]

Hi all

It has been a while.

I have a project using the T3.1 in conjunction with an East Rising 7" capacitive touchscreen (ER-TFTM070-5) and a SD card that operates as a PC peripheral. At times, depending on the PC and USB port available , this can represent a significant load, especially when the screen brightness is pumped up, so I am using an external power brick. However, due to the nature of how I use the device, it spends most of its time sleeping with the screen off and it usually only stays on for a few seconds at a time, up to a couple of minutes. Currently, as I said, I have a separate supply and have cut the VIN-VSUB link on the T3.1 and the PC connection is data only. This limits its use to when I have mains power available and negates portable use with a laptop. What I would like to do is use a single cell Lipo as a reservoir to avoid the external supply. The idea being, since the device spends most of its time sleeping, to use the USB power from the PC to charge the Lipo battery when the device is sleeping. I am planning on using the MCP73832 as the charge management system. When I touch the screen to wake it up I want to use the battery to power the screen and when the device switches back to low power mode once again top up the battery using the USB power to drive the above chip. There are many challenges to overcome, low voltage protection, switching, etc, most of which I am happy with. The one I am stuck with is a suitable 3.3V regulator for the screen. The screen data sheet shows that it can operate between 3.0V & 3.6V and consumes 480 mA max when supplied from a 3.3V supply. The Lipo when fully charged will be around 4.2V and should operate down to a minimum of 3V.

How can I power the screen from the Lipo and keep within the screen characteristics of 3.0V-3.6V? Most regulators with a 3.3V output expect a 5V input and accept a minimum around 4.5V, which I will never reach. In contrast I can't drive the screen straight from the battery as the battery will be at around 4.2V as the screen gets powered up, 0.6V over the maximum rating for the screen. I seem to be in no mans land.

Anyone have any ideas or guidance?

Thanks
Ex.

 

[MichaelMeissner]

First of all, you probably want to get Onehorse's High Current LiPo charger: https://www.tindie.com/products/onehorse/stbc08-high-current-lipo-battery-charger/. This is soldered underneath the Teensy, and it will automatically charge the lipo if there is USB power, and fall back to using the lipo battery if not.

Second, you might want to get a step-up/step-down power regulator, this will boost current under your target voltage and limit the voltage if it is over. These regulators aren't as efficient as one that only boosts or lowers the voltage, but it gives you more freedom with batteries that start out above the voltage and then drop over time. Here is one: https://www.pololu.com/product/2118. I imagine you would want to connect it between the lipo and the screen. Pololu also has one that is fixed to 3.3v: https://www.pololu.com/product/2122. Note, these regulators can get hot, so you might need to vent them and deal with the heat.

 

[Experimentalist]

First of all . . .

Thank you for you advice Michael, exactly what I needed. As ever the main advantage of using a Teensy over other offerings are Paul, PJRC in general, and the community of users, today namely you! I obviously managed to ask the question well enough to get the response I needed. Now to do some more reading . . .

Thanks again
Ex.

 

[pictographer]

If the prospect of shedding heat is a bother and you're willing to live with a buck regulator without the boost capability, there are definitely switching regulators that fit your specs. If you want to make your own board, you can find suitable parts using a parametric search at Mouser, et.al.

If you want a module, one small cheap module goes by the name Mini-360. A quick search turned up a Mini-360 for $2.18 with free shipping to the US. I've got one seemed just fine under light testing. I don't recall where I bought mine and I have no idea if the seller I linked to is good.

 

[Experimentalist]

there are definitely switching regulators that fit your specs . . . one small cheap module goes by the name Mini-360

This seems to have the same issue I have found with all of them, its minimum input voltage is 4.75V and the maximum the battery will ever reach is around 4.2V

I guess that leaves me with the other options suggested by @MichaelMeissner

Thanks go to both of you for your time

 

[MichaelMeissner]

I've seen suggestions elsewhere when using a 4 battery AA holder where freshly charged batteries bring the charge slightly higher than the device can handle to use a diode inline (which can also block reverse currents). Normal diodes drop about 0.7v. Of course you would have to tolerate the 0.7 loss when the battery settles down to 3.7v, and it does use a little extra power.