In August of last year, I wrote an article describing a design based on Texas Instrument’s TPS61200 low input boost converter. Soon after that, Sparkfun expressed interest in producing and selling this design. In just six short months, a LiPower has become available from Sparkfun store. It is a switchable 5V/3.3V boost converter designed to run from single cell Lithium-Polymer battery.
The TPS61200 converter is extremely versatile. It will start into full load from 0.5V, it can output decent current, it can operate when input voltage is higher than output, it can be programmed to switch off at certain input voltage level, preventing rechargeable battery from going into polarity reversal. It can also do many other neat things – to get an idea, take a look at Application Notes section on TI site. However, it is very hard to make a product capable of all this neatness at the same time. Out of the box, LiPower is exactly what is stated on the product page – the output can be switched from 5V to 3.3V and the undervoltage lockout (UVLO) is set to 2.6V, which is minimum safe voltage for LiPos. In this article, I will show you how to modify converter’s UVLO threshold to make it suitable for other types of batteries. I will start from very simple mod which eliminates UVLO completely and then explain more advanced modification, where UVLO can be set to a certain voltage.
1. Simple UVLO eliminator
If you want to use primary cells, such as alkaline or non-rechargeable lithium, you don’t want your switch to turn off at 2.6V. On the contrary, you want your supply to run until the last drop of juice gets sucked out of the battery. To set UVLO to the minimum possible value, which is 250mV, UVLO pin must be tied to Vin; the easiest way to do it is to place a short across resistor R3 (see schematic). A picture on the left shows how to achieve this – the green asterisk marks the place on the boards where mod is located.
Take a 4-6″ piece of bare thin wire, tin one end of it. If you have a vise, clamp the board so that JST connector is on the left and component side is on top. Apply some liquid flux to R3 and C2. Take wire in the left hand and place tinned end between R3 and C2. Heat up with soldering iron until soldered. Cut the excess wire leaving a little extra in case you later decide to revert to the original configuration.
To demonstrate that you don’t need to be NASA-certified electrical assembly technician to solder SMT parts I tried to produce as sloppy a soldering job as I possibly can – despite somewhat scary look modified circuit works just fine. Look closely and you will notice that I inadvertently de-soldered R3 from its place. Should this happen to you, solder it back somewhere so you won’t lose it.
This is it – the modified LiPower will work as long as input voltage is higher than 250mV. Note, however, that maximum output current is going to be low as well. According to figure 1 of the datasheet, it will be around 50ma depending on output voltage.
One last thing about this mod. Since R4 – the lower resistor of the voltage divider is left in place, some current will flow through it all the time. The amount of current depends on input voltage and can be calculated using Ohm’s law; for example, for 3V input the current through R4 is 13uA (it’s micro-amperes). If such amount of current is of concern, remove R4 from the board (it is located on the right of R3) and make sure its pads are not shorted together.