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.
Do not use this mod if your power source is of secondary type, such as a battery of NiMH cells. For rechargeable batteries, you do need UVLO and it has to be different from default UVLO setting. Keep reading – modifying UVLO is a liitle bit more involving.
2. Modifying UVLO. An introduction
The UVLO circuit of TPS61200 works like this: converter switches off when negative-going voltage on UVLO pin reaches 250mV. Converter switches on when positive-going voltage on UVLO pin reaches 350mV. This property is called a hysteresis. Hysteresis protects the converter circuit from oscillations when battery drains down to the threshold – we all know that when you remove the load, the battery voltage raises a little. Consequently, if there were no hysteresis, the converter would switch off at UVLO and then immediately switch on because unloaded battery voltage would raise above UVLO. It should be noted that purpose of capacitor C2 is to temporary disable the UVLO – it momentarily shorts R3 when power is applied making starting the converter from less than freshly charged battery possible.
To program UVLO threshold to a certain value, a voltage divider R3/R4 is used. It is set to output 250mV when it has desired voltage on the input. The input of a divider is connected to Vin, i.e., the battery. The formula for ULVO divider can be found on page 16 of the datasheet at the very top.
At present, two main types of rechargeable cells are used – Lithium and Nickel-Metal-Hydride. End of discharge voltage for NiMH is specified at 0.8V per cell and we are only really interested in UVLO values of 0.8V, 1.6V, and 2.4V for single, double and triple-cell MiMH batteries, respectively – the output voltage of four or more cell batteries is too high for TPS61200. 2.4V is close enough to default 2.6V, so LiPower can be used with 3-cell NiMH without modifications. We can see that we really only need to move UVLO voltage down from 2.6V and this can easily be done by soldering a resistor in parallel with R3. The value of resistance of R3 in parallel with this added resistor will be smaller than R3 alone, therefore, the output of the divider in relation to the input will “raise” and UVLO will go down.
3. Real life UVLO mod
Let’s say, we want to run 5V Arduino from single cell NiMH. We want LiPower to turn off when battery drains below 0.8V. First of all, we need to calculate new value for R3. From datasheet formula we find: R3 = 220K * ( 0.8/0.25 – 1 ) = 484K. Now we need to find a value for the resistor to put in parallel to existing 2M R3 to get 484K. The easiest way to do it is to launch this parallel resistor calculator, fill in R1 field with 2000, fill in Parallel resistance field with 484, press calculate and read result in R2 field. Regretfully, the 638.5224274406332 (kiloohms) value given is not very useful – we need real resistors. Two closest standard 1% resistor values (E96) are 634K and 649K. If we decide to use 634K, the UVLO will be set lower than 0.8V, run time will increase and battery life will decrease. Choosing 649K will have the opposite effect. There is no point in attempting to set UVLO precisely – NiMH cell has sharp voltage drop at the end of discharge and UVLO pin hysteresis will help keep converter off, so as long as UVLO is set in the ballpark, the cell will be OK.
When resistor value is known, it’s time to solder. Picture on the right shows the final result (click on the picture to make it bigger). Two 1/4W 5% resistors connected in series measure 325K-ish each. They are soldered across C2 instead of R3 because 0603 capacitor is slightly bigger and slightly less prone to de-soldering than 0603 resistor. Liberal application of solder flux is the key to a good solder joint. Quick testing of the modified circuit reveals that the TPS61200 switches off when input voltage falls to 0.863V and switches back on at 1.0V – given component tolerances this result is almost perfect.
Choosing parallel resistor for setting 1.6V is left as an exercise for the reader.
4. Final notes
Modifying LiPower’s UVLO settings board is simple, thanks to clever board layout and component value choice. With very little effort the board can be made suitable to work with one or two cell NiMH batteries. I am hoping that my explanations were helpful; if anything is not clear, please don’t hesitate to leave a comment with your questions. I am planning to write couple more articles explaining other LiPower board mods, such as solar cell MPPT tracker and white LED driver – stay tuned!