What is it?

This is Texas Instruments' TPS61088 boost converter reference design in a very small (25 mm x 14 mm) package (but look at the size of those capacitors!). It takes the ~4 V input from a single-cell LiPo battery and boosts it up to a higher voltage that can be programmed by changing one 0402 resistor (the yellow one next to the big caps). The soft start time, current limit, and switching frequency can also be programmed. Here I copied the reference circuit which sets these values (Tss ~ 0.125 s, Ilim ~ 10 A, fswitch ~ 1 MHz) to suit most applications. The mode pin is pulled to ground by a zero Ohm resistor to place the device in the forced PWM mode for all loads up to the current limit of 10.3 A. If this resistor is removed, the device will operate in the PFM mode under light loading and transition to the PWM mode at heavy loading with a current load limit of 11.9 A. Consult the data sheet for a detailed explanation of these modes. Unless you specify a particular voltage when ordering (anywhere from 4 to 12 V, I'll get as close as I can) the output voltage will be set for 4.4 V.

There are input and output pairs of through holes for soldering a LiPo battery connector and output leads. There is a slide switch which enables the TPS61088 chip. When disabled, the booster board passes the battery voltage minus ~0.3 V for the forward voltage of the TPS61088 internal diode. When the slide switch enables the TPS61088, the output voltage is whatever is programmed by the resistor divider. Since I use 1% or better resistors, the output voltage will start at 4.4 +/- 0.05 V. As the current draw increases, the voltage will slowly drop, linearly, up to the full 10 A load capacity. I have tested this up to 7 A and it works remarkably well, with the voltage dropping from ~4.2 at start to ~3.8 V at 7 A. This is much better performance than can be obtained with a single-cell LiPo battery alone.

Why did you make it?

A common problem when running motors that draw a lot of current from a single-cell LiPo battery is the current draw drops the battery voltage due to the internal resistance of the battery, which can be surprisingly large. It is common for the voltage to drop from ~4 V with no load to ~3 V when driving brushed motors at top speed. The problem is the change in voltage changes the efficiency of the motors (makes them less efficient). But more importantly, if the battery is also powering peripherals like a radio or a microcontroller and sensors the voltage drop can be so large that these simply stop working. This has happened to me when using the Edison which has a low voltage limit of 3.15 V below which it reboots! Nothing is more disappointing than having the controller on a vehicle decide to reboot when approaching full power!

This boost converter is designed to be so small it can be taped to a LiPo battery and simply become an integral part of the power source for your project. Think of it as an augmented LiPo battery, whose voltage will stay high even at very large current draws.

What makes it special?

I made this board as small as I dared while keeping a fair thermal resistance to avoid overheating the circuit during high current operation. It is small enough to attach directly to a common 350 mAH LiPo battery while maintaining the areal form factor. In other words, its not bigger than the battery most people use for robots and quadcopters. I didn't skimp on the circuit; it has the full complement of Brobdingnagian capacitors to maintain high voltage output and carry the 10 A current load.

Order the boards at OSH Park and put some together yourself, or order the fully assembled and tested board from me and see how much better an augmented LiPo battery can be!

(LiPo battery not included)