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TI boost converter - TPS61221

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Thought I'd share some thoughts on this chip since it's an obvious choice for powering low-power MSP430 type apps off a battery in some remote location.


The TI TPS6122x series are boost converter chips, i.e. you have to add capacitors and an inductor around it, which take a battery voltage from 0.7V-5.5V and boost it to the appropriate output -- 61220 is an adjustable model where you build a resistor network on the Feedback line to specify the voltage, 61221 is fixed to 3.3V, 61222 is fixed to 5.0V. Peak output is technically 200mA although you should keep it lower than that, maybe 120mA or lower to account for performance while the battery drains down.


TI's product page: http://www.ti.com/product/tps61221


The chip is tiny and only comes in surface-mount packages, the DCK package (0.65mm pitch, 6-pins). The pitch of the pins is similar to the pitch of the surface-mount MSP430 TSSOP packages, at least for the value line. Definitely DIY'able although I'd suggest soldering the TPS61221 chip first (so you have clear access to all the pins without other junk getting in the way of your iron) before you solder the rest of the components.


Anyway I've made 2 test boards (single-sided toner-transfer PCBs) to measure the actual characteristics of the chip. I've mainly only measured its current draw from the battery under no-load conditions, but I've confirmed it can power an LED (blue) drawing about 35mA with no limiting resistor. (The battery input side was showing weird results, about 220mA, which I suspect has to do with the 200mA switching current it uses to do its work... I'd probably need an oscilloscope to properly measure what's going on there. It's supposed to draw what it needs + 5.5uA in terms of net current draw, but does so using periodic/frequent 200-400mA swings in current across the inductor)


At zero load, my multimeter's uA range shows about 9-10uA draw from the battery, just about what the datasheet suggests. My first experimental board I used some Tantalum capacitors I had and I think they were a mistake; no-load power draw ranged from 400uA-3mA with those. I've had similar problems with them in past boards where they'd short circuit causing upwards to 1A draw off the power supply, so I have since thrown them away and stuck with ceramics instead. Using a 10uF ceramic cap on the input side (+Vbatt to GND), and a 1uF (close to the chip) + 10uF ceramic cap on the output side (Vout to GND) it behaves like the datasheet suggests. FWIW, I used a 4.7uH SMD inductor rated up to 650mA.


The datasheet mentions that if you switch the EN (enable) line off, to GND, it lets battery voltage pass through the onboard MOSFET's bypass diode. This seems to be the case, but it does net a ~0.3-0.5V voltage drop between Vbatt+ and what you see on Vout under those circumstances. Additionally, I noticed a ~10-11uA current draw from the battery with that EN line grounded. Seems like it's pointless to design a circuit where an MCU can shut off the boost converter (say, to save power while the MCU runs off the direct battery voltage bleeding through the boost converter)--because the voltage is dropped a bit and it's still sipping power anyhow. The boost converter OTOH can give the MSP430 a usable voltage until the battery's voltage is around 0.7V (and then, as I saw with a CR2032 + several LEDs toasting the thing, it still outputs some ~1.8V even with the battery run down to 0.4V)


Anyway that's my 2 cents on this chip. I'm going to use it in a design I'm coming up with (uses thermocouple amplifiers that require 3.0-3.6V), more on that later.


PS- Leaving the EN line floating doesn't sound like a good idea, my cheap 10uF ceramic caps seem to make noise (ceramic piezoelectric effect?) when a lot of switching is going on and while I hear it sometimes under normal operation with load (like that blue LED), they go bonkers and the LED flickers like crazy with the jumper removed from the EN line altogether (not GND'ing it). Guessing that destabilizes the switcher.

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