Hello all,
I'm in need of some pointers for a problem I have with a TPS61021A boost circuit. I've designed a small GPS tracker device that is supposed to run from a single AA battery. The boost circuit is supplies a 3.3V LDO with 3.6V, with a maximum draw of 150mA. Without the GPS unpowered, the entire circuit draws around 25mA, and the boost converter has no problem operating all the way down to 0.5V as the datasheet indicates, with around 80% efficiency at this voltage. But as soon as I connect the GPS module (which is using 25-30mA), the efficiency drops off a cliff. At 2.0V, the input current is 1A. If I decrease the input voltage further to 1.5V, input current quickly rises to 3A (which is the internal current limit) and output voltage falls out of regulation. L1 is BDHE002016101R0MQ1 1uH inductor from Pulse Electronics, with a saturation current of 3.6A. C1, C2, and C3 are CL21A476MQYNNNE, with an effective capacitance of 18.8uF at 3.6V. The output voltage of the boost converter is spot on 3.6V, so I can rule out the resistor divider being incorrect. One point that may or may not be relevant is that when I supply 3.6V on Vout with the the boost converter disabled, I get ~2V at Vin. The datasheet seems to indicate this shouldn't be happening due to the 'True output disconnect' feature.
Troubleshooting I've done so far- My first thought was the inductor was saturating for some reason, so I put an identical inductor in parallel with L1, but the efficiency numbers were the same.
- I forgot to add the feedforward capacitor to the current design, but I haven't experienced any instability issues. Just to rule this out, I've added a 10pF cap in parallel with R2, but I didn't see any difference in efficiency.
- I tried adding/removing output capacitance, still no effect.
- At higher input voltages like 3.0V, I'm seeing the expected 2MHz switching frequency on the scope, and nothing stands out as unusual. Tonight I will collect more datapoints with the scope.
- All of these findings have been replicated on multiple units, so I don't think faulty components are to blame.
At this point, I'm starting to wonder if there's a problem with the layout, given the relatively high switching frequency of 2MHz. The chip's ground pad extends on the same layer to the input and output caps. There's also an uninterrupted ground plane under the whole circuit. I tried to minimize the switching loop size, but maybe there's something I missed.
Any suggestions from power supply gurus out there?
Best Regards,
Caleb