Electronics > Power/Renewable Energy/EV's

TPS61021A boost regulator can't handle 50mA

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hlab:
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

artag:
Not sure about the reason for the high current draw.

However, applying voltage at Vout might well produce something at Vin. The output disconnect feature is so you can fully switch off the output (rather than getting the input passed through to the output as happens with a simple boost). I wouldn't say it offers any guarantees about backwards paths whether through the inductor or through internal circuitry (note that the gate drive circuit has both Vin and Vout connected to it but what happens inside isn't shown on the datasheet.

 

__steven:
Caleb, can you confirm that the PGND pad is indeed connected to ground in your layout (I believe this is pin 9)? I cannot tell from the photograph. Alternatively, have you attempted reflowing the IC such that you can validate that the PGND pad is making a good connection to the pad? I have had IC's that use this pad as an internal reference act funny when the part is not correctly soldered. It should dance under waving hot air.

Steven

mtwieg:

--- Quote from: hlab on April 29, 2024, 04:59:43 pm ---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

--- End quote ---
That's quite an extreme drop in efficiency. If your measurements are correct, that excess power is being dissipated somewhere, and it's probably getting very hot. Do you have access to a thermal camera?

If possible, I would try disconnecting the LDO and other load circuitry from the TPS61021A and test it with a simple dummy load (electronic load or dummy resistor), that way you will know exactly where power is going.


--- Quote from: hlab on April 29, 2024, 04:59:43 pm ---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.

--- End quote ---
If nothing else is connected to Vin, then it's not surprising to see some voltage there due to a few uA of leakage current. I'm betting if you add a 1K resistor from Vin to GND the voltage there will drop a great deal.

Also would suggest checking the PGND connection. In your layout screenshot it doesn't even look like there are any vias connecting it to anything...

hlab:
Thanks for the input guys. Yes the ground pour is continuous below the chip, you can't tell with that screenshot. Its definitely possible the ground pad isn't soldered since I didn't assemble these myself, I will have to check on that. The board does a good job of keeping the chip cool though, my finger says it's only 50-60C. I don't have an adjustable load, but I will test with various resistances on the output with the LDO disconnected. I have doubts that I'll learn anything from that, because when I supply 3.6V at the LDO's input, the entire circuit only draws 50-60mA as previously mentioned. All the datapoints I've collected so far indicate the power is being lost in the boost regulator itself. Very strange....

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