BQ24640: Low charging current

Brandon Tomblin

Part Number: BQ24640

Tool/software:

Hello,

I am trying to use the BQ24640RVAR to charge a super capacitor rated for 9000F at 4V and 7A. The input is 12V. The super capacitor is currently at 1.8V, but the output current to the capacitor is extremely low. I will attach the schematic for the circuit I am using, based on the typical application from the datasheet.

Probing the gate for the high side transistor gives me extremely short voltage spikes with a lot of ringing afterward.

Any insights on where to begin troubleshooting?

Thanks,

6 months ago

0 Christian Moyer 6 months ago

TI__Mastermind 31976 points

Hello Brandon,

I'm looking into this question and will provide an update after I gather more information.

Best Regards,

Christian.

0 Christian Moyer 6 months ago in reply to Christian Moyer

TI__Mastermind 31976 points

Hello Brandon,

What is the voltage on ISET pin?

What is the voltage on VFB pin?

Why do you have a cap on the CE pin?

Can you provide a scope capture of IBAT,VBAT,VIN,IIN?

Best Regards,

Christian.

+1 Brandon Tomblin 6 months ago in reply to Christian Moyer

Prodigy 10 points

Thank you Christian for trying to help!

Voltage on ISET pin is 1.38V

Voltage on VFB is 1.20V

Cap on CE pin is from the typical application schematic from the datasheet:

Scope of IBAT across 1Ω shunt:

Scope of VBAT:

Scope of VIN:

Scope of IIN across 1Ω shunt:

Let me know if you need more info.

0 Christian Moyer 6 months ago in reply to Brandon Tomblin

TI__Mastermind 31976 points

Hello Bradon,

The Diode between BTST1 and REGN should be a Schottky diode. Do you see any difference if you use a Schottky diode?

Best Regards,

Christian

0 Brandon Tomblin 5 months ago in reply to Christian Moyer

Prodigy 10 points

I have somehow managed to mark this issue resolved by accident. Apologies.

The BAT54C that I have there is a Schottky diode. Not sure why that isn't reflected in my CAD software. I did try severing the bottom right leg so that I no longer had two diodes in parallel. I have also tried halving the values of R1, R2, R7, and R8 hoping to get more stable voltages on inputs and using a 6.6uH inductor in place of the 3.3 that I had, all with no appreciable effect. I have found that I get a much more stable signal from HIDRV if the input voltage is much lower (around 5-7V), but at most I have been able to charge at about 600mA, rather than the 7A I am aiming for.

0 Christian Moyer 5 months ago in reply to Brandon Tomblin

TI__Mastermind 31976 points

Hello Brandon,

What are you using as the input adapter? Is your source current limited?

Best Regards,

Christian

0 Brandon Tomblin 5 months ago in reply to Christian Moyer

Prodigy 10 points

In the beginning I was testing at ~12V from a car battery, as that is the intended use case. Further testing has been with a programmable bench power supply that can go up to 30A. Neither supply should be causing this bottleneck.

+1 Christian Moyer 5 months ago in reply to Brandon Tomblin

TI__Mastermind 31976 points

Hello Brandon,

I don't see any obvious issue with your schematic,

Can you confirm that you followed the layout guidelines:

0 Brandon Tomblin 5 months ago in reply to Christian Moyer

Prodigy 10 points

I think that a failure to follow #6 might be the issue. I have ordered new PCBs with a new layout, and will report back when they arrive and I am able to test them.

0 Brandon Tomblin 4 months ago in reply to Christian Moyer

Prodigy 10 points

Poor ground connection between input and output capacitors was in fact the issue. I rearranged my layout to be similar to the layout found in https://www.ti.com/lit/pdf/sluuab3 reference design, and my circuit is now working as intended. Thanks for walking through this with me.

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BQ24640: Low charging current