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I'm using a L6902 (https://www.st.com/resource/en/datasheet/l6902.pdf) to charge a 4 cell lifepo4 pack (with a solid BMS).

Do I need a diode on the converter output (PACK+) to prevent the converter from being damaged when the ac adapter is disconnected, but the battery pack is connected?

Are there any other techniques besides a diode that I could use?

Here's my schematic:

kicad schematic

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  • \$\begingroup\$ After thinking about this more, could it be as simple as adding a pulldown resistor on VIN/VCC? \$\endgroup\$
    – alexlroemer
    Commented May 23 at 16:10
  • \$\begingroup\$ You have a DC path from Pack+ through your current sense resistor, the inductor, and the body diode of the top side switch, so it will power Vcc. I don't think that will damage anything, but it might not be desirable, and your pulldown won't fix the problem. Since it's not synchronous it can't boost back if the pack voltage is higher than the set voltage, but you might want see what the manufacturer says. \$\endgroup\$
    – John D
    Commented May 23 at 16:22
  • \$\begingroup\$ Do you know that disconnecting input is a problem at all? What do the abs. max. ratings say? Block diagram? \$\endgroup\$
    – Tim Williams
    Commented May 23 at 16:29
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    \$\begingroup\$ Also I hope this isn't intended to charge a lithium battery of any sort, (hooked directly to the pack) since it's not a proper charger and could easily cause issues by not using the correct charge algorithms and techniques. \$\endgroup\$
    – John D
    Commented May 23 at 16:31
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    \$\begingroup\$ @alexlroemer One example is if the battery is ever over-discharged. You can get (short circuit-like) dendrite growth that a charger IC will detect and go into "recovery" mode where the cells are charged at a very low rate until they reach a certain voltage where normal charging resumes. How and when do you terminate charging? You can't leave lithium cells on a "float". Other things a proper charger can do is modify the charge profile based on temperature. There's no standard definition of a "BMS" so I don't know what yours is capable of. \$\endgroup\$
    – John D
    Commented May 23 at 18:47

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Usually there would be two problems. Over current and over voltage.

It's unlikely that over current would be an issue as the DC/DC converter should be current limited and it already has a current sense resistor which would prevent large amounts of current transferring from out to PACK+.

The over voltage condition is a concern, it would be better to test and see if you see overshoot while disconnecting a battery on PACK+ with an oscilloscope. Another problem is if the battery is a higher voltage than the out pin, that would be bad, you need to ensure by design that the battery voltage never exceeds the max voltage of OUT.

If you do see a small amount of overshoot, then add a protection diode.

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  • \$\begingroup\$ Thanks, I will have to characterize the disconnect event. I was more worried about how the L6902 will handle the steady state where it's VCC is floating, but the OUT pin is seeing pack voltage. \$\endgroup\$
    – alexlroemer
    Commented May 23 at 17:59
  • \$\begingroup\$ If you like the answer, please use the voting system \$\endgroup\$
    – Voltage Spike ♦
    Commented May 23 at 20:22

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