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LM5177: Part choice for bidirectional single cell testing circuit

Lukas Obkircher
Prodigy 70 points
Part Number: LM5177

Tool/software:

I would like to build a bidirectional battery testing circuit and therefore I have been looking into the LM5177.
My desired battery voltage goes from 4.5V down to 1.5V, but the LM5177 just supports output voltage down to 3.3V.
When I take a look at the TI-Webpage with the parametric search and set the required feature of "bidirectional",
it outputs only a single part which is designed for use-cases where the Vout is higher than Vin and not even shows the LM5177.
Therefore I assume that there are more parts available, but at least not findable via the parametric search?!

My setup:
Input: bidirectional 24V power supply for increased efficiency (can be assumed like a really really big 24V battery)
Output: single cell battery (1.5V up to 4.5V)
Output current: ±30A bidirectional (charging & discharging)
Can someone recommend me a good alternative to this part?


Many thanks in advance Slight smile

  • 0
    TI__Guru 58661 points

    Hi Lukas,

    with that low battery voltage you will have a boost ratio of ~ 16   =   24V / 1.5V   this is very large and will be above what most devices can support.

    It would be much easier and efficient to use a 2S battery with gives you an min voltage of 3V or even a 3S with 4.5V

    Where is the 30A current mapped to?

    If this is on the application side this would be a current of 30A * 16 = 480A  out of the battery with 1.5V -> 24V

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 70 points

    Hi Stefan,
    the ±30A are mapped to the battery (1.5V to 4.5V), therefore the 24V side will be around ±6A (4.5V * 30A / 24V).
    Due to the test requirements the cells must be tested indivudually. The only thing I could change is the lower the input voltage to 12V, which would enhance the ratio to 8, but I cannot go below that. Can you recommend me a part which would be suitable for this application?
    Thanks in advance,
    Lukas

  • 0 in reply to Lukas Obkircher
    TI__Guru 58661 points

    Hi Lukas,

    the LM51772 might be fitting to this requirements when operation in Battery backup mode.

    I would recommend to have the input supply to 12V so that the boost ratio is only 8 with lowest battery.

    Also ensure the device gets the bias supply from the 12V so that it has enough operation supply to operate and drive the gate signals.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 70 points

    Hi Stefan,
    after some time I looked into it again and I still don't understand where the 3.3V limitation of the LM5177x is coming from.
    If I understand the datasheet the right way, your suggestion with battery backup-mode would mean I connect the battery to VIN, but than this leads to issues due to the UVLO on VIN, correct?
    For VOUT I coudn't find such a limitation, but why is VOUT limited to min. 3.3V on this part?

  • 0 in reply to Lukas Obkircher
    TI__Guru 58661 points

    Hi Lukas,

    UVLO can also be connected to the output, e.g. with some external logic you can also stop discharge at a certain level if discharging -> pull UVLO high if standard supply is available. (mIght be just an additional Diode and Resistor)

    We have limited the output min to 3.3 V as the performance (output voltage ripple) will get higher on that low voltage.

    To be able to charge an Capacitor for backup without violating the spec we have not added this limit there.

    Best regards,

     Stefan