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LM5171-Q1: Enabling FBLV Monitoring for Discharge on LM5171 Evaluation Board

António Maria Oliveira
Prodigy 20 points
Part Number: LM5171-Q1
Other Parts Discussed in Thread: LM5171

Tool/software:

Hi,

I intend to use the LM5171 to control both the charging and discharging of a Li-Ion cell and am currently testing the chip's capabilities with the evaluation board. To control the output voltage via the VSET pin, I removed R46 and connected an external voltage source to J37, which allows me to set VSET. For current control, I'm applying a voltage to J43 while keeping J42 shorted and J36 connected between positions 1 and 2.

In the charging mode, I can successfully implement constant current/constant voltage (CCCV) regulation, as the chip monitors the FBLV pin. However, during discharge, I'm unable to maintain CCCV control. When I set ISET to below 1V and the current starts flowing in the opposite direction, the chip begins monitoring the FBHV voltage instead. My goal is to have the chip monitor the FBLV voltage in both charging and discharging modes.

Could you advise on what modifications are necessary to the evaluation board to enable FBLV monitoring during discharge?

Thank you in advance for your assistance.

  • 0
    TI__Mastermind 22790 points

    Hi Antonio,

    Good to know that CCCV works well in charging mode.

    Did you configure DIR to low level in boost mode? The FBHV amplifier will be selected in this case.

    Please select FPWM to allow reverse current. You do not need to change anything else compared to charging mode.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 20 points

    Hi Feng,

    Thank you for the quick response. The FPWM mode has already been selected. According to the User's Guide, in FPWM mode, the direction can be automatically flipped by changing the ISET voltage, so J28 was left connected in positions 1 and 2. Should the jumper be removed, and the direction controlled solely by the ISET voltage? Will the FBLV feedback be used in both the charge and discharge directions?

    Best regards,
    Antonio

  • 0 in reply to António Maria Oliveira
    TI__Mastermind 22790 points

    Hi Antonio,

    J28 should connect 1,2 (buck mode) and only change ISET to change the direction of the current.

    A similar waveform is shown in Fig 4-14 ISET Tracking in the EVM user's guide.

    When DIR=buck mode, FBLV is always selected.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 20 points

    Hi Feng,

    Unfortunately, I'm still encountering the same issue during discharge. VSET seems to be ignored, and the DC-DC converter continues supplying a constant current until the cell is fully discharged. DIR was kept in buck mode. Do you have any other suggestions?

    Best regards,
    Antonio

  • 0 in reply to António Maria Oliveira
    TI__Mastermind 22790 points

    Hi Antonio,

    Can you share some waveforms like DIR, ISET, VSET, inductor current?

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 20 points

    Hi Feng,

    I don’t have the waveforms at the moment, but the DIR pin is consistently at 5V, and both VSET and ISET are set by an external power source. From my testing, it appears that FBLV only regulates upwards, which explains why the CV mode works during charging but not discharging. While ISET works in both directions, VSET only limits the current during charging when VLV reaches VSET. Is there a way to configure FBLV to also regulate downwards, so it can limit discharge current when the LV side reaches VSET?

  • 0 in reply to António Maria Oliveira
    TI__Mastermind 22790 points

    Hi Antonio,

    I don't see any reason why the loop is not working when discharging.

    Please note J43 is a positive voltage clamp.

    Best Regards,

    Feng