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LM5171: LM5171 - Dynamic current adjustments via DAC

Lukas Obkircher
Prodigy 50 points
Part Number: LM5171
Other Parts Discussed in Thread: PMP15038

Tool/software:

Hi,
the goal is to use the LM5171 to control the charging/discharging current and cutoff-voltage of a connected LV-battery via a DAC.
For example: target LV-Voltage is set to 13.8V, but at the same time the output current limit should be dynamically adjusted from for example 100mA to 30A via a DAC.
(digital potentiometers should not be used in this project)

Output voltage at LV: this seems to be quite easy, by controlling the VSET of the outer loop via the DAC
Output current at LV: this looks more complicated to me, because we still would like to use the outer loop error amplifier to prevent overvoltages/overcharging,
but compared to the reference designs, we would like to dynamically adjust the limit of the output current.
The eval-board is using a fixed resistor divider which from my understanding is just providing a fixed current limit
and the alternative circuit of the eval-board (by jumper-setting) is ignoring the outer voltage loop and let the user handle all kind of voltage controls.

Is there an easy way to implement this adjustable current limit via a DAC, while keeping the outer voltage control loop?
Thanks

  • 0
    TI__Genius 13545 points

    Hi Lukas,

    You may limit the current by limiting ISET voltage.

    You may refer to PMP15038 about how to build high precision CC/CV loop.

    If you don't need very high accuracy, you may use the circuit below in the LM5171EVM user's guide.

    Apply a voltage to J43, then ISETA will be clamped to this voltage, that means the current will also be clamped.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 50 points

    Hi Feng,
    thanks for your input. The PMP15038 looks pretty nice, but for now I would like to stick to the things which the LM5171EVM has to offer.
    I modified the EVM such as I can control the output voltage via VSET, which works fine, but now when I apply a voltage to J43, that voltage now becomes the main driving factor of the current loop and therefore also of the output voltage, which means that the output voltage gets higher than my desired voltage which was set via VSET. I tried it with connected J36 (1-2) and with J36 open. What am I missing there?

  • 0 in reply to Lukas Obkircher
    TI__Genius 13545 points

    Hi Lukas,

    Can you check ISETA voltage and see if it is clamped by the voltage to J43?

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    Prodigy 50 points

    I got it working, many thanks ! I just realized that J42 was not shorted...quite silly mistake, sorry for that :/
    The only thing which leaves some questionmarks on my side is a hysteresis for the current control.
    When I apply 1.015V on J43 no current flows. If I increase it to 1.020V the LV-current suddenly jumps. After overcoming that threshold I can also control the current between 1.000V and 1.020V. But after going below 1.000V I have to overcome the threshold of ~1.020V again. Where is this threshold coming from?

  • 0 in reply to Lukas Obkircher
    TI__Genius 13545 points

    Hi Lukas,

    Can you attach a waveform including the ISET, IL, COMP?

    I suppose the current at 1.02V is in burst mode, right?

    Best Regards,

    Feng