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WEBENCH® Tools/LM5170EVM-BIDIR: LM5170 CC mode control in boost mode

user5339996
Prodigy 80 points
Part Number: LM5170EVM-BIDIR
Other Parts Discussed in Thread: LM5170, INA281, TLV431

Tool/software: WEBENCH® Design Tools

Dear 

We have already designed Bidirectional buck-boost converter with LM5170.

Because the current sense in LM5170 is in low voltage sense, we have no idea how to design the CC mode in  boost mode.

Do you have design tools about CC mode in boost mode?

We have to use these conditions in boost mode :

Low voltage(input) : 9~16V

High voltage(output): 58.5V

CC mode in boost mode = 50W / 58.5V  = 0.855A

Current sense = 3mOhm

Thank you

  • 0
    TI__Guru 68675 points

    Hi,

    Thank you for designing with the LM5170.  The CC mode for boost operation will need a patch circuit because the current sense is on the low voltage rail side (which means the boost input). How accurate do you want the CC to be?

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 80 points

    Dear Youhao

    Output current accuracy is about 2%.

    Due to efficiency and input current control, the accuracy may not be accurate.

    It's fine.

    Our input voltage is 9V to 16V

    If you need more conditions, please let me know.

    Thank you.

  • 0 in reply to user5339996
    TI__Guru 68675 points

    Hi,

    Since your accuracy needs to be 2%, you may have to employ an INA part to directly sense and report the output current.  The LM5170's IOUT signal is for the input current, and translating it to the output may not meet your 2% accuracy requirement.  You may consider INA281 or something similar.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 80 points

    Dear Youhao

    Because we have already used LM5170 to design the circuit and tune buck mode, we just want to revise the layout.

    Our main controlling element is CV mode, CC mode on boost mode is auxiliary.

    Our boost mode control circuit is as shown in appendix.

    Please support us about the patch circuit.

    Thank you.

  • 0 in reply to user5339996
    TI__Guru 68675 points

    The attachment is corrupted and I cannot open it.  Do you still targeting 2% accuracy?

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 80 points

    Dear Youhao

    After discussion, We don't target 2% accuracy.

    The schematic is uploaded again.

    or you can use the website here:

    https://ppt.cc/fWtuPx

    thank you.

  • 0 in reply to user5339996
    TI__Guru 68675 points

    It seems to be a proposal that I recommended to some customers for the buck output, or boost input, CC limit.  It is an OPEN LOOP approach, relying on LM5170's accurate current regulation to ISETA setting. 

    For boost output, you will have to use an CLOSED LOOP approach.   See below concept.  It brings in a factor of (1-D) to the original IOUT signal (input current) to produce the boost output current:   Io = Iin x (1-D).  Adjusting the resistor values can adjust the CC level. LM5170 Boost CC Control 10A.pdf

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 80 points

    Dear Youhao

    I have some question about the schematic.

    1. Could you tell me how the TLV431 works while boost mode working ?

    2. How do I design the resistor on 20k and 12.4k Ohm? Do you have some design tools about it?

    3. Does the circuit effect on buck mode?

    Thank you

  • 0 in reply to user5339996
    TI__Guru 68675 points

    Hi,

    1. The TLV431 sets a threshold which can be programmed by the resistor divider at the ref pin.  It compares with the chopped IOUT signal.  Note that, you chopped the IOUT signal by LO, and IOUT is basically the inductor current, and LO has the duty cycle of the boost converter. Hence the current can pass the diode is  IOUT x (1-D).  The resulted current is filtered by the RC to get a dc signal proportional to Io, namely, proportional to  I(IN) x (1-D) = Io.  Ratio the resistor divider at the TLV431 ref pin will set your CC current limit threshold.

    2. You need to ratio the two resistors such that at your CC threshold, the voltage built up at the TLV431 is 1.245V.

    Basically,  [ (Io_CC / N) x Rcs x 50 x (250uA/50mV) + 25uA] x Rb / (Rt + Rb) = 1.245V

    Io_CC is your CC threshold

    N: you number of phases,

    Rcs: the current sense resistor

    Rt: the top side resistor at TLV431 ref pin

    Rb: the bottom side resistor at the TLV431 ref pin

    3. You should disable the FET at the IOUT pin for buck mode.  

    Thanks,

    Youhao