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LM5157EVM-SEPIC: Bias Pin Connection to Load Challenge

Raymond McIntyre
Prodigy 80 points
Part Number: LM5157EVM-SEPIC
Other Parts Discussed in Thread: LM5157

I am evaluating the LM5157 SEPIC.  I would like to run the circuit below the 4V, so I am trying to move the bias pin from the input to the load.  I am doing this by removing the J7 pin 2-3 jumper, then adding a J6 pin2-3 jumper.  When I do this the circuit will not turn on.  I am doing this following the datasheet figure 8-5.  Why does the eval board not run in this config?

  • 0
    Prodigy 80 points

    Realizing I am using a SEPIC circuit (input- output capacitior coupled), I also tried connecting bias to both VIN and LOAD each through its own diode by placing J6 jumper between pins 1-2 and a J5 jumper through pins 1-2, and still no luck.  My mention above to the figure 8-5 is in the part datasheet not the eval datasheet.

  • 0 in reply to Raymond McIntyre
    TI__Genius 16515 points

    Hello Raymond,

    Thanks for reaching out to us via e2e.

    The SEPIC EVM cannot start-up like that.
    As this is a SEPIC topology, there is no DC path from the input to the output.
    VOUT is 0V until the controller starts switching, so - if you only connect J6 pins 2 and 3 - then BIAS is also 0V and the controller has no supply.

    To be able to start-up the controller, please connect BIAS via both Diodes: J6 pins 1 and 2 and J7 pins 1 and 2.


    Also, on this EVM, R5 and R8 are defining an UVLO voltage of about 3.8V.
    Below this input voltage the controller is disabled.
    So you will need to modify these resistors (or simply remove R8 if you don't use any UVLO).


    Please keep in mind that the LM5157 will need a BIAS supply voltage of ≥ 2.9 V to be able to operate.
    Like stated below Figure 8-5: Decrease the Minimum Operating Voltage After Start-Up

    Therefore, to initially start the converter, you will need an input voltage of 2.9V plus diode drop (D3), which is around 3.6V initially.
    Once it starts boosting, it will generate its own BIAS supply on the output side and from then on VIN can go down to about 1.5V.

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    Prodigy 80 points

    Hello Harry:

    Tried your suggestion powering up at 12V with J6 1-2 and J7 1-2 jumpered.  Low voltage performance not improved.  Circuit shuts down at 3.6V.  This is the same as when bias was solely connected to input.

    Vbias is initially at 11.7V while running with jumpers as you suggested.  AS Vin is reduced to 3.5V, Vbias drops to 3.2V and circuit shuts off.

    Any suggestions?

  • +1 in reply to Raymond McIntyre
    TI__Genius 16515 points

    Hello Raymond,

    As stated earlier, setting some jumpers is not enough.
    You must remove R8, otherwise the UVLO will shut down the controller somewhere below 3.8V .. 3.6V.

    If BIAS does not remain at 11.xV, even if both diodes are connected, there is something broken on your board.
    As a test, remove the Jumper J6 and measure pin 1 after start-up. When you decrease the input voltage, pin 1 must remain at VOUT minus voltage drop of D2.
    Maybe D2 is broken or the jumper is not soldered properly ?

    Best regards,
    Harry