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LM5156EVM-BST: UVLO problem

Fred Cheng
Expert 4700 points
Part Number: LM5156EVM-BST
Other Parts Discussed in Thread: LM5156

Hi team,

this is UVLO spec in LM5156 EVM

I try to replicate it with calculator but..

but the result show totally different value. (25.886K not 80.6K)

why?

  • 0
    TI__Genius 14155 points

    Hello Fred,

    Thanks for reaching out to us again.

    I think there is some fundamental misconception what the range of the "programmable" hysteresis of the UVLO pin could be.
    The UVLO mechanism is meant to turn off the converter in case of an under-voltage at the input.
    When the supply is weak it could happen that the input voltage rises when the converter gets turned off and so it would get turned back on again.
    Therefore the hysteresis got introduced.   It is usually set to 0.4V .. 1V.

    This first table that you show does not tell that the 5V are determined by the UVLO mechanism.
    The UVLO level of 2.5V will result in the 60K / 80K resistors that are shown there.
    When you enter 2.6V and 2.2V in the calculator you will get similar resistor values.

    The calculator is trying to give you some freedom with the choice of the resistors and the resulting voltage level and hysteresis.

    The hysteresis is accomplished by feeding an extra current of 5 µA through the bottom resistor, which will increase the voltage that is seen on the UVLO pin.
    As this current is so low, you would need pretty big divider resistors to achieve a high hysteresis.
    Resistors in the range of hundreds of kiloohms may work, but might also pick up noise which will be counterproductive.

    Best regards,
    Harry

  • 0 in reply to Harry Hofner
    TI__Expert 4700 points

    Hi Harry,

    then what does the 5V in my first picture (EVM spec) mean if I got confused?

    I thought that was UVLO threshold?

  • 0 in reply to Fred Cheng
    TI__Genius 14155 points

    Hello Fred,

    I don't know where you got that picture from.
    Please clarify.

    Thanks,
    Harry

  • 0 in reply to Harry Hofner
    TI__Expert 4700 points

    Hi Harry,

    from LM5156EVM user guide, that's also the topic of this forum.. you can go check 

    so you're saying the real run mode threshold is 2.6V not 5V?

  • 0 in reply to Fred Cheng
    TI__Genius 14155 points

    Hello Fred,

    The numbers you refer to describe the EVM, not the part.

    The datasheet of the LM5156 states:
    – Operating range3.5V to 60V
    – 2.97V to 16V when BIAS = VCC
    – Minimum boost supply voltage 1.5V when BIAS ≥ 3.5V

    The numbers for the EVM differ, because customers could jumper the BIAS supply using the diode, there may be tolerances, temperature dependency, etc.
    To guarantee that every EVM will fulfill the rating under all conditions, these numbers are somewhat rounded.
    Instead of writing 3.5V + diode drop + margins, the board is rated for 5V startup voltage, which will always work.
    These 5V do not refer to the UVLO feature (see figure 11 Start-Up, VIN = 4V, IOUT = 3 A), which could not work if the UVLO would limit it to 5V.

    Best regards,
    Harry