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WEBENCH® Tools/LM5018: TI WEBENCH simulating UVLO incorrectly

George Sephton
Prodigy 10 points
Part Number: LM5018

Tool/software: WEBENCH® Design Tools

Hi,

I have the following circuit open in WEBENCH: 

According to the datasheet, using RUV1 = 14K and RUV2 = 511K, my UVLO threshold should be 45.9V.

If we take the circuit above, with VIN = 48V, the voltage at Node 1 and Node 10, should be 1.28, this is higher than the 1.225 UVLO reference and so the LM5018 should be active. However if I simulate this:

Note that the voltage at Node 1 is equal to around 5V, and the voltage at Node 10 is around 1.28. I know my potential divider is correct, but for some reason, if it connects to the UVLO pin on the LM5018, it stays at 5V. I re-simulated this by setting VIN to 40V instead of 48V:

This time Node 11 is at 1.06V, so the LM5018 should not be active, but again Node 1 is at 5V and sure enough the circuit will work, even though it shouldn't.

Is this a bug in the software or is my circuit incorrect? I can't see anything in the datasheet with regards to the UVLO pin except for the potential divider circuit.

  • 0
    TI__Mastermind 25516 points
    hi,

    I didn't get 100% what you are trying to simulate (in particular the purpose of R8/R9)...

    anyway, with reference to the block diagram on page 9 and the description on page 12 here is how UVLO works:

    a) the comparator look at the voltage of the divider Ruv1 / Ruv2. when this crosses over the value of 1.225V (typ.) then the device is enabled

    b) at this point the current generator on the UVLO pin turns on, lifting up the actual UVLO pin voltage by an amount that is proportional to the 20uA sourced and the value of the resistors Ruv1 / 2

    in this way you can program the hysteresis... let me make an example:

    if you want to enable the device when VIN >= 40V and want to turn it off when VIN falls below 38V then you set the equations:

    40V x Ruv1 / ( Ruv1 + Ruv2 ) = 1.225V
    ( 38V - 1.225V ) / Ruv2 + 20uA = 1.225V / Ruv1

    that gives you an unique pair Ruv1/2 (Ruv2 = 102K and Ruv1 = 3.2K, roughly speaking)

    hope this helped a bit
    KR
    Vincenzo