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LM5156H-Q1: CUVLO and RUVLOS choices

Josh Wei
Prodigy 291 points
Part Number: LM5156H-Q1
Other Parts Discussed in Thread: LM5156

Hi expert!

I want to know if I add 330nF CUVLO in parallel with RUVLOB. Should I add RUVLOS? I don't know if this capacitor is too large...Or what is your recommendation for this capacitor? Vsupply is 12V battery.

 

  1. RUVLOT=14.7K, RUVLOB=6.81K, Vsupply= 6~16V
  2. RUVLOT=14.7K, RUVLOB=11.3K, Vsupply= 6~16V

Can you help with this two combo if there are difference of RUVLOS?

besides, where can I find/calculate total power loss of LM5156H-Q1 as a flyback controller?

Many thanks!

  • 0
    TI__Mastermind 19650 points

    Hi Josh,

    Thanks for using the e2e forum.
    Both Cuvlo and Ruvlos are optional. The device has an internal hysteresis to avoid jittering between ENABLED and DISABLED state when the voltage is close to the UVLO threshold.

    Cuvlo can still be added to reduce noise and voltage spikes at the pin.
    Our LM5156 EVM design uses a Cuvlo of 220pF.
    https://www.ti.com/tool/LM5156HEVM-FLY
    I would recommend a cap in a similar range.

    Regarding power loss, the quickstart calculator tool comes with loss calculator.
    https://www.ti.com/tool/download/SNVC240
    There are additional losses like losses in the transformer or ESR of the caps, which has to be accounted for manually.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Prodigy 291 points

    Hi Niklas,

    Thanks for your illustration. Just wanted to add some info you may interested.

    In order to improve the performance of the start up just as you mentioned( voltage is close to UVLO threshold) , 330nF is already paralleled. Do you think 220pF is enough to withstand the start up and load transient impact on UVLO/SYNC pin? In my opinion, 330nF Cuvlo  is needed with Ruvlos but 220pF is optional. 

    Thanks!

  • +1 in reply to Josh Wei
    TI__Mastermind 19650 points

    Hi Josh,

    Thanks for the additional comments.

    There are no minimum requirements for Ruvlos or Cuvlo given.
    I would consider choice of these two components "fine tuning" of the turn-on/turn-off behavior of the LM5156 device.

    The main goal is to avoid that the device shuts down unintentionally due to VIN drop for a short time period. This is what Cuvlo is for.
    If larger Cuvlo is chosen, the delay between UVLO voltage rising above the enable threshold and the device actually turning on becomes longer, as the Cuvlo cap needs to be charged. This delay can be reduce with Ruvlos.

    Based on the resistor divider option you forwarded in the initial comment, both dividers should leave some margin to the UVLO trigger and the minimum VIN given. Therefore, the risk of accidental UVLO shutdown during operation is small.

    Best regards,
    Niklas