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LM5155-Q1: Could UVLO/SYNC connect MCU IO control?

Yuxi Liu(Tony)
Intellectual 1360 points
Part Number: LM5155-Q1
Other Parts Discussed in Thread: LM5155

Hi TIer

As shown in the figure below, the LM5155 circuit that we used previously was automatically turned on at power up. We now want to use the microcontroller to master the power supply on and off after power up. is it possible to pull down the 12-pin UVLO/sync by default and control it by serial-resistive IO to 3.3V or 1.8V? Looking at the specifications and design guidelines, it seems like they don't have a case statement like this, does this cause slow start each time you turn on? Also, looking at the timing diagram in the specification book, can the UVLO/sync pull low time be less than 35us, and the power output cannot be turned off?

  • 0
    TI__Mastermind 19620 points

    Hi Tony,

    Thanks for using the e2e forum.

    The UVLO pin of LM5155 can be used for disabling and enabling the device.
    When pulling UVLO low, the device will go into shutdown mode and no switching will happen.
    If UVLO is pulled high above the enable threshold, the device will start up with softstart again.

    If the turn-off and turn-on happens quickly, the voltage at SS will not fully discharge, so the softstart time is greatly reduced.

    For designing the external disable/enable circuit, I highly recommend to implement a hysteresis of e.g. 100mV. This will avoid chattering between the two states.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Intellectual 1360 points

    Thank you , Could you help to explain why is hysteresis circuit setting here? 

  • 0 in reply to Yuxi Liu(Tony)
    TI__Mastermind 19620 points

    Hi Tony,

    The device already has an internal hysteresis implemented.
    This is to prevent jumping between active and shutdown stage during turn-off and turn-on.

    My comment regarding additional, external hysteresis was meant for the case that their microcontroller turn on/off the device with high frequency.
    We recently had a customer that injected an UVLO signal that pulled the pin high and low several times within a very short timespan. This lead to some irregular switching cycles (but without any risk for the device).

    If this is not the case for your microcontroller, there should be no issues.

    Best regards,
    Niklas