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LM5045 Latch off operation

K.Hamamoto
Guru 12155 points
Other Parts Discussed in Thread: LM5045

Hi

Our customer is considering to use LM5045 with latch off mode when the over current protection works.

Is the following configuration possible?

The resistor which is put in parallel of Cres is to prevent rising to 4V.

Regards,

Koji Hamamoto

 

  • 0
    TI__Expert 3200 points
    Koji, I will look into this question for you and get back with an answer soon.
  • 0
    TI__Intellectual 1550 points

    Hi Koji,

    By latching off mode, I assume you mean permanently stopping the converter (SS cap is fully discharged and PWMs are turned off) and the converter does not restart automatically after the hiccup timer expires. Basically, it is not "hiccup mode" any more.

    With that said, simply putting a resistor across the RES cap may not work well because:

    1. If overcurrent condition is detected, 30uA current source is activated and charges the RES cap. If overcurrent condition is not detected, 5uA current sink is activated to discharge the RES cap. Putting a resistor across RES cap would affect this charging and discharging process. As a result, the time is takes to trigger hiccup mode (Vres>1) is affected.

    2. The added resistor also affects the RES cap charge/discharge process during the off period (Vres>1V).

    I suggest instead of using a resistor, you can try to use a zener diode to clamp the RES cap voltage to 2V-4V (e.g. 3V). This way it should not affect the amount of time before triggering hiccup mode. Also, hiccup counter would not reach 8 so the converter does not restart (i.e., latched off).

    Let me know how it works.

    Regards,

    Wangxin

  • 0 in reply to Wangxin Huang
    Guru 12155 points

    Hi Wangxin-san

    Thank you very much for your reply.

    I suggested to the customer about your answer. They understood.

    On the other hand, how much are the Min/Max of the threshold voltage(Vres) and the discharge current source(Ires-dis2 at Vcs<750mV and 2V<Vres<4V) at RES pin? (min/max of the following  characteristics)

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Intellectual 1550 points

    Hi Koji,

    I am looking into this and will get back to you soon.

    Regards,

    Wangxin

  • 0 in reply to K.Hamamoto
    TI__Intellectual 1550 points
    Koji,

    Unfortunately, the min/max rating of the RES pin thresholds and current sources are not available in the internal database. However, I can provide you with an estimation based on how these circuitries are implemented in LM5045.

    RES voltage thresholds are derived from the REF (5V with +-3% tolerance), so their tolerance should be close.

    The RES pin current sources should have similar tolerance as UVLO pin current source (20uA with +-20% tolerance).

    Note that this is an estimation and the exact numbers should be verified in the experimental tests.

    Regards,
    Wangxin