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LM46002-Q1: Behavior if input voltage drops below output voltage

Alexander Bichert
Prodigy 30 points
Part Number: LM46002-Q1

Hi community,

is there any information about the behavior of the LM46002-Q1 if the input voltage falls below the desired output voltage. This happens for example during cranking of an engine. In this case the input would be ca. 3.8 V and the output is set to 4.5 V. The enable pin is tied to the input voltage. The output current is approximately 1.5 A.

My LTSpice Simulation shows that the output voltage decreases to zero and starts ramping up the voltage again after ca. 8 ms. Then this sequence repeats....

  • 0
    TI__Genius 11085 points
    I am not sure why the simulation shows Vout will be turned off after Vin drops to about 4.0V. Please check with the simulation model support team.

    For the behavior of the part, I believe it will continue operation in dropout mode when Vin is below Vout set point. The dropout voltage (Vin-Vout) depends on the load level and the frequency setting and the losses in the design. If the load current is not over the rated level, the part should not hiccup. And if VCC is not below UVLO level, Vout shouldn't be turned off by the UVLO either.

    At the transition when Vin drops suddenly, there might be a moment that the output voltage back feed the input side via the high side FET parasitic diode. After Vout is discharged to below (Vin+0.7V), the diode shouldn't turn on any more. Please refer to the dropout curves on the datasheet to estimate the dropout voltage under such conditions.

    -Yang
  • 0 in reply to YangZhang
    Prodigy 30 points

    Hello Yang,

    thank you for your fast response! I found an interesting statement in the datasheet. Is my interpretation correct, that if the output voltage drops to a value below the power good condition, this hiccup behavior starts?

    How can I prevent it without lowering the nominal output voltage?

  • 0 in reply to Alexander Bichert
    TI__Genius 11085 points
    Hiccup will only happen when LS current limit has been tripped for 32 consecutive cycles AND PGood = low. When LS current limit is tripped, the DC current will be LS current limit level + 1/2 current ripple. It will be higher than 2A. If your load current is 1.5A, it should not trip hiccup behavior even if Pgood is low.

    In simulation, could you also plot the inductor current and load current waveforms to see the LS limit has been tripped or not. The other thing to monitor is the VCC voltage. If VCC voltage drops with the VIN, it could trip VCC UVLO and it could turn off the output voltage as well.

    Let me know your findings.

    -Yang
  • 0 in reply to YangZhang
    Prodigy 30 points

    Hi Yang,

    thank you for your answer. The current has always been below the LS limit. Meanwhile we had to reduce the output voltage of the regulator due to other reasons, which also solves this problem.


    Thanks four your support!

    -Alex