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Output Ramp time and sequence for LP3891EMR-1.2

Sameer Manikfan
Prodigy 20 points

Hello,

I have a question on output voltage ramp up time and sequence for LP3891EMR-1.2. Vin = 3.3V, VBias = 5V and Vout = 1.2V. Ramp up time for 5V Bias voltage is 5us. Ramp up time for 3.3V is 3ms. (3.3V is generated from 5V using TPS54232D to achieve slow start).

What would be the ramp up time for 1.2V output? Also what would be the sequence between 3.3V input and 1.2V output from LP3891EMR-1.2. There is no specific sequence requirement, but ramp up time should be slow (min 200us) for both 3.3V and 1.2V.

Thank you.

Regards,

 

Sameer.

  • 0
    TI__Mastermind 22610 points

    I will presume that the ramp up for Vbias=5.0 (1V/us) and Vin=3.3V (3.3V/3ms = 1.1mV/us) both start at the same time (t=0).

    Once Vbias reaches the minimum operating voltage (t=5us) another 10us or so is needed for the internal circuitry to be operational. At this point we would be at t=15us, and the 3.3V input would be at 16.5mV.

    As Vin continues to rise, Vout depends on Vin - Vdo, and Vdo depends on load current.

    Guessing that you might have a load current such that the average Vdo is 200mV, then Vin needs to be at Vout+Vdo= 1.2V+0.2V= 1.4V.

    Within this operating range Vout will track behing a rising Vin due to the voltage loss to Vdo.

    With Vin rising at a 1.1mV/us rate it will take an additional 1.26ms ((1.4V-16.5mV) / (1.1mV/us)) to reach Vin=1.4V for a regulated output voltage of 1.2V. (t= 15us+1.26ms= 1.275ms))

     

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Thank you Donald.

    Regards,

    Sameer.