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LM78L: voltage at power on transient

david sun1
Intellectual 260 points
Part Number: LM78L

Dear Support and Expert, 

I have a design which has an intermittent problem which may relate to the LM78L05 , as shown in the waveform, the dark blue is 12V input, the pink is 5V output, 

the observation is that after the input voltage over ~3.5V, the output of the LM78l05 starts to output ~2.4V until the input voltage is over 7V, the output starts to regulate to 5V. 

I couldn't find a chart of the power-on transient on the datasheet, is this startup waveform make sense?

Best Regards,

David Sun

power-on transient process

  • 0
    TI__Mastermind 37965 points

    Hi David,

    What you are seeing here is the LDO starting up in dropout. Once the device begins turning on, the output ramps up much more quickly than the input is ramping, so once it catches up to the input (minus the dropout voltage), the output tracks the input until its target value is reached. 

    What is the load current for this test? Figure 3 shows that for Iout = 70mA at room temperature the dropout voltage is about 1.8V, which looks pretty close to the dropout voltage you are seeing.

    A solution to this problem is to ramp the input up faster. Is that possible in your application? 

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 260 points

    Hi Nick, 

    Thank you for your prompt response, It is not a big issue for the design, I just want to confirm it is the characteristic of the Chip. 

    have a great day. 

    David Sun