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LP2992 electrical characteristics question

Leo Mirkin
Prodigy 50 points
Other Parts Discussed in Thread: LP2992

Greetings,

At the bottom of page 4 of the LP2992 datasheet there is a specification for Von/off; the specification for the Low state (output off) is 0.55V typical and 0.15V maximum. The 0.15V maximum is in boldface type, which as mentioned at the top of the page applies over the full operating temperature range (-40°C to +125°C).

The DSP on the board that I'm working on, that would provide this input, can guaranty its low output to 0.4V, which satisfies the 0.55V typical requirement, but not the 0.15V maximum requirement. In general I find the 0.15V requirement rather low and difficult to meet.

Has anyone else encountered this problem? Is there a specification for a narrower temperature range? In my application the regulator would not see temperatures outside -10°C to +50°C.

Thanks in advance for your input!

Leo

  • 0
    TI__Mastermind 22610 points

    The low OFF voltage threshold requirement is not so much due to temperature as it is to a rather poorly designed on/off circuit. There is a large linear range in the Vout vs Von/off transition where the output voltage more or less tracks the on/off pin voltage. What this would mean to you is that if you cannot get the on/off pin all the way down below 150mV, the output voltage will be low, but not off. Hence all the verbage and the 'Caution' in the "ON/OFF INPUT OPERATION" section in the datasheet

    Following image shows the typical Vout (red) vs Von/off (yellow) behavior for LP2992-5.0

  • 0 in reply to Donald Jones
    Prodigy 50 points

    Thank you for your reply, Donald! It's interesting that in your plot the threshold appears much closer to the maximum specification i.e. 150mV than the typical one, 550mV. What do you make of the slew rate requirement (40mV/us) in the ON/OFF INPUT OPERATION section? Is there something in the design that requires this, or is it just a ballpark value: suggesting that if the input transition is completed in less than 100us the changing output will be less likely to cause problems in ICs downstream (which have V+ requirements of their own)?

  • 0 in reply to Leo Mirkin
    TI__Mastermind 22610 points

    I noticed the threshold seemed a tad lower than what I expected, possibly due to setup/calibration issues, but I wanted to show the general behavior of the LP2992 shutdown.

    The 40mV/us slew rate value is a ballpark value. The slew rate requirement is to prevent (minimize) the output voltage 'chattering' on-off-on-off-etc as the internal on/off threshold starts to move. What looks like low level noise on the output voltage is actually quite a bit worse than it appears in the image provided as the image has averaged data. If the shutdown voltage rises/falls fast enough the output chattering is avoided, If the output capacitor is large enough, and/or the load current is low enough, the chattering can be masked.

  • 0 in reply to Donald Jones
    Prodigy 50 points

    Ok, understood. Thanks again, Donald!