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TPS706: UVLO limit?

Nate Enos
Genius 9420 points
Part Number: TPS706

Hi team,

I'm looking at the TPS706 for a battery operated application. I'm curious how it will operate at 2.7V if the output is 2.5V. I saw in the datasheet there's UVLO but it doesn't go into any detail about when it sets in. Does it trigger UVLO as soon as its within drop out and shut the output off?

Thanks,

Nate


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  • 0
    TI__Prodigy 280 points
    Hi Nate,

    The dropout voltage is related to the output current of the TPS706. What is the current for your application?
    The dropout characteristics will be similar for the entire TPS706 family. So while we do not test dropout for the 2.5V output version (due to the minimum Vin requirements of TPS706), for example at 50mA output current the typical dropout voltage would be ~300mV. Therefore, for 2.5V output voltage the minimum input voltage would be ~2.8V which is well within the recommended operating region of theTPS706 family.

    Best regards.
    Song
  • 0 in reply to Andonis Wang
    TI__Genius 9420 points
    Hi Song,

    My question was regarding what happens when operating below dropout. Let's use your 300mV example. If the current is 50mA and the dropout is 300mV, Vout is 2.5V and the input voltage is 2.7V what happens? If the UVLO kicks in, then the output will be zero (this is bad for the design). If it doesn't, then the LDO will try to regulate, but Vout will start to track Vin (this is okay, because it'll be close enough).

    I'd like to know which it is.

    Thanks,
    Nate
  • 0 in reply to Nate Enos
    TI__Mastermind 22610 points
    See "Figure 28 and Figure 29 - Power-Up and Power-Down Response" in the TPS706 datasheet.

    When Vin is below the UVLO threshold the output is off.
  • 0 in reply to Donald Jones
    TI__Prodigy 280 points
    Hi Nate,

    UVLO is below the minimum input voltage; therefore, as long as you are within the recommended operating conditions, the device won’t shut down.
    When the device operates in the dropout region Vout=Vin-Vdropout.
    Please note when the device operate at the dropout region, the PSRR would be greatly decreased.

    Best regards.
    Song
  • 0 in reply to Andonis Wang
    TI__Genius 9420 points

    Hi Song and Donald,

    Thanks for the extra info. The piece of information I was looking for was the exact threshold UVLO kicks in, which isn't stated in the datasheet. Figure 28 and 29 show a single part's UVLO (which happens to be around 2.5V), but from what I understand the UVLO is guaranteed to be under 2.7V (the lowest recommended input), but it could be lower (2.5V or 2.3V for example) due to process variation.

    This was important for this design because the voltage could momentarily drop below 2.7V depending on battery life and temperature. If UVLO kicked in the whole system would reset, which would be an issue. If UVLO was lower, let's say 2.5V then it's no problem, we would just be in dropout and the system is fine. 

    Thanks for the help! That's everything I was trying to figure out.

    Nate