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LM2941: implement ON/FF pin on LM2941

wayne zhou1
Prodigy 50 points
Part Number: LM2941
Other Parts Discussed in Thread: TPS3700, TPS3710

I would like to use LM2941 LDO regulator. there is ON/OFF pin, normally people just tied it to ground to enable it. but we need to use it to monitor the output voltage, suppose we would like to shut the power when output voltage drop to 3 V from 5 V.

ON/OFF pin is TTL/CMOS compatible, which is

when <0.8V, ON

when >2V, OFF

normally, there is a digital power monitor chip which tap the output of the LDO and digitize it, send to microcontroller by I2C/SPI/PMbus, microcontroller send back TTL/CMOS signal to ON/OFF based on the data from I2C bus

we only have one battery of 9V on the board as input power, after LDO regulator, convert to 5V output, could you tell us how to create a signal to the ON/OFF pin when output drops from 5V to 3V?

notice that LM2941 is an old chip, do you have alternative new one?

  • 0
    TI__Guru 55940 points

    Hi Wayne,

    While you can certainly use a voltage detector or supervisor to monitor the output voltage, it is important to keep in mind that you will not be able to directly drive the ON/OFF pin of the regulator based on the output of the same regulator.  If you attempt this, your regulator will never turn on as the output will initially be below 3 V.   

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 50 points

    Hi Ryan,

    Thanks for quick reply. I understand the dilemma. do you have a solution or application note for it? 

    V/R,

    Wayne

  • 0 in reply to wayne zhou1
    TI__Guru 55940 points

    Hi Wayne,

    You may be able to find a better solution for this type of question on a logic or supervisor forum as the question is more of how to drive the LDO enable pin rather than a function of the LDO.  That being said a voltage detector such as TPS3700 may be useful for your application if you tie both outputs of the detector together so that you only see a logic high in the region where you want the regulator to turn off.  Again the difficulty is getting the regulator to turn on the first time due, you effectively need to find a logic device that would allow you to ignore the first startup.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 50 points

    Thanks Ryan. vey appreciate your help.

    V/R,

    Wayne

  • 0 in reply to wayne zhou1
    Prodigy 50 points

    Hi Ryan,

    thanks for your previous help. you recommendation of TPS3700 will do the job. this time, we would like to know if you have a chip can monitor a power with two under voltage, for example,

    if the power supply is 9V,

    we would like to monitor when it drop to 7V

    also would like to monitor further drop to 6V

    do you have a chip to this dual monitoring?

    you can open a new ticket for this one.

    Very appreciate your help.

    Wayne

  • 0 in reply to wayne zhou1
    TI__Genius 14245 points

    Hey Wayne,

    The challenge is finding a supervisor that can monitor two supplies & operate from a 9V supply but I'm going to suggest two options. 

    1. TPS3780: dual supervisor, however max VDD is 6.5V so you would need a resistor divider from 9V to GND which would be used to power the VDD of the supervisor. As usual with adjustable threshold supervisors you would use two more resistor dividers to set the thresholds for the supplies being monitored by SENSE1 and SENSE2. 
    2. TPS3710: This is a single supervisor so you would need two of them but the max VDD is 18V so you wouldn't need to divide down your input supply.