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LM22672 Minimum VIN

Richard Evans
Prodigy 20 points
Other Parts Discussed in Thread: LM22672

It isn't clear from the documentation how much higher VIN needs to be over VOUT. I can only make inference from the duty-cycle calculation.

My application is VIN = 2x LION batteries in series, sum voltage of 8.4V Max down to 6.0V, VOUT = 5.0V with load of 200mA.

Ran a battery life test with static resistive load on +5V output at 100mA.  Over 24 hour period, the pair of batteries discharged from 8.29V to 7.0V at which time one of the batteries failed due to overcurrent.  The surviving battery was still good at 3.5V.  This has happened twice.

How can I determine at what point the LM22672 will enter what the datasheet under "Precision Enable and UVLO" calls "excessive battery discharge...or abnormal device operation in applications where the input voltage falls to low"?

  • 0
    TI__Guru 57555 points

     

    This device has a minimum operating input voltage of 4.5V.

    An input UVLO threshold of about 4.2V rising and 3.9V falling is also built in to the regulator.  The device will not turn-on until the input voltage reaches 4.2V and will then turn-off when the input falls to below 3.9V.  If you need the device to respond to different input voltage levels, then use the method shown on page 8.  This way you can set an input voltage threshold that will shut off the regulator before your batteries are discharged too far.

    Of course, as with any DC/DC the input current will increase as the input voltage falls, until the regulator drops out of regulation.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 20 points

    Thanks for your reply.

    While the device needs 4.5V to operate, I don't believe that is enough 'headroom' for a 5.0V output.  

    I am trying to find out how much VIN must be above VOUT for reliable, efficient operation.

    RWE

  • 0 in reply to Richard Evans
    TI__Guru 57555 points

    Equation 4 will give you an estimate of the minimum input needed.  For more accuracy you can include the battery resistance.

    FD