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LP3985: Minimum Vin

Charles Cammarata45
Intellectual 270 points
Part Number: LP3985

Section 7 of the data sheet states:

7.4 Device Functional Modes

7.4.1 Operation with VOUT(TARGET) + 0.3 V ≤ VIN ≤ 6 V The device operates if the input voltage is equal to, or exceeds, VOUT(TARGET) + 0.3 V. At input voltages below the minimum VIN requirement, the devices does not operate correctly, and output voltage may not reach target value.

7.4.2 Operation Using the EN Pin If the voltage on the EN pin is less than 0.4 V, the device is disabled, and in this state shutdown current does not exceed 1.5 μA. Raising VEN above 1.4 V initiates the start-up sequence of the device.

This is a bit confusing. Does this mean if the EN pin is used and is above 1.4V, then the the Vin requirement is not applicable? Are they two separate enable modes? It is not clear.

Thanks!

  • 0
    Intellectual 270 points

    Additionally, the performance characteristic curves on page 10 (for example) show characteristics with Vin=Vout+0.2V. This would seem to contradict the 0.3V Vin requirement.

  • 0
    TI__Mastermind 18466 points

    Hi Charles, 

    The 0.4V voltage is the maximum guaranteed value that the enable will read as an "off" value - regardless of the input. 

    The 1.4V voltage is the minimum guaranteed value that the enable will read as an "on" value - regardless of the input.

    it is recommended to keep the enable voltage at a value at or below enable.

    Vin, however, is set to be 2.5V or VOUT(nom) + the rated dropout voltage for the load current. 

    A nominal value of Vin used in our data sheets is Vin = Vout+0.3V. But a functional Vin can be lower than that. 

    Best,

    Juliette

  • 0 in reply to Juliette
    Prodigy 130 points

    Hi Juliette

    So the statement:

    7.4.1 Operation with VOUT(TARGET) + 0.3 V ≤ VIN ≤ 6 V The device operates if the input voltage is equal to, or exceeds, VOUT(TARGET) + 0.3 V. At input voltages below the minimum VIN requirement, the devices does not operate correctly, and output voltage may not reach target value.

    I was interpreting "Vin must be >= Vout(target)+0.3V" to mean that if the condition is not satisfied that the regulator may not turn on at all. Can you confirm this is not the case?

    I have an application with an LP3985 5V device powered from a 5.37V supply (5.22V-5.48V) with its 5V typical load of 50mA, max of ~ 85mA.The load can work down to 4.5V. I would expect that if the input voltage gets to low that the regulator would behave as a saturated switch with some dropout, and by definition would not operate as a "regulator", but would still conduct current like a switch. Am I correct?

    Best Regards, Charlie

     

  • 0 in reply to Charles Cammarata
    TI__Mastermind 18466 points

    Hey Charlie, 

    You are correct. You seem to have caught an error in the datasheet and I have made note of it. 

    VDO at 100mA is about 70mV so you would need a Vin >= Vout+70mV.

    In dropout the LDO behaves like a switch and VOUT tracks VIN. When the device transitions from dropout to linear mode, the output will overshoots because the pass element is at its point of least resistance. 

    Best,

    Juliette