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TPS79918 ENABLE

Suresh Samynathan
Intellectual 360 points
Other Parts Discussed in Thread: TPS3808

Hi,

In my application, 3.3V should come first and then 1.8V. I'm using output of 3.3V to enable the TPS79918 regulator through R and C. However the TPS79918 regulator is enabled before the EN pin reaches 0.4V (Shutdown threshold maximum is 0.4V). If I pull down the enable pin, then the regulator remains in OFF condition.

Please clarify why the regulator starts when the enable is in shutdown threshold.

 

  • 0
    TI__Mastermind 41770 points

    Just to clarify, the device begins to start up when the enable reaches the minimum threshold VEN(HI) which is 1.2V. The device shuts down when the enable pin reaches as high as 0.4V.

    What is your input voltage? How is the NR pin terminated?

    There may be an issue with the RC (timing) circuit connecting to the EN pin.

    Please provide a circuit of your application.

     

    I suggest  trying to turn the device on with just the input voltage first without the RC and see if it turns on properly.

     

  • 0 in reply to Darwin Fernandez
    Intellectual 360 points

    Input voltage is 3.3V.

    NR pin connected to GND through 0.01uF.

    Please find the INPUT (3.3V), Eable and Output waveforms.

    The issue is devic truns ON before the enable pin reaches the turn ON threshold.

     

  • 0 in reply to Suresh Samynathan
    TI__Mastermind 41770 points

    Suresh,

    This may be do to the RC circuit connected to the EN pin. The EN pin is usually driven by much faster digital high and low signals. If timing is the application for TPS79918, I recommend using a supervisor to monitor the 3.3V rail. When it reaches regulation a logic signal triggers the enable pin to turn on. The TPS3808 is good example and has a timing feature to delay the output before going high.

    Here is the TPS3808 datasheet:

    http://www.ti.com/product/tps3808-ep

     

  • 0 in reply to Darwin Fernandez
    TI__Mastermind 41770 points

    Suresh,

    The TPS79918 is an LDO optimized for fast VIN ramp rates applications. Looking at the VIN profile on the waveform you have attached, the ramp rate is too slow. For your application, I recommend using the TPS73418. This has the same specifications as the TPS79918 except it has a higher current rating (250mA). Also, this follows pin-for-pin the TPS79918. Attached is the datasheet. The TPS73518 also accomplishes your power requirements (500mA). Attached is the datasheet.

    http://www.ti.com/lit/ds/symlink/tps73418.pdf

    http://www.ti.com/lit/ds/symlink/tps73518.pdf

     

    Again, please consider the voltage supervisor recommended in the previous post. The RC enable configuration creates a delayed turn-on signal, however, this means the signals will be above logic LOW and below logic HIGH for a longer amout of time. In this range, the converter can be either on or off.