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TPS81256: Using enable pin to drive device in PWM

PATRICK OHARA
Genius 3185 points
Part Number: TPS81256
Other Parts Discussed in Thread: BQ24075

Hello,

I'd like to drive the enable pin on the TPS81256 with a digital I/O pin from a CC2640r2. I'd like to drive the enable pin at <10 KHz. I don't see info about input impedance or capacitance for the enable pin in the specification. The voltage input on this device will be from the output of a BQ24075 (4.2-5.0 V depending on if running on battery or USB). The idea is to provide a regulated 5V to a voltage sensitive device (drawing ~20mA) regardless of if it's driven by the battery or USB. I want to control the PWM parameters (frequency & duty cycle) from the CC2640r2 I/O. Can the enable pin on the TPS81256 be used this way?

Thanks for your help,

Patrick

  • +1
    TI__Genius 9046 points

    Hi Patrick,

    I do not think it is ok.

    One problem I can see is the start-up time for TPS81256 from EN=H to VOUT established. The typical time is 400us. So if you have 10k PWM, then the start-up time itself will be larger than a duty cycle (100us). And 400us is just a typical value. We do not promise the max value here. In addition the falling time is not controlled by TPS81256. It is rather influenced by load resistance and output capacitance.

    -Wenhao

  • 0 in reply to Wenhao Wu
    Genius 3185 points

    Hi Wenhao,

    I decided to use a MOSFET on the low side of my load to switch it rather than using the enable pin for PWM. Thanks for your help!

    Patrick