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TPS81256: Driving a capacitive load

PATRICK OHARA
Genius 3185 points
Part Number: TPS81256
Other Parts Discussed in Thread: CSD16301Q2

Hello,

I'm using the TPS81256 to supply power to a laser diode module. I'm modulating the laser by using a CSD16301Q2 MOSFET on the low side of the module to modulate the connection to ground - traditional LED type driver. The laser module has an impedance of ~312 ohms. I cannot measure the capacitance of the module - measures basically 0 pF. From the scope trace, I see the device is turning off fast, likely due to the internal 312 ohms, but charging the module/capacitor is very slow causing the output signal to never attain the 5VDC supplied by the TPS81256. Any suggestions as to how I can make this module turn on quicker?

Thanks,

Patrick

  • 0
    TI__Genius 9046 points

    Hi Patrick,

    Could you please share me details how you realize the turning ON and OFF? Do you know what is the load current when laser module has power? Many load has larger impedance when it is in UVLO than when it is enabled. Some even you measured 312ohm when VCC is UVLO, it does not mean it only draws 5V/312Ohm current all the time.

    I drew a simplified block diagram below. Please confirm.

    -Wenhao

  • 0 in reply to Wenhao Wu
    Genius 3185 points

    Hello Wenhao,

    Thank you for your help. You don't quite have the diagram correct. The ground on TPS81256 is always connected to ground (uninterrupted). The MOSFET's gate is driven from I/O from CC2640r2, the drain is attached to the output of the TPS81256, and the source is tied to ground. It seems odd that voltage takes so long to ramp up and then collapses immediately. I attached a Keithley bench power supply in place of the TPS81256 and I get the same behavior. There must be something strange happening in the laser module.

    The load current is ~15mA.

    Thanks again for the help. If you have further ideas, I'd like to hear them.

    Patrick

  • +1 in reply to PATRICK OHARA
    TI__Genius 9046 points

    Hi Patrick,

    So your gate is actually floating right? If your gate is connecting to CC2640R2, if you need to get CSD16301Q2 turn ON when I/O is high, it should be higher than 5+1.1V in order to get CSD16301Q2's VGS always higher than 1.1V.

    You could place CSD16301 to low side as I drew above to get VGS correctly.  Or you can use a load switch or effuse in TI's portfolio. They have internal charger bump to force GATE higher than source.

    -Wenhao