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TPS709: Stabilize 5V output in a wireless charging system

JT@PB
Prodigy 50 points
Part Number: TPS709

Hello,

We are using a wireless charging coil and rectifier as the input to the TPS70950DRVR. The output powers the battery charger IC, which powers the rest of the system. I probed the voltage on the output pin and notice it varies from ~1V to 5V as the wireless charging (Rx) coil is moved around over the Tx coil. We believe this is causing undefined behavior with the battery charger IC, so looking for help to stabilize the output from the LDO. I'm also wondering if this is expected behavior for the output?

The picture below is the output from TPS70950DRVR (input to battery charger) as the coil is moved around.

Thanks!

  • 0
    TI__Guru 50025 points

    Hello,

    This is not expected behavior if the input to the LDO is not also falling to ~1V, so I wonder if the output voltage of the rectifier is just not sufficiently high to keep the LDO out of dropout and the output at 5V. Even if the rectified voltage was moving around a lot but was always high enough to avoid the LDO going into dropout, the output would not behave like this. Since these changes in voltage are very low frequency, the output would remain very stable so long as there is sufficient headroom.

    Can you please provide another scope shot with the LDO Vin and Vout together? 

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 50 points

    Thanks for the quick reply, here is Vin (ch1) and Vout (ch2) as the Rx coil is moved around.

    It does look like the output drops below only when the input is too low. Should the output be disabled in this case?

  • 0 in reply to JT@PB
    TI__Guru 50025 points

    Hello,

    The LDO doesn't care, but that depends on your specific application. If the downstream ICs cannot properly function over the voltage range you are seeing, then you may want to have the LDO turn off if the input is not sufficient to maintain the output. You can do this by using a resistor divider at the input for the EN pin to shut the part off when the the input falls too low. 

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 50 points

    Thanks Nick. Is there an example design or app note or something I could reference for that configuration?

  • 0 in reply to JT@PB
    TI__Guru 50025 points

    Hello,

    Now that I am thinking about it more, I don't think using a resistor divider would work in this application because the hysteresis is too wide for EN. 

    Are there any other voltage rails present? Is there a way you could use a supervisor or something similar to disable the LDO? I suppose we are getting away from the LDO, which is my specialty. You may want to post in another forum - perhaps for supervisors - for help constructing a circuit to provide the enable signal more precisely.

    Best regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 50 points

    There are no other voltage rails present unfortunately. The coil + rectifier (input to the TPS709) generates the 5V rail for the rest of the system.