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TPS709: Reverse Current Waveform

Lucas Wassenius
Intellectual 1445 points
Part Number: TPS709

Hi,

I am asking on behalf of a customer that uses the TPS70933 in a battery powered application to step-down the voltage from 9V input to 3.3V.

When testing, the output of the TPS70933 was connected to a 5V rail through a 10kOhm resistor(with no load on the 3.3V output). This caused an oscillating waveform on the output side. Oscillating between 3.3V to 3.7V and a current of 170uA can be detected through the 10kOhm pull-up resistor. Which indicates that there is a 170uA leakage into the LDO.

We are interested in the internal structure of the device to determine what can cause this oscillation. We know that this is not a recommended way to design the circuit and it will not be a issue in this design. 

Do you have any ideas what can cause this waveform? When simulating this scenario in WEBENCH, the fault cannot be detected.

  • 0
    TI__Mastermind 37025 points

    Hi Lucas,

    The 170uA current you describe is due to the voltage drop from 5V, down to 3.3V, through a 10k ohm resistor.

    170uA * 10k = 1.7V, which is the voltage drop between the two rails.

    So 170uA is not leakage per se.
    Also keep in mind that linear regulators do not sink current. 
    They can only source current. 
    So if something starts charging up the output, a standard LDO has no means to hold it down.

    So what is likely happening is the 170uA is charging up the output capacitance of the LDO, and once the output reaches some overvoltage threshold, an additional circuit internal to the LDO pulls down on the load to prevent an over voltage condition.  This would be something like a fault protection circuit.  If this were the case, I would expect something just over 10% of the output voltage would be the over voltage trip point, and that is what you are seeing.  3.3V *1.1 = 3.63V, so 3.7V would be in the range I would anticipate an over voltage circuit to kick in.  This over voltage circuit is not described in the datasheet, but I've seen it in other linear regulators. 

    I checked our archives and did not find anything related to this question from other customers.
    However this type of circuit you described is uncommon.

    Can the customer replace the resistor with a diode?
    If the anode was connected to the 3.3V output and the cathode was connected to the 5V, this problem may very well go away.

    Thanks,

    - Stephen