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TPS82085: Unloaded Current

Mike Carlos
Prodigy 180 points
Part Number: TPS82085

Hi,

I have just starting soldering down TPS82085 modules. Below is my schematic.

I have soldered U5, R13,R15, R14, C52.   I have NOT soldered C154,C130. There is NO load on the device so nothing is fed by 3.6V

When I apply 5V, the voltage I measure at the output is 3.6 Volts which is correct. However the current drawn from the power supply is ~200mA.

Measuring VOUT with the scope I get the image below. The period of the sawtooth is about 13ms.

Is this expected? Why so much current is drawn without any load?

Regards

  • 0
    TI__Intellectual 880 points
    Hello,
    The scope plot is not very clear (AC or DC coupling? What is the vertical and horizontal resolution?).
    Please repeat the test by soldering the mandatory input capacitor. The PCB layout should follow the recommendations in the data sheet.

    Best regards,
    Ivo
  • 0 in reply to Ivo
    Prodigy 180 points

    Hi Ivo,

    Ok I will insert the input CAP as well as you say that it is mandatory. 

    The vertical resolution is 10mV/Div and 10msec/Div. The trigger level shown as T on the vertical axis is at 3.62V. So the scope is DC coupled. The waveform has 3omV peak to peak ripple around 3.6V with a ~100Hz frequency.

    Regards

  • 0 in reply to Ivo
    Prodigy 180 points

    Hi Ivo,

    I have soldered all the components. However the unloaded current is still ~200mA. When I insert a 56Ohm load the current drawn from the supply becomes ~270mA.

    Do you have any idea why the unloaded current is such high?

    Regards

  • 0 in reply to Mike Carlos
    TI__Intellectual 880 points
    Hi Mike,
    this is definitely not the expected behavior of the power module. At not load condition the quiescent current should be in the range of 20uA and when applying a 56 Ohm resistive load the input current should be below 80mA.
    Please check if there is any further load at the output (remove all connections to your remaining circuity on your PCB), check soldering if there are any shorts. You could order an evaluation module for reference.
    Best regards,
    Ivo