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TPS92520-Q1: TPS92520 PWM waveform

zhang lin
Intellectual 310 points
Part Number: TPS92520-Q1

Tool/software:

Hello expert,

I am testing TPS92520, the waveform of output voltage and output current. Why is the waveform of current PWM waveform and the waveform of voltage not PWM waveform? What is going on?

  • 0
    Intellectual 310 points

    To add, we used an internal PWM with a frequency of 215hz.

  • 0 in reply to zhang lin
    TI__Mastermind 23326 points

    The 520 creates a constant current source and when it gets turned on that current goes into the output capacitance which raises the output voltage and once the output voltage is higher than the Vf of the LEDs that current goes into the LEDs.  Once the current is turned off the LEDs stop to conduct and then the output capacitance starts to discharge.  That is what you are seeing as the output voltage.  The current in the LED will look more like the PWM signal.  You can use a current probe to take a look at it.  

    -fhoude

  • 0 in reply to fhoude
    Intellectual 310 points

    I still don't understand. The waveform of the output current in channel 3 looks like a square wave, but the output voltage in channel 1 is not a square wave, and the voltage did not drop to 0V during the PWM off period. What's going on.

  • +1 in reply to zhang lin
    TI__Mastermind 23326 points

    Zhang, once the output current stops (turns off)  the voltage on the output will go down due to leakage.  If the voltage is below the LED diode Vf then it become high impedance and the output capacitor holds the charge and slowly decay due to leakage current on the output.  It's really that simple.