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TPS7A4101 input capacitor when driving LED chain

marek hajek
Prodigy 40 points
Other Parts Discussed in Thread: TPS7A4101

The datasheet for the  TPS7A4101 requires at least a 1uF cap on the input for stability. However, Figure 13 (see below) in the datasheet does not show an input capacitor. When the device is used for driving LEDs with a PWM signal connected to the EN pin, is an input capacitor required? What is the range of the switching frequency allowed on the EN pin?

  • 0
    TI__Expert 8905 points

    Marek,

    Thank you for you post and bringing this to my attention. I will look into this.

    Regards,

    David

  • 0 in reply to David Hopkins
    TI__Expert 8905 points

    Marek,

    The schematic provided in the datasheet was a simplified schematic to demonstrate a unique use of an LDO. A minimum of 1uF of input capacitance is still required, as per the datasheet, for the LDO to be stable.


    The timing requirements on the PWM input to the enable pin would be a function of the input and output capacitance, along with the time it takes for the LDO to turn on and off. We will look more into this.

    Regards,

    David

  • 0 in reply to David Hopkins
    TI__Expert 8905 points

    Marek,

    Sorry for the delay, but I was able to bench test the device on the EVM with 8 LEDs daisy chained together, Cin at 1uF, Cout at 4.7uF, Cbypass removed, R1 107k, R2 at 59k, and a 75ohm load resistor.

    The smallest pulse on the enable bit that would enable the LED's was 1.25 milliseconds, with up to a 50% duty cycle. After that, the LDO either would not fully start-up as the pulse was too small, or as the duty cycle increased the output capacitor would not discharge fully, so the LDO was not fully off.

    Regards,

    David  Hopkins

  • 0 in reply to David Hopkins
    Prodigy 40 points

    Thanks David,

    This suggests that the LED frequency has to stay below 400 Hz (assuming 50% duty cycle). The LEDs would not have a wide dimming range as the pulse width could only go down to 1.25ms and the frequency has to stay up in order to avoid flicker.

    I assume, based on the datasheet note, if the load resistor is ~60 ohms, R1 and R2 would not be needed at all (provided the feedback pin is tied to the output).

     

     - Marek

  • 0 in reply to marek hajek
    TI__Expert 8905 points

    Marek,

    Thank you for your patience on waiting for a response to this posting.

    The LED frequency and duty cycle would be quite limited, but for some applications it could be more than sufficient.

    Removing R1 and R2 will allow at most 20mA to power the LEDs (as Vout would go to Vref), so if that is sufficient current than yes, they would not be needed.

    Regards,

    David