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TPS92641EVM: Voltage at LEDs higher than 30 V

Werner Eisenbarth
Prodigy 30 points
Part Number: TPS92641EVM
Other Parts Discussed in Thread: TPS92641

Hello,

We have the development board with added Q3 for external Shunt FET PWM Dimming.
R1 is decreased to 15 kOhm to get a switching frequency of about 1 MHz.
Vin = 48 V

With an LED string with Vo = 29 V everything is good. The current is regulated to 1 A.

yellow: PWM input     magenta: LED current 1 A

With an LED string with Vo = 33 V the current breaks down to less then 0.5 A.

Is there a minimum difference or a minimum factor between input voltage and the LED voltage?
Or is something in the dimensining of the evaluation board not suitable for our application?

We would like to use LED voltages up to 42 V with an input voltage of 48 V.

Thank you.

Best regards

   Werner

  • 0
    TI__Mastermind 22745 points

    Hi Sir,

        Could you share the test circuit with TPS92641? And for TPS92641, it can support Vin=48V and Vout=42V application.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Prodigy 30 points

    Hello,

    The test circuit is the standard evaluation board. Only change is a decreased Ron to get a switching frequency of about 1 MHz.

    I assume that it should work with Vin=48V and Vout=42V. But obviously it does not. Already with Vout=33V the current breaks down.

    Unfortunatelly in my first post the diagramms are not shown. So I try again to include them to ilustrade what happens.

    With an LED string with Vout = 29 V everything is good. The current is regulated to 1 A.
    yellow: PWM input
    magenta: LED current 1 A

    With an LED string with Vout = 33 V the current breaks down to less then 0.5 A.

    Best regards

        Werner

  • 0 in reply to Werner Eisenbarth
    TI__Mastermind 22745 points

    Hi Werner,

        Could you please increase the Ron to decrease the switching frequency and please provide the waveform of inductor current and Vout? Thanks.

    BR

    Sean

  • 0 in reply to Sean Zhou
    Prodigy 30 points

    Hello,

    now I've a new evaluation board with the original switching frequency of 500 kHz. I only added Q3 for using faster PWM via SDIM.

    With this it works with higher Vout up to about 40 V (with Vin = 48 V).

    So, there exists an essential interrelation between the switching frequency and the difference between Vin and Vout. I didn't find information about this in the data sheets and manuals.

    The voltage problem can be solved with 500 kHz switching frequency. But we need to use a big output capacitor of 470 nF to reduce the ripple. Without this or with smaller capacity our LEDs begin to flicker when we are dimming down. But this big capacity increases the switching delay of the PWM dimming from 3 µs to 15 µs. Which means that we cannot use the first 2 % of the PWM range (1280 Hz PWM frequency). Beside this the inductor generates more noise. But these are minor problems for our application.

    Thank you very much.

    Best regards

       Werner Eisenbarth