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TPS92692: PWM dimming LED very slow

Janson Lin
Intellectual 340 points
Part Number: TPS92692

1. I am trying to use PWM dimming to light 15 LED on . My input voltage is 19V and output 0.02V to  IADJ pin then I output 1/50 duty PWM signal to light LED. When power on and light first time,LED start to light up about  10 seconds later, It's very slow to light up. How to solve it?

2. When LED is light up, then light off and light on again with 1/50 duty PWM. LED can light up in 1 seconds . But when I light LED with 10/50 duty PWM and light offI then light up with 1/50 duty PWM immediately, LED would start to light in 2 ~ 3 seconds. It have a delay between PWM signal. How to solve it

My SW frequency is 600KHz by using external clock source,direct PWM frequency is 2KHz, Compensation capacitor and Resistor is the same as EVM, Rcs is 0.2ohm, Ris is 0.02 ohm, Rslp is 274K ohm, Css is 0.1uf, CDM is 0.027uF, CIMON is 10pF.

Best regards

Janson

  • 0
    TI__Mastermind 28655 points
    Hello Janson,

    1) Are you saying Viadj is 0.02V? Is there an output capacitor and what is the output voltage?

    2) I need to know your operating condition as above, what is Vout, Viadj, the topology.

    Ris = 0.02 ohm, means a 12.5A peak current setting.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Intellectual 340 points

    Hi Irwin :

    I try to duplicate this issue in EVB board , and this phenomenon can duplicate in EVB.

    1) Viadj is 0.04V, EVB output capacitor is 4.7uF x4 , my board is 4.7uF x2 ，output voltage is around 31V

    2) Vout is 31V ,Viadj is 0.04V, Boost topology.

    I use smaller Ris  for initial test. I out maximum is 750mA and peak current around 2.6A. L is 33uH.

    Best regards

    Janson

  • 0 in reply to Janson Lin
    TI__Mastermind 28655 points

    Hello Janson,

    1)  You are trying to charge the output capacitors with a very low current as well as trying to start-up with 2% PWM duty cycle.  Just charging the capacitors will take 0.4 seconds depending on what the output current actually is This is to charge from 19V to 31V.  Soft start and Comp will also charge very slow if the PWM duty cycle is 2%.  The actual error can be up to +/-7 mV and you are running down at 14 mV.

    I = C*(dv/dt), dt is calculated this way with 14 mA, 9.4 uF, and (31V-19V), dt does not include soft start and comp charging.

    I calculate Ioutmax at 170 mV/ 0.2 ohms = 850 mA

    You can speed this up by Enabling at 100% PWM until the output capacitors are charged.

    You can speed this up by reducing the output capacitors, reducing the soft start and compensation capacitors though you will have to make sure it is still stable over all operating conditions.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 340 points

    Hi Irwin :

    Remove soft-start doesn't improve this delay time but decrease compensation capacitor can improve. The compensation network and value follow EVB design. I calculate these value from "TPS92692 boost cal" again and get  Rcomp, Ccomp and Chf is N/A. I modify C36 to 0.01uF but power unstable when I out over 300mA. How do I get right compensation capacitor value?

  • 0 in reply to Janson Lin
    TI__Mastermind 28655 points
    Hello Janson,

    This requires looking at loop stability to improve loop response. What you are trying to do is difficult because you need stability at higher continuous current and want it to start up dimmed very low at 2% pwm.

    Regards,