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TPS92641 flicker when shunt pwm diming

Ruben Jönsson
Prodigy 220 points
Other Parts Discussed in Thread: TPS92641

I am working on a commercial product that is using TPS92641 as led drivers.

I have some problems with visible flicker when shunt pwm diming LEDs with a total forward voltage above around 30V (input voltage above around 40V). This seems to originate from the inductor charging while the shunt transistor is conducting (SDIM pin low). Sometimes and seemingly randomly the inductor gets charged to a much higher current than it should which disturbs the regulation when shunting is switched off again resulting in a visible flicker.

I think this is because the on-time for the high side switch transistor sometimes becomes too long during the slewing up of the inductor. The highest inductor charging current mostly comes right when shunting is activated. I have even seen the driver issuing two charging pulses within a short time right after the shunt transistor switches on.

PWM period time is 610Hz. Linear dim current is set to 300mA while PWM shunting. Above 300mA the current is linear dimmed which works fine. Max current is 1.5A.

I have tried smaller values for the COMP capacitor, thinking it could make the regulation reach the target current faster but it only makes things worse when switching off the shunt transistor. Actually, right now the COMP capacitor is set to 1uF which seems to give the best results.

I have done a workaround which helps a lot - At the same time as the SDIM pin is set low I am setting the UDIM pin low for about 70us which discharges the inductor completely. But then the first pulse can still sometimes and randomly be pretty high compared to the following pulses but the behavior still seems much calmer when looking at the oscilloscope.

Have you seen this before and what can be done about it?

Thanks!

  • 0
    TI__Mastermind 25500 points
    It's hard to say without waveforms but it sounds like you could be hitting or exceeding a minimum effective duty cycle situation meaning the switch is on longer than it should be and/or off shorter than it should be. That would cause the inductor current to ramp beyond the regulation set point. Shunt FET dimming can become limited at higher input voltages but often if you use a lower switching frequency it will help. Another option is to add resistance in series with the shunt FET so that the output does not drop all the way to zero but to some level above zero and below the LED stack conduction voltage.
  • 0 in reply to Clinton Jensen
    Prodigy 220 points

    Thanks for your reply.

    The duty cycle is just below 80% with normal regulation (not shunt dimmed) and within the limits for max/min on and off times so that should be good.

    I think the main problems are disturbance when switching on the shunt dim transistor and high input voltage together with low output voltage when shunt dimming.

    The disturbance will vary a bit depending on when in the regulation cycle the shunt dim transistor is switched on. This is probably where the randomness comes from. Worst is when the high side transistor is conducting and it is at the end of the on-time. At this time the current is already at max through the inductor and there is a short delay between HG pin low and high side switch completely off which means it can still conduct when the shunt transistor activates. This together with the high current in the inductor results in a short high peak current which can disturb the regulation and directly start a new charging cycle.

    When input voltage is high and output voltage is low while shunt dimming, the resulting minimum on-time, which is limited to a minimum value by the driver, is still too long which results in too high peak current through the inductor.

    I will try to put a resistor in series with the shunt transistor to limit the peak current. I also UDIM the driver for part of the time when the shunt dimming is active in order to power it off which always start the inductor current at 0 level which in turn reduces the resulting disturbance from the current spike. A 1000 Ohm @ 100MHz ferrite bead on the CS pin also seems to have helped here.

  • 0 in reply to Ruben Jönsson
    Prodigy 220 points
    Hi,

    3.3 Ohm in series with the shunt transistor did the trick. Now I can drive LED stacks from 3V to 40V @ 1.5A with input voltages up to around 60V (adjusted to the LED stack voltage) without any flicker.