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TPS92641 high PWM frequency, low duty

Keith Weller
Prodigy 100 points
Other Parts Discussed in Thread: TPS92641, LM3409

Hi,

I need an LED driver that can deliver 2.5A at about 12V from a supply of about 13.5 to 36V. The challenging part is that we want a PWM frequency of around 30KHz, and contrast ratio of at least 30:1 (so on pulses < 1us ideally).

Is this realistic with the TPS92641 using the shunt FET mode? I'm wondering how accurate the LED current regulation would be at such short pulse duration, since maintaining good color balance is important in our application.

  • 0
    TI__Mastermind 25500 points

    30kHz is pretty fast, but shunt FET dimming is very fast with a part like this. 30:1 should attainable. If you need really good linearity you may need to have a pretty high switching frequency, maybe 1MHz or more, so choose low Qg FETs.

  • 0 in reply to Clinton Jensen
    Prodigy 100 points

    Thanks for the advice Clinton. I've ordered a demo board and will see if it does what we need. 

    I also discovered the LM3434, which look ideal with its shunt FET dimming, but unfortunately we need up to 40V supply input; do you know of any other parts I should be considering?

  • 0 in reply to Keith Weller
    TI__Mastermind 25500 points

    The LM3409 is a similar hysteretic buck, but not synchronous. But it can achieve similar dimming with a shunt FET.

  • 0 in reply to Clinton Jensen
    Prodigy 100 points
    Yes, I've just been been playing with a demo board for that device that I had lying around, and it does indeed look capable. The constant off time scheme seems well suited to the task, especially if I trigger it synchronously with the PWM to keep the inductor current well defined when the shunt FET turns off.
    A synchronous converter with constant off time would be ever nicer, but I suspect we can live with the somewhat lower efficiency if it delivers on the other fronts.
  • 0 in reply to Keith Weller
    TI__Mastermind 25500 points
    Good to hear. I'm not sure what version of the datasheet you have, but there is better information on shunt FET dimming in the latest datasheet. There are some cases where you may want to derive the off timer from another source if the output voltage is too low to charge it while the FET is on. But if you have some scheme to trigger an on time already this may be irrelevant.
  • 0 in reply to Clinton Jensen
    Prodigy 100 points

    I've been referring to SNVS602K (MARCH 2009–REVISED JULY 2015). Something I'm not clear about though is why the off time is modulated by Vout, especially since it then needs to be overridden with a fixed voltage when the shunt FET is on (page 17). Why not just have a fixed off time irrespective of the output voltage?

  • 0 in reply to Keith Weller
    TI__Mastermind 25500 points
    That is a good question. The reason the off timer is usually referenced to Vout is so that if the output voltage changes, either from the LEDs heating up and the Vf dropping or for using the same circuit with different output voltage stacks, the switching frequency will remain relatively constant. The reason another supply is sometimes used when shunt FET dimming is that if the FET Rds(on) is low enough you cannot charge the COFF pin past its threshold. So it could potentially cause a maximum off time condition (this depends on PWM frequency and duty cycle of course) and it could cause a delay in the response when the FET is opened. At 30kHz it likely will not be an issue, but if you do see any unwanted delays that is likely the cause. I just wanted you to know in case you see it so you will know how to fix it.
  • 0 in reply to Clinton Jensen
    Prodigy 100 points
    Hi Clinton,

    I'm pleased to report the LM3409 solution is coming together very nicely! I do have a question about the 5uA 'IADJ' current source that you may be able to advise me on; the data sheets indicates a spread of 3.8 to 6.4uA and I'm hoping that's mainly manufacturing spread. Is there any information of it's variation with temperature?
  • 0 in reply to Keith Weller
    TI__Mastermind 25500 points

    The IADJ current spread includes manufacturing spread and over the full temperature range. But it's not a very important spec. The 5uA is just there to force IADJ high and clamp at 1.24V. It makes the circuit more simple if you do not need analog adjust. But if you look at the current sense voltage spec you will notice a bit more variation than if you drive IADJ. That is because the clamp variation and IADJ current variation are built in to that spec. So accuracy is better if you drive IADJ or use your own precision reference for it.