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PWM dimming limit for LED Driver(Base on TPS92511 Application)

Kuo Duke
Expert 2040 points
Other Parts Discussed in Thread: TPS92511, LM3402, LM3404, LM3406

Hi All,

I have question of PWM dimming limit for LED Driver(Based on TPS92511 Application),

1.

The TPS92511 has very fast PWM dimming response time.
For example, if the switching frequency is 500 kHz,
the minimum DIM pulse width is 6µs and the
dimming frequency is 150Hz,
a contrast ratio of more than 1000:1 can be achieved.

can we have use mathematics to explain this relation ?? 

2.

In fact, duty of TPS92511, can implement  PWM dimming freq 30kHz@Vin 13V, Vout 7.2V, Iout 0.5A

D = 7.2/12 approximately 0.55

fast PWM dimming freq 30kHz, Do I need add external shunt FET ??

Thanks all.



  • 0
    TI__Mastermind 25500 points

    1. You can go through the math if you like, but I have not done that practice. But the datasheet just recommends 3 switching cycles minimum. So it all depends on your switching frequency, PWM frequency, and current rise/fall times. If no output capacitor is used the rise/fall times are governed by the inductor value by v=Ldi/dt.

    2. This part is not really ideal for shunt FET dimming. But the 30kHz dimming can be achieved with high switching frequency and a low inductor value. You will not achieve 1000:1 however. Again it depends on a minimum of 3 switching cycles and the rise/fall time of the current. If you need very fast shunt FET dimming it can be achieved easily with the LM3402, LM3404, or LM3406 depending on the maximum current you need.

  • 0 in reply to Clinton Jensen
    Expert 2040 points

    Hi sir,

    another problem, could you help answer to me?

    How about the Duty cycle limit of TPS92511?

    can implement Vin 36V Vout 33V (D = 33 / 36 = 0.91)

  • 0 in reply to Kuo Duke
    TI__Mastermind 25500 points
    As a low side switch floating buck it is capable of 100% duty cycle. So that should be no problem.