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TPS61165 PWM control

Shannon
Prodigy 10 points
Other Parts Discussed in Thread: TPS61165

Hi, I'm using a TPS61165 to drive high brightness LEDs, and I'm not sure from the datasheet how the PWM control works.  I'm hoping to flash the LEDs at a low frequency (visible to the human eye, so less than 60 Hz), and vary the dimming at the same time to create an interesting display for a class project.  On page 10 of the datasheet, it says that the optimum PWM frequency is 5 kHz to 100 kHz, but it also mentions that an external RC network can be used for lower frequencies (also see Fig. 17).  The question that comes to mind is--why?  Since the dimming is controlled by the duty cycle and not the frequency, why would someone want to use a frequency outside the recommended range?

If I use a layout similar to that of Fig. 19, will it be possible to use the low frequency to flash the LEDs?  I'm a little concerned that the chip will enter shutdown mode every time I turn the LEDs off--is that an issue?  (See page 8.)

A quick reply would be greatly appreciated.  Thanks!

  • 0
    TI__Mastermind 36110 points

    The TPS61165 does not provide the PWM dimming you desire.  The IC takes a PWM dimming signal and reduces the internal reference voltage to a value equal to 0.2V times the duty cycle of the PWM signal.  The resulting LED current is reduced to ILED=VFB/RSET*D, where D is the duty cycle of the applied PWM signal.  The LED current does not turn on and off.  The external RC network you refer to in figure 17 allows analog dimming with lower frequency PWM signal.  This circuit injects a voltage into the FB pin, effectively reducing the FB voltage and and lowering the analog output current. 

    For PWM dimming, you have several options.

    You can achieve PWM dimming by turning the IC on and off with the CTRL pin.  Make sure your required dimming frequency is slow enough that the IC can actually turn on and off faster than the frequency of the signal you apply.

    You can remove the 0.1uF capacitor in Figure 17 to achieve PWM dimming.

    You can place a resistor and switch in parallel with Rset.  Turning the switch on and off effectively changes the value of Rset.  This method can be used to change the LED current between two values.