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TPS92691: wide current range (20mA to 2A with analog dimming)

Ryuji
Guru 20090 points
Part Number: TPS92691

Hello,

Our customer would like to set the output current as 20mA(min) and 2A(max).
In order to avoid the flicker phenomenon, I would like to use Analog Dimming.
However, since the IADJ range is 140mV to 2.25V, we can't achieve 2A(max) and 20mA(min) with analog dimming.
Thus, I would like to set to the following settings to achieve this requirement.

Rcs=0.075ohm, VIADJ=140mV, then ILED will be 133mA, and PWM duty dimming 15%.

Then I have the following questions.

(1) PWM frequency
Could you please let me know the maximum PWM frequency for this case?
How much should I set the PWM dimming frequency to avoid flicker?
I want to prevent movie shooting from being affected.
Current Fsw setting is 390kHz.

(2) Analog dimming accuracy at low voltage setting
If VIADJ is set to 21 mV, 20 mA is set with Rcs of 0.075 ohm.
What is the current accuracy in this case?


(3)Other method
Could you please let me know more better way for this requirement if there is?


Best Regards,
Ryuji

 

  • 0
    TI__Mastermind 25500 points

    Hello Ryuji-san,

    You can dim lower but as you suspect the accuracy will not be good. The 140mV spec is to make sure you stay in the linear range. But the current sense voltage there would be 10mV and there is approximately +/-5mV of offset over all conditions so it could vary +/-50% there. If you go down to 5mV current sense voltage some may dim further and some may not as you try to go even lower due to the offsets and the accuracy gets even worse. You might even be limited by the minimum on time (leading edge blanking time) depending on input and output voltages.

    PWM dimming to go lower is a possibility. You can get great linearity up to about 1kHz with the disconnect FET and pretty good linearity up to about 10kHz. The lower the better generally as long as it is fast enough to not interfere with video refresh rates.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Guru 20090 points
    Hello Clint san,

    Thank you for the reply.
    Is it possible to switch the resistance value of Rcs between 750 mohm and 75 mohm with SW to achieve this wide current range?
    Please let me know if you have any concerns.

    Best Regards,
    Ryuji
  • 0 in reply to Ryuji
    TI__Mastermind 25500 points

    Hello Ryuji-san,

    Yes. It will need to be tested for the particular application to see how far it can dim. Depending on the switching frequency the low end can be limited by the minimum duty cycle (the lower Fsw the further it can dim).

    Regards,

    Clint