• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 62 subscribers
  • Views 482 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

how to calculate external pull-up resistor for tlc 5971

Nurudeen Osanyinpeju1
Prodigy 20 points
Other Parts Discussed in Thread: TLC5971, LM3404

Good day to all ,

Please i need your help on how to calculate the correct value of External Pull-Up resistor to be used for  the Output channels in TLC 5971 chip.I have  4 pieces of tlc5971 on my board.the voltage supply to each chip is 4.1V(Vcc-Pin19)while my IREF(Pin1) is 24.8k Ohms.

I am using the signals generated from the tlc 5971 to modulate the pwm of an Led Driver with a constant current Output of 700mA and 3.8V which  is used to control the brightness of Hihg Power Leds.The Problem i am having is that the PWM Signal is not stable and clean.

For instance when i used 3.9k Ohms for the Pull Resistor,the  Leds were ON only @ 5% and there was error  of  3% but when i used 1.5K Ohms for one of the channels,the Led was ON @ 1% and at the same time the  error was reduced to 0.07%.Because of this result.i changed all the Pull-Up resistors to 1.5k Ohms but to my suprise all the Leds did not ON,so i have to Change the Pull-Up resistors back to  3.9K Ohms.

 Sir,I would be very glad if you could guide me on how to calculate the correct value of the Pull-Up resistor taking into consideration the Information i provided above.

thank you for the usual Support. Best regards

  • 0
    TI__Intellectual 1820 points

    Hello,

    I'm still not very clear about your problem. It will be better if you can share your schematic.
    You said you use the output waveform of 5971 as PWM signal to control a high current(700mA) LED driver(like LM3404)?
    If this is the case, I'm afraid that the frequency of output signal of 5971 may be too high.

    TLC5971 implement ES-PWM with internal 10MHz grayscale clock, this means the PWM signal frequency at OUTx pin is 19.5kHz typically.

    Please confirm if this 19.5kHz PWM frequency is acceptable by your high current LED driver.

    Thank you
    Mike