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TLV9001: How to design breathing LED function by current sink circuits?

Hugo Lee
Intellectual 530 points
Part Number: TLV9001
Other Parts Discussed in Thread: SN74LVC1G07

Tool/software:

Hello,

We would like to design a breathing LED function. Can we implement breathing function by TLV9001 current sink circuit as below circuits? Is this correct?
Could breathing PWM be controlled by Vx or by 3.3V power source?

Or....Could breathing PWM be controlled by 3.3V power source to circuit?

Thanks,
Hugo

  • 0
    TI__Guru*** 145126 points

    Hello Hugo,

    The left circuit is good for DC current control, the right side is fine in theory but could be problematic in practice.

    For PWM input, make sure loop is fast enough for acceptable performance on left side. Almost certain problems on right side.

    Current control is best way to drive LED. 3.3V should be enough but know that 3.3V = VLED + VDS + Vx and VDS can't be 0 or negative. 

    Also due to op amp Vos error, LED off might not be fully off with 0V input. Vx = IN + Vos

  • 0
    Guru 268180 points

    Such a current sink circuit is appropriate for an analog control signal.

    When you have a PWM control signal, you can simply use a plain current-limiting resistor. (If the PWM control signal is too weak to drive the LED, add a simple buffer like the SN74LVC1G07.)

  • 0 in reply to Clemens Ladisch
    Intellectual 530 points

    Due to the WLED used has wide Vf range 2.55V~3.25V, so I can't simply use current-limiting resistor.Vf range

  • 0 in reply to Ron Michallick
    Intellectual 530 points

    TLV9001S with shutdown pin seems a good way to prevent not fully off. Could I use TLV9001S shutdown pin for PWM breathing LED effect?
    With 70us enable time and 4us disable time, LED ON period could be [ON time - 70us - 4us] for each PWM cycle, right?

  • 0 in reply to Hugo Lee
    Guru 268180 points

    So you want to use the current-sink circuit to just set the 100 % current, and then use the PWM control signal to switch between that current and 0 A.

    It might be a better idea to use a low-pass filter to convert the PWM signal into an analog signal, and use that as control signal for the current sink. Then you would not need to care about how fast the current sink can start and stop regulating. Additionally, there would be no flicker even with a low PWM frequency.

    Please note that in circuit B, the supply current of the opamp (60 µA) is visible as a voltage drop over RSET. If RSET is at least 33 Ω, then that voltage drop is always larger than the input offset voltage (< 2 mV), so a control signal of 0 V would be guaranteed to switch off the LED completely.