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LED controller or driver for a special application

FZ37
Intellectual 450 points
Other Parts Discussed in Thread: LM3550, TPS61196, LM3409

The application is to drive a string of 3.5V LEDs at 5-10KHz with ~50% duty cycle (On and Off). The drive current need to be adjustable or programmable (I2C bus preferred) from 100mA to 1A (2-300mA if combining outputs).

It requires the controller or driver to be able to switch on and off the LEDs in a few us at full load (100mA to 1A).

I am looking at

1) LM3550, not sure if I can switch on and off the external driver without disstablizing the control loop in torch mode.

2) TPS61196, looks promising. Need resistor to set the current and combine 2-3 outputs to drive one LED string.

3) LM3409, data sheet shows clear fast enough on/off time (page 9).

... ...

Any suggestions or recommendations are appreciated.

F.Z.

  • 0
    TI__Mastermind 25500 points
    I would need to know how many LEDs (total string voltage) and the input voltage to be sure, but if you need a buck converter the LM3409 will absolutely be able to do that. It has analog adjust and a very fast PWM dimming feature. It can achieve rise and fall times in the us range just using its standard PWM dimming, but could achieve rise and fall times in the ns range using shunt FET PWM dimming. It does not have I2C however. But I've used this part in many designs and it is very good.
  • 0 in reply to Clinton Jensen
    Intellectual 450 points

    Thanks for your reply.

    There are 5 LEDs (3V to 3.5V) on each string. And the drive current is 100mA to 500mA for now with potential going up to 1A. I went over more details of the data sheet and user guide of the demo board for LM3409. One concern is the fluctuation of LED current during on-state is quite large. In order to lower that it requires using a large inductor or  a much higher switching frequency. Is the part going to behave the same when the switching freq. is set to 1MHz or higher? It says up to 2 MHz.

  • 0 in reply to FZ37
    TI__Mastermind 25500 points

    I'm not quite sure what you mean by large LED current fluctuation during on-state? Are you referring to the LED current ripple? That can be reduced by increasing the inductor value and/or increasing the switching frequency. But there is a limit, the datasheet describes a minimum amount of current sense voltage ripple for proper operation. If further reduction is needed an output cap can be used, but it would need to be small for fast enough rise/fall times, you may need to shunt FET dim in that case. The other parts you mentioned in your post I am not familiar with so I couldn't comment.

  • 0 in reply to Clinton Jensen
    Intellectual 450 points
    Yes, it's the LED current ripple. In AN-1954 demo guide, the calculation is for a 1A LED current with 450mA ripple. I have to recalculate all the numbers if I want to set LED current to 350mA with ~100mA ripple. And it does require a minimum ripple current for about 24mV/Rsns.

    I probably have to get both demo boards for LM3409 and TPS61196 to test them out after change some of the R, C and L.

    Thanks for bringing up the minimum ripple current. I am not quite sure how a output Cap help on current ripple.
  • 0 in reply to FZ37
    TI__Mastermind 25500 points
    An output cap would not affect the inductor ripple current. But adding even a small amount of capacitance across the LED (output cap) will reduce the ripple current in the LED itself.
  • 0 in reply to Clinton Jensen
    Intellectual 450 points

    I will try that once I get the demo boards. Thanks again for your replies.