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LM3409: LM3409/6 Application Question

Kuo John
Expert 1075 points
Part Number: LM3409
Other Parts Discussed in Thread: LM3406

Hi team,

Could you help me to know LM3406(CONT) and LM3409(COFT) benefit about different control mode?

CONT and COFT both have good responds for PWM dimming. why to crate two different but simulate control mode device? 

Looks different just high side protect in COFT mode device?

  • 0
    TI__Mastermind 25500 points

    Hello Kuo,

    There are a number of differences and picking one over the other really depends on the requirements of the application. I will try to summarize some of the main differences:

    The LM3406 with constant on time and monitoring of the output voltage gives certain benefits. The main benefit is that the switching frequency will be pretty constant regardless of the input and output voltage. The LM3409 will have a somewhat constant switching frequency versus output voltage, but not input voltage. However, this limits the range of operation for the LM3406 versus the 09, for example see the graphs in the 06 datasheet of "Vo-max vs Vin". The 09 will be able to operate at a much wider range as it can adjust the Fsw as needed. That also allows the 09 to be more suitable to using the exact same design for a number of different LED loads without changing any component values, you may need to with the 06.

    The LM3406 also uses an error amplifier versus a high speed comparator for the 09. This results in perhaps a little better line/load regulation, but much more limited bandwidth. The 09 is much faster reacting to any transients and better with PWM/shunt FET dimming. The LM3409 is therefore better for very low frequency and very high frequency PWM dimming with either method.

    The 06 is also better suited for PWM dimming only, not really analog dimming as it likes to operate in CCM. The 09 is designed for analog dimming if needed, it is fine with DCM operation if you are using an output capacitor.

    The current sense is different also in that one is high side and one is low. Some applications require connecting the LED string to ground, that is where high side comes in.

    So the 06 is more predictable with Fsw and perhaps a little better line regulation, but the 09 is more versatile and much faster for PWM dimming applications that require it to be very slow/fast. Of course if you need two wire input dimming the 06 is designed specifically for that.

    Regards,

    Clint