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LM3409HVEVAL: Fast LED Driver for Machine Vision Applications

CT
Prodigy 40 points
Part Number: LM3409HVEVAL
Other Parts Discussed in Thread: LM3409EVAL, LM3409

Good morning,

I'm looking to create a fast LED driver for a machine vision application and wondered if you'd be able to help with specifying a suitable driver and reviewing my current setup.

Our requirements are as follows:

> VIN: 15V - 30V max

> VOUT: VF = 12V - 14V max

> ILED: Must swing between a maximum of 0 - 1A with minimal ripple (ideally, a rise and settling time of between 1 - 2us with LED ripple ~1% would be great but possibly unachievable)

> MAXIMUM LED SWITCHING SPEED: We'd like to have the LED on for 10us and then off for say 10-20us. (ton = 10us, toff = 10-20us, trise/fall = 1 - 2us)

> CONTROL METHOD: Ideally analog dimming (whilst not as fast as PWM, the LED output current tends to be more stable when using analog dimming opposed to PWM)

We've been testing the LM3409EVAL to try and achieve this response however, we notice a significant rise in LED current ripple once we try to move past 20kHZ LED switching frequency when using PWM. Analog dimming is noticeably worse at lower switching frequencies.

Thanks for your help in advance.

Kind regards,

Connor

  • 0
    TI__Genius 14940 points

    Connor, 

    I'm afraid that LM3409 can't meet the requirements all the same time. Actually, it's seems hard to achieve the requirement with buck converter as LED ripple ~1% requires large inductance or capacitance,  which limits rising & settling time on the other side. 

    You may use shunt FET dimming to achieve rise and settling time of between 1 - 2us with small output cap or no output cap, or you can have larger output capacitance to reduce LED ripple current. While you can hardly do both.  

  • 0 in reply to Xiaoguang1
    Prodigy 40 points

    Thank you for your feedback.

    Is there an alternative solution that you would recommend for our problem? We've considered using a heavily compensated discrete current source driver but this has it's own problems and requires a lot of components.

    Alternatively, what could be classed as "best case" for the LM3409 (or similar devices you have available)? For example, if we removed the output capacitor and used a shunt FET topology to achieve say a 5us setting time, what levels of ripple current could we expect? 5%? 10%?

    Kind regards,

    Connor

  • +1 in reply to CT
    TI__Genius 14940 points

    Hi, Connor, 

    For LM3409, you may use below equation to calculate ripple current. 

    Thus, if you select lower Roff/Coff and/or larger inductance, you may achieve 5% ~ 10% ripple. 

    Two concerns:

    1. For LM3409, it is constant off time device, thus, frequency various a lot. For example, frequency @ Vin=30V is much higher than frequency@ Vin =15 V with Vo=12~14V. Thermal consider to be considered at Vin high; 

    2. There is minimum inductor current ripple request to ensure accurate ILED regulation. Please refer to 8.3.4 Inductor Current Ripple for details. If you don't care about it, then it is ok; 

    You may also check TPS92642, which is fixed frequency device with int. FETs.