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LT3474 in parallel

Hi, 

I'd like to use the LT3474 as a current source, but need max 3A, so I'm thinking of using them in parallel. How would you recommend to do that?

Thanks for your help.

Kind regards

  • Hi, just a bump, any recommendation for using 3xLT3474 in parallel to increase max output current?

    Kind regards

  • Hi GuillaumeB,

    May I know what are your Vin Vout specifications? Also, may I know if you are gonna use the PWM dimming capability of this device or just its constant current capability?

    If you are not gonna use the PWM dimming capability I would suggest onto looking at the part LT8613 as it has a reference design of an LED driver capable of delivering at a max output current of 5A.

    Thanks

    Siglo

  • Hi Siglo, 

    Thanks for your answer.

    I have an input of typically 24V (20V to 28V) and an output of max 12V to 14V at 3A.

    What I need is a digitally controlled current source able to source 5mA to 3A with good precision, hence the choice of LT3474 for the precision at very light output current, and the idea to put 3 of them in parallel to achieve 3A output current. Each of them would be driven by its own DAC.

    I won't use the PWM dimming as the current control will be done using the 3 DACs. LT8613 seems interesting but if I'm not wrong, it is not a current source and needs an output voltage to be set. Please correct me if I'm not understanding the capability of this IC correctly.

    Regarding the 3xLT3474 in parallel, do you think there would be in issue in doing so?

    Kind regards,

    Guillaume

  •  Hi GuillaumeB,

    In the datasheet of LT8613, is a current mode control converter and the current regulation loop is controlled by the ISN, ISP, and ICTRL. The ISP/ ISN pins monitor the voltage across an external sense resistor such that the VISP-VISN does not exceed 50mV by limiting the peak inductor current. Which means the current regulation loop will try to maintain the 50mV drop across VISP-VISN (which is proportional to the output current) regardless of the output voltage set by the Vfb and the new "Vout" will be proportional to the output current, regulated by the control loop) and the output resistor. (you can check on the data sheet typical applications part on the configuration of how current limit is established and an application for a digital current source)

    You can override the 50mV voltage drop in the VISP-VISN pin that allows you to control the voltage of the ISP-ISN Pin (which is proportional to the output current) below 50mV and 0mV by controlling the voltage on the ICTRL pin between 1V to 0V. Referring to the graph below, since the graph shows a very linear ISP-ISN to ICTRL pin relationship, this can give a precise control on the IC as long as the DAC that's going to be used is going to produce a precise output voltage. Just be careful with the Abs Max current rating of the IC in order to prevent malfunction or problems.

    The problem with the LT3474 is there are no applications done and presented in the data sheet regarding configuring in parallel. With that, it is not best to operate the 3 LT3474 in parallel as we cannot ensure the behavior of the IC when connected in parallel. One thing I could think of is that one may source higher current compared to the others that could potentially damage the IC that is sourcing the higher output current since it is operated beyond its max output current capacity. So I think it is best not to operate 3x LT3474 and opt with the LT8613 with a higher output current capability.

    Regards,

    Siglo

  • Hi Guillaume,

    Ideally, parallel connection of current sources is straightforward: set-up each LT3474 independently and tie LED pins of all the three parts together. Easier said than done so I recommend testing in bench.