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LT3517 low output current, why?

Hello,

I'm testing boost convertor with LT3517 and I have lower output current than expected. Input voltage is 12V or higher, output voltage depend on condition, but max 37V and out current should be 200mA. I have 0R47 resistor between pin ISP and ISN, so without correction on pin CTRL should be approx. 210mA. Problem is, I have only 180mA on real circuit, so why? Wrong inductor or something else?

Thaks for help.

  • Hello Yakobium,

    The problem is with your FB pin resistor setting. If you expect 37V of LEDs, you drive these LEDs with the ISP and ISN  sense resistor, not with the FB resistor setting. The FB resistor setting is an overvoltage protection setting used when the LEDs string is removed. This voltage needs to be set higher than the maximum LED string setting by about 10%.

    The issue here is that currently, you have this setting about about 36-37V. That is incorrect.

    For a 37V LED string, it should be set at about 41V.

    However, 41V OVP is a bit high for this part with its absolute maximum switch voltage rating. There is some overshoot to account for if the LEDs are ever removed, and that is risky with 37V of LEDs and 41V OVP setting.

    You either need a very big output cap and about 40.5V OVP setting (with the FB resistors), or you need to decrease your LED string voltage (less LEDs) with this IC or use a different IC with higher peak switch voltage setting.

    Good luck,

    KSz

  • Hello,

    thanks for advice, you have absolutely right, I test it today. I expect less than 10%, but that's it.

     

    And I have second issue to confirm. I notice, device is pretty hot. I measured almost 60°C on package and if I consider device is up to 1,5A..... So, is this warming normal? (Yes, I have 2-layer board and thermal pad drain heat off).

     

    Thanks.

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