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LT3743 shuts down when I switch from a low current to a high, while increasing slowly works

Question asked by lexel on Jul 23, 2018
Latest reply on Jul 23, 2018 by ARad

Hello,

 

I am developing a 400W buck driver for a big flashlight company.

 

I use the LT3743 buck chip with 4 LEDs in series at 30A shown as TYPICAL APPLICATIONS 24V, 20A 3-LED Driver on page 24 of the datasheet, MOSFETS are doubled to manage this current, all other components are calculated like the datasheet gives me the formulas and values.

I used the calculations to design the driver.

 

It works if I slowly increase or decrease the PWM value  and or Ctrl_H input voltage.


It is regulated for very low brightness using PWM input cycle decreasing with a fixed 10%(150mV) on CTRL_H, then going higher output the PWM cycle increases to 100% after that CTRL_H increasing slowly to 1.5V max control voltage.

So I can regulate the output from 50mA to 30A

 

The problem comes when I am not slowly increase the brightness jumping from a low to a high output setting from the MCU.

At this point jumping to a current more than 2A I see only a short flash then there is no more output.

When I slowly decrease output the LED output comes back alive at about 1.7A and can be slowly increased to 30A.

 

I am not sure which protection trips here, I disabled capacitor output voltage protection put on every part a cap to block noise or spikes, but I cant fix the problem.


Best regards

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