strange problem with Serial.print

Hi guys,

I experience a strange problem with serial communication with my project.
I realized a buckboost switched power supply with Arduino to power HB LEDs (see this post : to see schemes).
The project works very well and I use Serial functions to get debug informations.

When LEDs are driven with current bellow 0.7A. Serial.print function seem to works as expected. After that, serial communication display something totally wrong (you can see screenshot attached) !
For information, I do not use a USB connector directly attached to the arduino uno board. But use a FTDI board with Rx & Tx connected to the arduino uno (i do this to avoid USB power & battery to be connected at the same time on the board).

Furthermore, when I change the current flawing to the LED (by changing duty cycle/OCR1A register), serial communications return to normal operations. So the arduino is not crashed !

Did anyone understand what’s going on ?

Thanks for your help.