Why is Arduino resetting while driving motors? I am driving motors for my line-follower project . The circuit schematic is given below 

The power source is a 11.1V 2200mAh 25C Lipo . Practically the cells give 11.5-12V. I used a switching buck regulator to step down the voltage. However , when i run my robot on track after few seconds,the arduino tends to reset and run again. This problem typically arises when i drive the motors at higher PWM(above 180 on analogWrite function). Since i use PID, limiting the PWM means i have to drive at lower speeds which i do not want.

I am providing links for the items i used for my circuit:


buck converter;
motor driver;
IR sensor module;
DC motors.


The sensor array draws 150mA at peak. Any suggestions as to where the problem might lie would be of great help. Thank you . 

EDIT 1: Changed the input voltage Vin to 7.9V through the buck converter . 
        The problem still persists .
The VIN pin goes to a 5V voltage regulator on the Arduino and needs at least about 7V minimum to work properly.  If you want to supply 5V to an Arduino do it either on the 5V pin or via the USB connector.  The VIN pin should receive 7V to 12V.