A Digital Constant Current Power LED Driver

AN1138 A Digital Constant Current Power LED Driver drive current for this type of LED would be 350 mA and Author: Stephen Bowling a typical forward voltage would be 3.5V. Secondly, we Microchip Technology Inc. will assume that the circuit input voltage is 12V. The inductor value will be chosen to allow a maximum current ripple of +/- 20%. The eye will not be able to INTRODUCTION perceive this current ripple since the switching frequency is much faster than can be detected. This document describes a power LED driver solution Considering this fact, you might be tempted to further using the PIC12HV615 microcontroller (MCU). The increase the allowable current ripple to reduce the PIC12HV615 is an 8-pin MCU with many integrated inductor value. However, the +/- 20% limit is a rule of analog features. The LED driver circuit is a buck (step- thumb to minimize efficiency losses in the LED. You will down) solution and the circuit presented here can want to check the LED manufacturer's data sheet to operate from most any input voltage source as long as verify the maximum peak current. it exceeds the forward voltage of the LEDs to be driven. The switching frequency of the buck regulator is A proportional-integral (PI) controller algorithm is used chosen to be 125 kHz. This frequency was chosen to regulate the LED current to a constant value. The PI based on the selected operating frequency of the MCU controller is executed at a rate of 976 Hz, leaving plenty and the resulting control resolution that can be of CPU time available for other tasks. Although this obtained from the hardware PWM module. When sample rate would provide inadequate control operated from the internal 8 MHz RC oscillator, the response for most power supply applications, it works ECCP module can provide 6 bits (64 steps) of duty well for LED applications because the LED presents a cycle resolution at 125 kHz. If this control resolution is constant load to the power stage. Therefore, the not sufficient, the PWM frequency can be lowered or controller does not need to make frequent adjustments. the MCU can be operated with an external oscillator The LED current is sampled using a resistor in series circuit to provide a higher operating frequency. with the source of the MOSFET in the buck circuit and A summary of the operating parameters used for amplified using a single op amp. The LED current is design calculations is provided in Table 1 below. sampled using one of the available ADC inputs on the PIC12HV615. The Enhanced Capture Compare PWM (ECCP) module of the MCU is used in PWM mode to TABLE 1: BUCK REGULATOR drive the buck circuit. Since the MCU has an internal OPERATING PARAMETERS voltage regulator and 8 MHz oscillator, very few Parameter Value external components are required to complete the circuit. Input Voltage, V 12V IN LED Forward Voltage, V 3.5V f CIRCUIT DESCRIPTION LED Current, I 0.35A f Current Ripple, ?I+/- 20% A detailed schematic