Microchip Technology APGRD004 Data Sheet

        LED Lighting Solutions Design Guide 

www.microchip.com/lighting

Some power supply applications require fast dynamic 
response to compensate for load changes. In these 
applications, a fast ADC and fast calculation performance 
are required. However, a LED provides a stable load for a 
constant-current power supply. Therefore, a fast ADC and fast 
processing power are not always required to implement a 
digital control loop for a LED driver application. 

A low-cost device in the PIC12F or PIC16F family with a CCP 
peripheral and an ADC can be used to implement a LED 
driver using digital control. The CCP peripheral is used in 
PWM mode to control the power supply circuit. Operating 
from the internal 8 MHz device oscillator, the CCP can 
provide PWM frequencies above 100 KHz to keep power 
component sizes small. Since the LED provides a constant 
load, it is sufficient to sample the output current and adjust 
the PWM duty cycle at a much lower rate. A sample rate of 
1000 Hz is ideal for many applications.

LEDs can be driven with a fully digital control loop. Instead 
of measuring the LED current with an op amp or comparator 
circuit, the LED current is sampled using an ADC. Some 
type of digital algorithm replaces the analog control loop. 
A proportional-integral-derivative (PID) control algorithm is 
commonly used because it has software coefficients that 
can readily be adjusted to affect the controller behavior. A 
digital PWM peripheral is used to drive the LED. The digital 
algorithm computes an output based on its inputs and 
provides the duty cycle for the PWM peripheral.

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AN1138 – A Digital Constant Current Power LED Driver,

 DS01138

Set Point

Feedback

Controller
Output

Feedback

Controller
Output

Feedback Loop

Set

Set Point

ADC

PWM

PID or

Digital Filter

Algorithm

–

+

LED
String

15V

5V

Current

Sense

Filter

ADC Input

DC Bus

125 KHz

PWM