AN2841 datasheet(5/31 Pages) STMICROELECTRONICS

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AN2841

Description of LED dimming

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Description of LED dimming

There are two categories of LED dimming:

●

Analog: Uses a linear device to set the current flow through the LED.

Advantages: Absence of digital noise; a simple implementation.

Limitations: Color shift, low dynamics.

●

Digital: Uses a switch to set nominal or zero current flow through the LED.

Advantages: Accurate and fast color control.

Limitations: The presence of digital noise and necessity to use a frequency high

enough to prevent flickering; a complex implementation.

This document focuses on digital dimming and the use of PWM (pulse width modulation) in

common applications.

When using PWM for lighting applications, the following rules must be kept in mind:

●

For the LED light to be seen as non-flickering by the human eye, in most cases the

PWM frequency must be higher than 100 Hz.

●

The LED light brightness depends on the PWM duty cycle. The brightness depends on

the resolution of the PWM duty cycle.

Figure 1.

PWM signal with 16 steps of possible voltage values

Using a 4-bit PWM resolution, there can be 16 different light intensity (energy) values.

Concerning color applications, 16 million (256 × 256 × 256) colors can be reached using

tricolor LEDs with an 8-bit dimming resolution for each single color LED.

Step: 0

9 10 11 12 13 14 15 0

1 ...

TPWM

AM00341

AN2841 Datasheet(PDF) 5 Page - STMicroelectronics