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PWM Output Current Control

An LED operates at its maximum efficiency when

operated at the rated drive current specified by

the manufacturer. Operating an LED at lower than

its rated forward current not only decreases the

system efficiency; but may cause color (or wave-

length) shifting. In illumination applications, this

could cause visible changes to lighting.

A preferred method is using pulse width modula-

tion (PWM). As shown at left, the output current

input. By varying the signal duty cycle the average

output current is adjusted up or down. To avoid

visible flicker, the PWM signal should be greater

than 200 Hz.

For duty cycles (DPWM) between 0.1 and 1, the

output current is derived by the formula:

II

X D

NOM

MAX

PWM

=

tor transistor (as shown). The diode and resistor

suppress high amplitude negative spikes that may

be caused by the drain-source capacitance of the

transistor. Negative spikes on the control input of

the unit could cause errors in output current or

erratic operation.

output of a microcontroller. Again, any high amplitude

negative spikes that may be caused by the drain-

source capacitance of the FET must be supressed.

PWM Control Signals

PWM Dimming Application

A simple method of achieving digital (or PWM) dimming is by using a 555 timer

ates over a supply voltage range of 4.5 VDC to 18VDC. Here it is connected

Excess ripple could cause timing errors.

The timer is connected for astable (free run) operation. The frequency is set

again. The formulas for calculating the frequency and duty cycle are included

in the MPD application note “Driving LEDs”.

allow for duty cycles over a range of approximately 5% to 95%. If manual

adjustment of the circuit).

sible. In order to avoid excessive current flow through the internal discharge