Datasheet

LT3590
10
3590f
Direct PWM Dimming
Changing the forward current fl owing in the LEDs not only
changes the intensity of the LEDs, it also changes the color.
The chromaticity of the LEDs changes with the change in
forward current. Many applications cannot tolerate any
shift in the color of the LEDs. Controlling the intensity of
the LEDs with a direct PWM signal allows dimming of the
LEDs without changing the color. In addition, direct PWM
dimming offers a wider dimming range to the user.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second.
By increasing the PWM frequency to ~80Hz or higher,
the eye will interpret that the pulsed light source is con-
tinuously on. Additionally, by modulating the duty cycle
(amount of “on-time”), the intensity of the LEDs can be
controlled. The color of the LEDs remains unchanged in
this scheme since the LED current value is either zero or
a constant value.
The time it takes for the LED current to reach its pro-
grammed value sets the achievable dimming range for a
given PWM frequency. For example, the settling time of
the LED current in Figure 6 is approximately 50μs for a
48V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
ƒ
ƒ
==
== =
100 50
11
100
001
Hz t μs
ts
D
SETTLE
PERIOD
,
.
iim Range
t
t
s
μs
Min
PERIOD
SETTLE
.
:===
001
50
200 1
•
.
•Duty Cycle
t
t
μs
s
SETTLE
PERIOD
==100
50
001
1000 0 5
100 0 5 100
=
=→
.%
%.%Duty Cycle Range at Hz
The calculations show that for a 100Hz signal the dimming
range is 200 to 1. In addition, the minimum PWM duty
cycle of 0.5% ensures that the LED current has enough
time to settle to its fi nal value. Figure 7 shows the dim-
ming range achievable for three different frequencies with
a settling time of 50μs.
APPLICATIONS INFORMATION
Figure 7. Dimming Range Comparison
of Three PWM Frequencies
Figure 6. Direct PWM Dimming Waveforms
I
LED
20mA/DIV
V
SW
20V/DIV
PWM
5V/DIV
V
IN
= 48V
4 LEDs
2ms/DIV
3590 F06
PWM DIMMING RANGE
1 10 100 1000
3590 F07
100Hz
10kHz
1kHz