LT3590
APPLICATIONS INFORMATION
Direct PWM Dimming
48V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Changing the forward current flowing in the LEDs not only
changestheintensityoftheLEDs,italsochangesthecolor.
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.
Example:
ƒ =100Hz, tSETTLE = 50μs
1
1
tPERIOD = =
ƒ 100
= 0.01s
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.
tPERIOD
tSETTLE 50μs
0.01s
Dim Range=
=
= 200:1
tSETTLE
tPERIOD
50μs
0.01s
Min Duty Cycle =
•100=
•100= 0.5%
Duty Cycle Range =100%→0.5% at 100Hz
Thecalculationsshowthatfora100Hzsignalthedimming
range is 200 to 1. In addition, the minimum PWM duty
cycle of 0.5ꢀ ensures that the LED current has enough
time to settle to its final value. Figure 7 shows the dim-
ming range achievable for three different frequencies with
a settling time of 50μs.
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
PWM
5V/DIV
100Hz
V
SW
20V/DIV
1kHz
10kHz
I
LED
20mA/DIV
3590 F06
V
= 48V
2ms/DIV
IN
1
10
100
1000
4 LEDs
PWM DIMMING RANGE
3590 F07
Figure 6. Direct PWM Dimming Waveforms
Figure 7. Dimming Range Comparison
of Three PWM Frequencies
3590f
10
Linear [ Linear ]
