LT3590EDC#PBF Linear Technology, LT3590EDC#PBF Datasheet - Page 10

• Manufacturer Part Number: LT3590EDC#PBF
• Description: IC, LED DRIVER, BULK MODE, 48V
• Manufacturer: Linear Technology

Specifications of LT3590EDC#PBF

• Led Driver Application: LED Fixed Signage, Traffic Signs
• No. Of Outputs: 1
• Output Current: 50mA
• Output Voltage: 55V
• Input Voltage: 4.5V To 55V
• Dimming Control Type: PWM / DC
• Operating: RoHS Compliant
• Topology: Buck

LT3590
APPLICATIONS INFORMATION
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
10
20mA/DIV
20V/DIV
5V/DIV
PWM
Figure 6. Direct PWM Dimming Waveforms
V
I
LED
SW
V
4 LEDs
IN
= 48V
2ms/DIV
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48V input voltage. The achievable dimming range for this
application and 100Hz PWM frequency can be determined
using the following method.
Example:
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.
The calculations show that for a 100Hz signal the dimming
D
Min
Duty Cycle Range
ƒ
t
PERIOD
i i m Range
=
100
Duty Cycle
Hz t
=
Figure 7. Dimming Range Comparison
of Three PWM Frequencies
1
,
ƒ
1
=
=
10kHz
SETTLE
t
t
100
PERIOD
SETTLE
1
=
PWM DIMMING RANGE
10
t
t
=
=
PERIOD
SETTLE
=
100
0 01
50
=
1kHz
.
0 01
μs
50
%
s
.
•
μs
→
100
100
s
0 5
=
. %
100Hz
=
200 1
0 01
50
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.
at
:
1000
μs
s
100
•
10 0 0 0 5
Hz
=
. %
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