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LT3754
APPLICATIONS INFORMATION
Programming LED Current Derating (Breakpoint and
Slope) versus LED Ambient Temperature (CTRL Pin)
LED data sheets provide curves of maximum allowed
LED current versus ambient temperature to warn against
damaging of the LED (Figure 6). The LT3754 LED driver
improves the utilization and reliability of the LED(s) by al-
lowing the programming of an LED current derating curve
versus the ambient temperature of the LED(s). Without the
ability to back off LED currents as temperature increases,
many LED drivers are limited to driving the LED(s) at 50%
or less of their maximum rated currents. This limitation
requires more LEDs to obtain the intended brightness
for the application. The LT3754 allows the LED(s) to be
programmed for maximum allowable current while still
protecting the LED(s) from excessive currents at high tem-
perature. The temperature breakpoint and the slope of LED
current versus ambient temperature can be programmed
using a simple resistor network shown in Figure 7.
This is achieved by programming a voltage at the CTRL
pin with a negative temperature coefficient using a resistor
divider with temperature dependent resistance (Figures 7
and 8). A variety of resistor networks and NTC resistors
with different temperature coefficients can be used to
achieve the desired CTRL pin voltage behavior versus
temperature. The current derating curve in Figure 6 uses
the resistor network shown in option C of Figure 7.
Table 7 shows a list of NTC resistor manufacturers/ distribu-
tors. There are several other manufacturers available and
the chosen supplier should be contacted for more detailed
information. To use an NTC resistor to monitor the ambient
temperature of the LED(s) it should be placed as close as
possible to the LED(s). Since the temperature dependency
of an NTC resistor can be non-linear over a wide range of
temperatures it is important to obtain a resistor’s exact
values over temperature from the manufacturer. Hand
calculations of CTRL voltage can then be performed at
each given temperature and the resulting CTRL voltage
plotted versus temperature.
Table 7. NTC Resistor Manufacturer s
MANUFACTURER WEB
Murata Electronics North America www.murata.com
TDK Corporation www.tdk.com
Digi-key www.digikey.com
90
80
70
8 LED STRINGS (2 × 40mA PER STRING)
RESISTOR
OPTION A
R1
31
29
V REF
CTRL
LT3754
60
50
40
30
20
10
LT3754
PROGRAMMED LED
CURRENT DERATING
CURVE
R NTC
R2
R NTC
OPTION A TO D
R X R NTC
R Y
R NTC
R Y
R X
0
0
10
20 30 40 50 60
70
80
T A -TEMPERATURE (°C)
3754 F06
A
B
C
D
3754 F07
Figure 6. LED Current Derating vs LED Ambient Temperature
Figure 7. Programming LED Current Derating Curve
vs Ambient Temperature (R NTC Located on LED PCB)
3754fc
18
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