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LT3754 Datasheet(PDF) 18 Page - Linear Technology

Part No. LT3754
Description  16-Channel × 50mA LED Driver
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT3754 Datasheet(HTML) 18 Page - Linear Technology

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LT3754
18
3754f
Programming LED Current Derating (Breakpoint and
Slope) versus LED Ambient Temperature (CTRL Pin)
LED datasheets 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
allowing 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. The LT3754 allows
the temperature breakpoint and the slope of LED current
versus ambient temperature to be programmed using a
simple resistor network shown in Figure 7.
Without the ability to back off LED current as the ambient
temperature of the LED(s) increases, many LED drivers are
limited to driving the LED(s) at only 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 temperature. This
is achieved by programming a voltage at the CTRL pin
with a negative temperature coefficient using a resistor
APPLICATIONS INFORMATION
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 behaviour 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/
distributors. 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 Manufacturers
MANUFACTURER
WEB
Murata Electronics North America
www.murata.com
TDK Corporation
www.tdk.com
Digi-key
www.digikey.com
LT3754
REF
CTRL
31
29
R1
3754 F07
R2
RY
RY
RX
RX
RNTC
RNTC
RNTC
RNTC
D
C
B
A
OPTION A TO D
Figure 7. Programming LED Current Derating Curve
vs Ambient Temperature (RNTC Located on LED PCB)
Figure 6. LED Current Derating vs LED Ambient Temperature
TA-TEMPERATURE (°C)
–50
250
225
200
175
150
125
100
25
100
75
050
3754 F06
125
–25
RESISTOR
OPTION A
LT3754
PROGRAMMED LED
CURRENT DERATING
CURVE


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