Datasheet

LT3760
18
3760fc
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 LT3760 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 A 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
in
formation. To use an NTC resistor to monitor the ambient
temperature of the LED(s) it should be placed as close as
applicaTions inForMaTion
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
If calculation of CTRL voltage at various temperatures gives
a downward slope that is too strong, alternative resistor
networks can be chosen (B,C,D in Figure 7) which use
temperature independent resistance to reduce the effects
of the NTC resistor over temperature. Murata Electronics
provides a selection of NTC resistors with complete data
over a wide range of temperatures. In addition, a software
tool is available which allows the user to select from dif-
ferent resistor networks and NTC resistor values and then
simulate the exact output voltage curve (CTRL pin behav-
ior) over temperature. Referred to on the website as the
‘Murata Chip NTC Thermistor Output Voltage Simulator’,
LT3760
V
REF
CTRL
3
1
R1
3760 F07
R2
R
Y
R
Y
R
X
R
X
R
NTC
R
NTC
R
NTC
R
NTC
DCBA
OPTION A TO D
Figure 7. Programming LED Current Derating Curve
vs Ambient Temperature (R
NTC
Located on LED PCB)
Figure 8. Programmed CTRL Voltage vs Temperature
T
A
- AMBIENT TEMPERATURE (°C)
0
CTRL VOLTAGE (V)
1.50
1.25
1.00
0.75
0.50
0.25
30 40 706020 50
3760 F08
8010
RESISTOR
OPTION A