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LT3754
APPLICATIONS INFORMATION
1.50
1.25
1.00
RESISTOR
OPTION A
0.75
0.50
0.25
0 10 20 30 40 50 60 70 80
T A - AMBIENT TEMPERATURE (°C)
3754 F08
Figure 8. Programmed CTRL Voltage vs Temperature
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
different resistor networks and NTC resistor values and
then simulate the exact output voltage curve (CTRL pin
behavior) over temperature. Referred to on the website as
the ‘Murata Chip NTC Thermistor Output Voltage Simula-
tor’, users can log onto www.murata.com/designlib and
download the software followed by instructions for creating
an output voltage ‘V OUT ’ (LT3754 CTRL pin voltage) from
a specified V CC supply (LT3754 V REF pin voltage). At any
time during selection of circuit parameters the user can
access data on the chosen NTC resistor by clicking on
the link to the Murata catalog. For a detailed example of
hand calculations using an NTC type resistor divider to
program CTRL pin voltage, read the LT3478 LED driver
data sheet section Programming LED Current Derating vs
Temperature under Applications Information.
Using the T SET Pin for Thermal Protection
The LT3754 contains a special programmable thermal
regulation loop that limits the internal junction temperature
of the part. Since the LT3754 topology consists of a single
boost controller with sixteen linear current sources, any LED
string voltage mismatch will cause additional power to be
dissipated in the package. This topology provides excellent
current matching between LED strings and allows a single
power stage to drive a large number of LEDs, but at the
price of additional power dissipation inside the part (which
means a higher junction temperature). Being able to limit
the maximum junction temperature allows the benefits of
this topology to be fully realized. This thermal regulation
feature provides important protection at high ambient tem-
peratures, and allows a given application to be optimized
for typical, not worst-case, ambient temperatures with
the assurance that the LT3754 will automatically protect
itself and the LED strings under worst-case conditions.
The operation of the thermal loop is simple. As the ambi-
ent temperature increases, so does the internal junction
temperature of the part. Once the programmed maximum
junction temperature is reached, the LT3754 begins to
linearly reduce the LED current, as needed, to try and
maintain this temperature. This can only be achieved
when the ambient temperature stays below the desired
maximum junction temperature. If the ambient tempera-
ture continues to rise past the programmed maximum
junction temperature, the LEDs current will be reduced
to approximately 5% of the full LED current.
While this feature is intended to directly protect the LT3754,
it can also be used to derate the LED current at high tem-
peratures. Since there is a direct relationship between the
LED temperature and LT3754 junction temperature, the
TSET function also provides some LED current derating
at high temperatures.
3754fc
19
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