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LT3760
APPLICATIONS INFORMATION
JUNCTION TEMPERATURE (°C)
userscanlogonto www.murata.com/designlib anddown-
load the software followed by instructions for creating an
output voltage ‘V OUT ’ (LT3760 CTRL pin voltage) from a
specified V CC supply (LT3760 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 LT3760 contains a special programmable thermal
regulation loop that limits the internal junction temperature
of the part. Since the LT3760 topology consists of a single
boost controller with eight 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 LT3760 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 LT3760 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 LT3760,
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 LT3760 junction temperature, the
TSET function also provides some LED current derating
at high temperatures.
Two external resistors program the maximum IC junction
temperature using a resistor divider from the V REF pin,
as shown in Figure 9. Choose the ratio of R1 and R2 for
the desired junction temperature. Figure 10 shows the
relationship of T SET voltage to junction temperature, and
Table 8 shows commonly used values for R1 and R2.
3
V REF
R2 LT3760
2
T SET
R1
3760 F09
Figure 9. Programming the T SET Pin
950
900
850
800
750 V PTAT
700
650
600
550
500
0 25 50 75 100 125 150
3760 F10
Figure 10. Programing the T SET Pin Threshold
Table 8. Resistor Values to Program Maximum IC Junction
Temperature (V REF (Typical) = 1.485V)
T J (°C) R1 (kΩ) R2 (kΩ) T SET (V)
100 24.9 20 0.824
115 28.0 20 0.866
130 30.9 20 0.902
3760fc
19
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