Datasheet

LT3760
19
3760fc
applicaTions inForMaTion
users can log onto www.murata.com/designlib and down-
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.
Tw o
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.
Figure 9. Programming the T
SET
Pin
3760 F09
LT3760
V
REF
T
SET
R2
R1
3
2
Figure 10. Programing the T
SET
Pin Threshold
JUNCTION TEMPERATURE (°C)
0
500
V
TSET
THRESHOLD (mV)
600
700
800
50
100 125
25
75
150
950
550
650
750
850
900
3760 F10
V
PTAT
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