LT3598
13
3598fb
Table 5. NTC Resistor Manufacturers/Distributors
Murata Electronics North America 770-436-1300
www.murata.com
TDK Corporation 516-535-2600
www.tdk.com
Digi-Key 800-344-4539
www.digikey.com
If calculating the 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 overtemperature.
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 behavior) overtemperature. Referred to as the
“Murata Chip NTC Thermistor Output Voltage Simulator,”
users can log onto www.murata.com and download
the software followed by instructions for creating an
output voltage V
OUT
(CTRL) from a specifi ed V
CC
supply
(V
REF
).
Using the T
SET
Pin for Thermal Protection
The LT3598 contains a special programmable thermal
regulation loop that limits the internal junction temperature
of the part. Since the LT3598 topology consists of a single
boost converter with six 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
APPLICATIONS INFORMATION
(which means a higher junction temperature). Being able
to limit the maximum junction temperature allows the
benefi ts of this topology to be fully realized. This thermal
regulation feature provides important protection at high
ambient temperatures, and allows a given application
to be optimized for typical, not worst-case, ambient
temperatures with the assurance that the LT3598 will
automatically protect itself and the LED strings under
worst-case conditions.
The operation of the thermal loop is simple. As the ambient
temperature increases, so does the internal junction
temperature of the part. An internal voltage is developed
that’s proportional to the junction temperature (V
PTAT
).
Once the programmed maximum junction temperature
is reached, the LT3598 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 temperature 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 LT3598,
it can also be used to derate the LED current at high
temperatures. Since there is a direct relationship between
the LED temperature and LT3598 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 8. Choose the ratio of R1 and R2 for
the desired junction temperature. Figure 9 shows the
relationship of T
SET
voltage to junction temperature, and
Table 6 shows commonly used values for R1 and R2.
Figure 7 . LED Current Derating vs Temperature Using NTC Resistor
3598 F07
R
Y
R
Y
R
X
R
X
R
NTC
R
NTC
R
NTC
R
NTC
DCBA
LT3598
V
REF
CTRL
R2
R1
(OPTION A TO D)
