Datasheet

LT3599
14
3599fe
APPLICATIONS INFORMATION
Using the T
SET
Pin for Thermal Protection
The LT3599 contains a special programmable thermal
regulation loop that limits the internal junction temperature
of the part. Since the LT3599 topology consists of a single
boost converter with four 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 benefi ts 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 LT3599 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 LT3599 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 LT3599,
it can also be used to derate the LED current at high
temperatures. Since there is a direct relationship between
the LED temperature and LT3599 junction temperature, the
T
SET
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. Table 6 shows commonly
used values for R1 and R2 (see T
SET
graph).
Table 6. Resistor Values to Program Maximum IC Junction
Temperature
T
J
(°C) R1 (k) R2 (k)
100 80.6 53.6
105 82.5 53.6
110 82.5 51.1
115 84.5 51.1
120 84.5 49.9
135 84.5 44.2
145 90.9 44.2
Programming Switching Frequency
The switching frequency of the LT3599 is set between
200kHz and 2.1MHz by an external resistor connected
between the RT pin and ground (see Table 7). Do not
leave this pin open.
Selecting the optimum switching frequency depends
on several factors. Inductor size is reduced with higher
frequency, but effi ciency drops due to higher switching
losses. In addition, some applications require very high duty
cycles to drive a large number of LEDs from a low supply.
Low switching frequency allows a greater operational duty
cycle and, hence, a greater number of LEDs to be driven.
In each case, the switching frequency can be tailored to
provide the optimum solution. When programming the
switching frequency, the total power losses within the IC
should be considered.
Table 7. Switching Frequency
SWITCHING FREQUENCY (MHz) R
T
(k)
2.1 20
2.0 21.5
1.5 31.6
1.0 53.6
0.5 121
0.4 154
0.3 210
0.2 324
Figure 8. Programming the T
SET
Pin
3599 F08
LT3599
V
REF
T
SET
R2
R1