Datasheet

LT3597
14
3597fa
applicaTions inForMaTion
Table 6 shows commonly used values for R1 and R2.
Choose the ratio of R1 and R2 for the desired junction
temperature limit as described in Figure 7.
Table 6. T
SET
Programmed Junction Temperature
T
J
(°C) R1 (kΩ) R2 (kΩ)
85 49.9 97.6
100 49.9 90.9
115 49.9 84.5
The T
SET
pin must be tied to V
REF
if the temperature pro-
tection feature is not desired.
Figure 8. Programming the CTRLM Pin
LED Current Derating Using the CTRLM Pin
Another feature of the LT3597 is its ability to program a
derating curve for maximum LED current versus tem-
perature. LED data sheets provide curves of maximum
allowable LED current versus temperature to warn against
exceeding this current limit and damaging the LED. The
LT3597 allows the output LEDs to be programmed for
maximum allowable current while still protecting the
LEDs from excessive currents at high temperature. This
is achieved by programming a voltage at the CTRLM pin
with a negative temperature coefficient using a resistor
divider with temperature dependent resistance (Figure 8).
As ambient temperature increases, the CTRLM voltage
will fall below the internal 1V voltage reference, causing
LED currents to be controlled by the CTRLM pin voltage.
The LED current curve breakpoint and slope versus tem-
perature are defined by the choice of resistor ratios and
use of temperature-dependent resistance in the divider
for the CTRLM pin.
LT3597
3597 F08
CTRLM
R2
R1
(OPTION
A TO D)
V
REF
A
R
NTC
C
R
NTC
B
R
NTC
R
X
D
R
NTC
R
X
R
Y
R
Y
–50
T
SET
VOLTAGE (V)
0.4
0.5
75 125
3597 F07
0.3
0.8
0.6
0.7
50250–25 100
TEMPERATURE (°C)
Figure 7. T
SET
Voltage for Temperature Derating