LT3754 datasheet(14/24 Pages) LINER

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LT3597

3597f

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. TSET Programmed Junction Temperature

TJ (°C)

R1 (kΩ)

R2 (kΩ)

49.9

97.6

100

49.9

90.9

115

49.9

84.5

The TSET pin must be tied to VREF 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

(OPTION

A TO D)

VREF

RNTC

–50

0.4

0.5

125

3597 F07

0.3

0.8

0.6

0.7

–25

100

TEMPERATURE (°C)

Figure 7. TSET Voltage for Temperature Derating

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