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LT3760

3760fc

applicaTions inForMaTion

userscanlogontowww.murata.com/designlibanddown-

load the software followed by instructions for creating an

output voltage ‘VOUT’ (LT3760 CTRL pin voltage) from a

specified VCC supply (LT3760 VREF 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 TSET Pin for Thermal Protection

The LT3760 contains a special programmable thermal

regulationloopthatlimitstheinternaljunctiontemperature

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

dissipatedinthepackage.Thistopologyprovidesexcellent

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

featureprovidesimportantprotectionathighambienttem-

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.

WhilethisfeatureisintendedtodirectlyprotecttheLT3760,

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.

Two external resistors program the maximum IC junction

temperature using a resistor divider from the VREF pin,

as shown in Figure 9. Choose the ratio of R1 and R2 for

the desired junction temperature. Figure 10 shows the

relationship of TSET voltage to junction temperature, and

Table 8 shows commonly used values for R1 and R2.

Figure 9. Programming the TSET Pin

3760 F09

LT3760

VREF

TSET

Figure 10. Programing the TSET Pin Threshold

JUNCTION TEMPERATURE (°C)

500

600

700

800

100

125

150

950

550

650

750

850

900

3760 F10

VPTAT

Table 8. Resistor Values to Program Maximum IC Junction

Temperature (VREF (Typical) = 1.485V)

TJ (°C)

R1 (kΩ)

R2 (kΩ)

TSET (V)

100

24.9

0.824

115

28.0

0.866

130

30.9

0.902

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