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LT3754

3754f

APPLICATIONS INFORMATION

If calculation of 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 over temperature. 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 pin

behavior) over temperature. Referred to on the website

as the ‘Murata Chip NTC Thermistor Output Voltage

Simulator’, users can log onto www.murata.com/designlib

and download the software followed by instructions for

creating an output voltage ‘VOUT’(LT3754CTRLpinvoltage)

from a speciﬁed VCC supply (LT3754 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.

Figure 8. Programmed CTRL Voltage vs Temperature

Using the TSET Pin for Thermal Protection

The LT3754 contains a special programmable thermal

regulation loop that limits the internal junction temperature

of the part. Since the LT3754 topology consists of a single

boost controller with sixteen 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 beneﬁts 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 LT3754 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 LT3754 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 LT3754,

it can also be used to derate the LED current at high

temperatures. Since there is a direct relationship between

the LED temperature and LT3754 junction temperature, the

TSET function also provides some LED current derating

at high temperatures.

TA - AMBIENT TEMPERATURE (°C)

1.50

1.25

1.00

0.75

0.50

0.25

3754 F08

RESISTOR

OPTION A

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