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LT3599
APPLICATIONS INFORMATION
PROGRAMMING LED CURRENT DERATING
vs TEMPERATURE
Programming LED Current Derating Using the CTRL Pin
A useful feature of the LT3599 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
LT3599 allows the output LEDs to be programmed for
maximum allowable current while still protecting the
LEDs from excessive currents at high temperature. This is
temperature dependency of an NTC resistor can be non-
linear over a wide range of temperatures, it is important
to obtain a resistor’s exact values over temperature from
the manufacturer. Hand calculations of CTRL voltage can
then be performed at each given temperature, resulting
in the CTRL versus temperature plotted curve. Several
iterations of resistor value calculations may be required
to achieve the desired breakpoint and slope of the LED
current derating curve.
Table 5. NTC Resistor Manufacturers/Distributors
Murata Electronics North America (770) 436-1300
www.murata.com
achieved by programming a voltage at the CTRL pin with
a negative temperature coef?cient using a resistor divider
with temperature dependent resistance (Figure 7). As the
temperature increases, the CTRL voltage will fall below the
TDK Corporation
Digi-Key
(516) 535-2600
www.tdk.com
(800) 344-4539
www.digikey.com
internal 1V voltage reference, causing LED currents to be
controlled by the CTRL pin voltage. The LED current curve
breakpoint and slope versus temperature is de?ned by the
choice of resistor ratios and use of temperature-dependent
resistance in the divider for the CTRL pin.
A variety of resistor networks and NTC resistors with
different temperature coef?cients can be used for pro-
gramming CTRL to achieve the desired CTRL curve vs
temperature.
Table 5 shows a list of manufacturers/distributors of NTC
resistors. There are several other manufacturers avail-
able and the chosen supplier should be contacted for
more detailed information. If an NTC resistor is used to
indicate LED temperature, it is effective only if the resistor
is connected as closely as possible to the LED strings.
LED derating curves shown by manufacturers are listed
for ambient temperature. The NTC resistor should have
the same ambient temperature as the LEDs. Since the
If calculating the 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 overtemperature.
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 behavior) over temperature. Referred to as
the “Murata Chip NTC Thermistor Output Voltage Simula-
tor,” users can log onto www.murata.com/designlib and
download the software followed by instructions for creat-
ing an output voltage V OUT (CTRL) from a speci?ed V CC
supply (V REF ). At any time during the selection of circuit
parameters, the user can access data on the chosen NTC
resistor by clicking on a link to the Murata catalog.
R2
V REF
LT3599
R Y
R Y
CTRL
R NTC
R NTC
R X R NTC
R NTC
R X
R1
(OPTION A TO D)
A
B
C
D
3599 F07
Figure 7 . LED Current Derating vs Temperature Using NTC Resistor
3599fe
13
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