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LT3598

3598fa

LED Current Derating Using the CTRL Pin

A useful feature of the LT3598 is its ability to program a

derating curve for maximum LED current versus tempera-

ture. LED data sheets provide curves of maximum-allowable

LED current versus temperature to warn against exceed-

ing this current limit and damaging the LED. The LT3598

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 CTRL pin with a nega-

tive temperature coefﬁcient using a resistor divider with

temperature dependent resistance (Figure 7). As ambient

temperature increases, the CTRL voltage will fall below the

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 deﬁned by the

choice of resistor ratios and use of temperature-dependent

resistance in the divider for the CTRL pin.

Table 5 shows a list of manufacturers/distributors of

NTC resistors. There are several other manufacturers

available 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 temperature dependency of an NTC resistor can be

nonlinear over a wide range of temperatures, it is important

to obtain a resistor’s exact value over temperature from

APPLICATIONS INFORMATION

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

TDK Corporation

516-535-2600

www.tdk.com

Digi-Key

800-344-4539

www.digikey.com

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 volt-

age curve (CTRL behavior) overtemperature. Referred

to as the “Murata Chip NTC Thermistor Output Voltage

Simulator,” users can log onto www.murata.com and

download the software followed by instructions for creat-

ing an output voltage VOUT (CTRL) from a speciﬁed VCC

supply (VREF).

Figure 7 . LED Current Derating vs Temperature Using NTC Resistor

3598 F07

RNTC

LT3598

VREF

CTRL

(OPTION A TO D)

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