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LT3478/LT3478-1

34781f

LT3478/LT3478-1

VREF

CTRL2

CTRL1

3478 F07

RNTC

OPTION A TO D

APPLICATIO S I FOR ATIO

CTRL2 to achieve the desired CTRL2 curve vs temperature.

The current derating curve shown in Figure 6 uses the

resistor network shown in option C of Figure 7.

to obtain a resistor’s exact values over temperature from

the manufacturer. Hand calculations of CTRL2 voltage

can then be performed at each given temperature and the

resulting CTRL2 curve plotted versus temperature. 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

MANUFACTURER

Murata Electronics North America

www.murata.com

TDK Corporation

www.tdk.com

Digi-key

www.digikey.com

If calculation of CTRL2 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

(CTRL2 behavior) over temperature. Referred to as the

‘Murata Chip NTC Thermistor Output Voltage Simulator’,

users can log onto www.murata.com/designlib and down-

load the software followed by instructions for creating an

output voltage VOUT (CTRL2) from a speciﬁed VCC supply

(VREF). At any time during selection of circuit parameters

the user can access data on the chosen NTC resistor by

clicking on a link to the Murata catalog.

The following example uses hand calculations to derive

the resistor values required for CTRL1 and CTRL2 pin

voltages to achieve a given LED current derating curve.

The resistor values obtained using the Murata simulation

tool are also provided and were used to create the derating

curve shown in Figure 6. The simulation tool illustrates

the non-linear nature of the NTC resistor temperature

coefﬁcient at temperatures exceeding 50°C ambient. In

addition, the resistor divider technique using an NTC

resistor to derive CTRL2 voltage inherently has a ﬂatten-

ing characteristic (reduced downward slope) at higher

temperatures. To avoid LED current exceeding a maximum

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. To use an NTC resistor to indicate

LED temperature it is only effective if the resistor is con-

nected as close as possible to the LED(s). LED derating

curves shown by manufacturers are listed for ambient

temperature. The NTC resistor should be submitted to

the same ambient temperature as the LED(s). Since the

temperature dependency of an NTC resistor can be non-

linear over a wide range of temperatures it is important

Figure 7. Programming LED Current Derating Curve

vs Temperature (RNTC Located on LEDs PCB)

Figure 8. CTRL1, 2 Programmed Voltages vs Temperature

TA AMBIENT TEMPERATURE (°C)

1100

1000

900

700

800

500

400

300

200

100

600

3478 F08

100

LED CURRENT = MINIMUM

OF CTRL1, CTRL2

R3 = OPTION C

CTRL1

CTRL2

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