LT3478/LT3478-1
13
34781f
LT3478/LT3478-1
V
REF
CTRL2
CTRL1
10
13
12
R4
3478 F07
R2
R1
R3
R
Y
R
Y
R
X
R
X
R
NTC
R
NTC
R
NTC
R
NTC
DCBA
OPTION A TO D
APPLICATIO S I FOR ATIO
WUU
U
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 V
OUT
(CTRL2) from a specifi ed V
CC
supply
(V
REF
). 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
coeffi cient at temperatures exceeding 50°C ambient. In
addition, the resistor divider technique using an NTC
resistor to derive CTRL2 voltage inherently has a fl atten-
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 (R
NTC
Located on LEDs PCB)
Figure 8. CTRL1, 2 Programmed Voltages vs Temperature
T
A
AMBIENT TEMPERATURE (°C)
0
CTRL1, CTRL2 PIN VOLTAGES (mV)
1100
1000
900
700
800
0
500
400
300
200
100
600
25
3478 F08
50 75 100
LED CURRENT = MINIMUM
OF CTRL1, CTRL2
R3 = OPTION C
CTRL1
CTRL2
