LT3599(RevD) データシートの表示（PDF） - Linear Technology

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LT3599
(Rev.:RevD)

4-Channel 120mA LED Driver with ±1.5% Current Matching

Linear Technology 

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

achieved by programming a voltage at the CTRL pin with

a negative temperature coefficient using a resistor divider

with temperature dependent resistance (Figure 7). As the

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.

A variety of resistor networks and NTC resistors with

different temperature coefficients 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

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

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 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 VOUT (CTRL) from a specified VCC

supply (VREF). 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

R1

(OPTION A TO D)

VREF

LT3599

CTRL

RY

RY

RNTC

RNTC

RX RNTC

RNTC

RX

A

B

C

D

3599 F07

Figure 7 . LED Current Derating vs Temperature Using NTC Resistor

3599fd

13

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