LT3754 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT3754

16-Channel × 50mA LED Driver

Linear Technology 

LT3754

Applications Information

Programming LED Current Derating (Breakpoint and

Slope) versus LED Ambient Temperature (CTRL Pin)

LED data sheets provide curves of maximum allowed

LED current versus ambient temperature to warn against

damaging of the LED (Figure 6). The LT3754 LED driver

improves the utilization and reliability of the LED(s) by al-

lowing the programming of an LED current derating curve

versus the ambient temperature of the LED(s). Without the

ability to back off LED currents as temperature increases,

many LED drivers are limited to driving the LED(s) at 50%

or less of their maximum rated currents. This limitation

requires more LEDs to obtain the intended brightness

for the application. The LT3754 allows the LED(s) to be

programmed for maximum allowable current while still

protecting the LED(s) from excessive currents at high tem-

perature. The temperature breakpoint and the slope of LED

current versus ambient temperature can be programmed

using a simple resistor network shown in Figure 7.

This is achieved by programming a voltage at the CTRL

pin with a negative temperature coefficient using a resistor

divider with temperature dependent resistance (Figures 7

and 8). A variety of resistor networks and NTC resistors

with different temperature coefficients can be used to

achieve the desired CTRL pin voltage behavior versus

temperature. The current derating curve in Figure 6 uses

the resistor network shown in option C of Figure 7.

Table 7 shows a list of NTC resistor manufacturers/ distribu-

tors. There are several other manufacturers available and

the chosen supplier should be contacted for more detailed

information. To use an NTC resistor to monitor the ambient

temperature of the LED(s) it should be placed as close as

possible to the LED(s). 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 and the resulting CTRL voltage

plotted versus temperature.

Table 7. NTC Resistor Manufacturers

Manufacturer

Murata Electronics North America

TDK Corporation

Digi-key

WEB

www.murata.com

www.tdk.com

www.digikey.com

90

8 LED STRINGS (2 × 40mA PER STRING)

80

70

RESISTOR

OPTION A

60

50

40

LT3754

PROGRAMMED LED

30 CURRENT DERATING

CURVE

20

10

0

0 10 20 30 40 50 60 70 80

TA-TEMPERATURE (°C)

3754 F06

Figure 6. LED Current Derating vs LED Ambient Temperature

31

VREF

R1

LT3754

29

CTRL

R2

OPTION A TO D

RY

RY

RNTC

RNTC

RX RNTC

RNTC

RX

A

B

C

D

3754 F07

Figure 7. Programming LED Current Derating Curve

vs Ambient Temperature (RNTC Located on LED PCB)

3754fc

18

Share Link: