Datasheet AD8240 (Analog Devices) - 8

AD8240 ADVANTAGES OF DRIVING LED LAMPS WITH CONSTANT VOLTAGE
The advantages of driving LED lamps with constant voltage are
DRIVING AUTOMOTIVE LEDS
• Low system cost There are two different architectures for driving LEDs in • left/right/center brake lamps, running lamps, and turn signals. Accurate monitoring • Proven strategy
Constant Current
The most common method for driving LEDs is with a constant
BACKGROUND
current. This current can be supplied from a constant current A great variety of LED lamps are being used in automotive source or from a constant voltage source in series with a ballast applications. The most popular application is center brake resistor. Driving LEDs without some form of ballast carries lamps. Currently, many manufacturers are developing some risk of premature LED failure due to thermal runaway in technology to use LEDs for left/right brake lamps, running high temperature ambient conditions. lamps, and turn signals. There are also plans to use high power For example, in the simplest application, the center brake lamp LEDs for forward lighting fog lamps and low beams. is driven from a relatively constant voltage with brightness There are two fundamental types of LEDs used in these applica- controlled by a series ballast resistor. This simple driving tions. The first is the low power bright LED. The second type is method has been used in a wide variety of automotive platforms the high power, extremely bright LED in the 1 W to 10 W range. for some time. With this method, the LEDs and ballast resistors are preselected for brightness as part of the manufacturing While the following information can be applied to applications strategy. using the high power LED, or incandescent lamps, the constant voltage method is designed for applications typically using the When driving with a constant current source, LED driving low power bright LEDs. This type of LED is used in arrays that and monitoring cannot be done using two or fewer wires form LED lamps. (shared ground). Since the current is constant, it does not change with partial LED failure. Instead, the current is divided
MONITORING THE LEDS
among the remaining functional LEDs, causing them to fail In addition to driving the LED lamp, the electronics in the prematurely at an unpredictable rate. Additionally, it is not control module must include a method for monitoring partial possible to detect partial failure by measuring the voltage LED failure in the lamp. Certain factors, such as overdriving change. The voltage does not change by a detectable amount and mechanical stress, can cause LED failures. because of the steep V/I curve exhibited by bright LEDs. When using a constant current scheme, at least one additional wire Auto manufacturers are using LED lamps as a way to differen- per lamp must be added to the harness to monitor partial or tiate themselves and give a car a unique appearance. Several total LED failure. Additionally, electronic modules must be failed LEDs in the lamp would ruin the aesthetics of the lamp. added to each lamp.

As a result, manufacturers are demanding the ability to monitor the LED lamps for partial failure.
Constant Voltage
Driving LEDs with a constant voltage allows for easy, low cost In addition to monitoring the LEDs for aesthetic reasons, detection of partial failure, an advantage not available with a monitoring must also be included as a result of automotive constant-current architecture. This is because the current from regulations. These regulations specify the minimum light the voltage source changes in direct proportion to the number output of external lamps. For example, if half of the LEDs in of LEDs that have failed. This current can be measured with a a particular lamp failed, the lamp would still operate, but the low cost shunt and an amplifier back at the body control light output would be insufficient to meet automotive module. This detection scheme is implemented in the AD8240 regulations for brightness. This concern is not an issue for LED driver/monitor through the use of a high-side, current- incandescent bulbs, because they are either completely on or sensing amplifier. The current is measured on the high side in completely off. The ability of the LED lamp to provide some order to separate the current from those combined in the light output in the case of partial outage, however, allows for an chassis ground return or shared-wire ground return. extra degree of safety over incandescent lamps. Additionally, there are automotive regulations requiring the monitoring of the turn signals regardless of the type of light source. Rev. 0 | Page 8 of 12 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS PRODUCT DESCRIPTION LINEAR REGULATOR (BLOCK A) HIGH-SIDE CURRENT-SENSE AMPLIFIER WITH OPEN LED DETECTION (B COMPARATOR WITH LATCH-OFF OVERCURRENT PROTECTION (BLOCK C) INTELLIGENT DRIVER (BLOCK D) USING/EVALUATING THE AD8240 LED DRIVER MONITOR SETUP Shunt Resistor Selection Circuit Configuration CONTROLLING THE LED LAMP USING/EVALUATING THE VSENSE OUTPUT Using/Evaluating the Short-Circuit Protection Feature ADVANTAGES OF DRIVING LED LAMPS WITH CONSTANT VOLTAGE BACKGROUND MONITORING THE LEDS DRIVING AUTOMOTIVE LEDS Constant Current Constant Voltage OUTLINE DIMENSIONS ORDERING GUIDE