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AD8240 Datasheet(PDF) 8 Page - Analog Devices |
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AD8240 Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD8240 Rev. 0 | Page 8 of 12 ADVANTAGES OF DRIVING LED LAMPS WITH CONSTANT VOLTAGE The advantages of driving LED lamps with constant voltage are • Low system cost • Accurate monitoring • Proven strategy BACKGROUND A great variety of LED lamps are being used in automotive applications. The most popular application is center brake lamps. Currently, many manufacturers are developing technology to use LEDs for left/right brake lamps, running lamps, and turn signals. There are also plans to use high power LEDs for forward lighting fog lamps and low beams. There are two fundamental types of LEDs used in these applica- tions. The first is the low power bright LED. The second type is the high power, extremely bright LED in the 1 W to 10 W range. While the following information can be applied to applications using the high power LED, or incandescent lamps, the constant voltage method is designed for applications typically using the low power bright LEDs. This type of LED is used in arrays that form LED lamps. MONITORING THE LEDS In addition to driving the LED lamp, the electronics in the control module must include a method for monitoring partial LED failure in the lamp. Certain factors, such as overdriving and mechanical stress, can cause LED failures. Auto manufacturers are using LED lamps as a way to differen- tiate themselves and give a car a unique appearance. Several failed LEDs in the lamp would ruin the aesthetics of the lamp. As a result, manufacturers are demanding the ability to monitor the LED lamps for partial failure. In addition to monitoring the LEDs for aesthetic reasons, monitoring must also be included as a result of automotive regulations. These regulations specify the minimum light output of external lamps. For example, if half of the LEDs in a particular lamp failed, the lamp would still operate, but the light output would be insufficient to meet automotive regulations for brightness. This concern is not an issue for incandescent bulbs, because they are either completely on or completely off. The ability of the LED lamp to provide some light output in the case of partial outage, however, allows for an 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. DRIVING AUTOMOTIVE LEDS There are two different architectures for driving LEDs in left/right/center brake lamps, running lamps, and turn signals. Constant Current The most common method for driving LEDs is with a constant current. This current can be supplied from a constant current source or from a constant voltage source in series with a ballast resistor. Driving LEDs without some form of ballast carries some risk of premature LED failure due to thermal runaway in high temperature ambient conditions. For example, in the simplest application, the center brake lamp is driven from a relatively constant voltage with brightness controlled by a series ballast resistor. This simple driving method has been used in a wide variety of automotive platforms for some time. With this method, the LEDs and ballast resistors are preselected for brightness as part of the manufacturing strategy. When driving with a constant current source, LED driving and monitoring cannot be done using two or fewer wires (shared ground). Since the current is constant, it does not change with partial LED failure. Instead, the current is divided among the remaining functional LEDs, causing them to fail prematurely at an unpredictable rate. Additionally, it is not possible to detect partial failure by measuring the voltage change. The voltage does not change by a detectable amount because of the steep V/I curve exhibited by bright LEDs. When using a constant current scheme, at least one additional wire per lamp must be added to the harness to monitor partial or total LED failure. Additionally, electronic modules must be added to each lamp. Constant Voltage Driving LEDs with a constant voltage allows for easy, low cost detection of partial failure, an advantage not available with a constant-current architecture. This is because the current from the voltage source changes in direct proportion to the number of LEDs that have failed. This current can be measured with a low cost shunt and an amplifier back at the body control module. This detection scheme is implemented in the AD8240 LED driver/monitor through the use of a high-side, current- sensing amplifier. The current is measured on the high side in order to separate the current from those combined in the chassis ground return or shared-wire ground return. |
Similar Part No. - AD8240_15 |
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Similar Description - AD8240_15 |
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