V PLUS LED Driver/Monitor AD8240 FEATURES FUNCTIONAL BLOCK DIAGRAM PWM input for LED brightness control V SHUNT BASE O Open LED detection 5 6 7 8 Latch-off overcurrent protection R SENSE V 1 Constant voltage regulated output SENSE 10k Supply range: 9 V to 27 V Regulated voltage range: 12.5 V to 27 V Operating current: 300 A 5V 2 Shutdown current: 10 A REFERENCE R1 350k Temperature range 40C to +125C LATCH-OFF 8-lead MSOP package DRIVER 3 PWM R2 250k AD8240 GENERAL DESCRIPTION 4 GND The AD8240 LED driver/monitor, in combination with an external transistor, supplies a constant 12 V to drive LED Figure 1. lamps. This allows cost-effective LED lamp monitoring and PRODUCT HIGHLIGHTS short-circuit protection. The output is regulated at 12 V when 1. Partial LED lamp failure detection. the supply voltage is between 12.5 V and 27 V. Allows for compliance with automotive regulations for turn signal functionality detection and minimum A CMOS compatible, level-dependent, digital input can be used brightness, as well as running/brake light minimum for PWM control of the LED brightness. VO is turned on when brightness compliance. the PWM input is high and turned off when the input is low. The AD8240 is designed to work with a PWM frequency up 2. Current limiting/latch-off protection. to 500 Hz, and a typical PWM range from 5% to 95%. Limiting and latching off the LED current protects vehicle wiring and prevents lamp damage Open LED detection is accomplished by measuring the change 3. PWM input. in LED lamp current caused by an open LED(s) through the use Provides brightness control. of an internal high-side current-sense amplifier that amplifies the voltage across an external current shunt. The voltage across 4. Constant voltage output. the shunt resistor is amplified to a level that can be measured by Saves cost by minimizing wiring and system design a microcontroller A/D converter or a comparator. The ability to complexity. measure the change in LED lamp current is the key benefit of 5. Drives an external transistor for low power operation. constant-voltage LED lamp driving. Providing for an external power transistor allows the AD8240 to be a low cost solution in a small package. A The output is current-limited by latching off the output voltage more efficient design is made possible when the system when the current reaches a preset level. The current limit is set designer can select a power device with specifications by selecting the value of the external current shunt that causes that match the application requirements. the output of the sense amplifier to slightly exceed the 5 V reference level when the current exceeds a maximum level. 6. Linear regulation. When the sense amplifier output exceeds 5 V, it trips an internal Minimizes EMI, which allows faster system integration, comparator that causes the driver to latch off the output voltage. qualification, and time to market. Additionally, costs are The latch is reset during the next PWM cycle. The overcurrent reduced by eliminating the inductor required for a condition can also be detected by a microcontroller or external switching design. Because of the power-saving nature of comparator by measuring the sense amplifier output. LED lamps as compared to incandescent lamps, a switching driver is typically not required. Rev. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Specifications subject to change without notice. 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All rights reserved. 04824-0-001AD8240 TABLE OF CONTENTS Using/Evaluating the AD8240 LED Driver Monitor ....................6 Specifications..................................................................................... 3 Setup................................................................................................6 Absolute Maximum Ratings............................................................ 4 Controlling the LED lamp............................................................7 Product Description......................................................................... 5 Using/Evaluating the VSense Output .............................................7 Linear Regulator (Block A) ......................................................... 5 Advantages of Driving LED Lamps with Constant Voltage ........8 High-Side Current-Sense Amplifier with Open LED Detection (Block B)...................................................................... 5 Background ....................................................................................8 Comparator with Latch-off OverCurrent Protection Monitoring the LEDs ....................................................................8 (Block C)........................................................................................ 5 Driving Automotive LEDs............................................................8 Intelligent Driver (Block D) ........................................................ 5 Outline Dimensions ....................................................................... 10 Ordering Guide .......................................................................... 11 REVISION HISTORY 4/04Revision 0: Initial Version Rev. 0 Page 2 of 12