The AD8240 LED driver/monitor, in combination with an external transistor, supplies a constant 12 V to drive LED lamps. This allows cost-effective LED lamp monitoring and short-circuit protection. The output is regulated at 12 V when the supply voltage is between 12.5 V and 27 V.
A CMOS compatible, level-dependent, digital input can be used for PWM control of the LED brightness. VO is turned on when the PWM input is high and turned off when the input is low. The AD8240 is designed to work with a PWM frequency up to 500 Hz, and a typical PWM range from 5% to 95%.
Open LED detection is accomplished by measuring the change in LED lamp current caused by an open LED(s) through the use of an internal high-side current-sense amplifier that amplifies the voltage across an external current shunt. The voltage across the shunt resistor is amplified to a level that can be measured by a microcontroller A/D converter or a comparator. The ability to measure the change in LED lamp current is the key benefit of constant-voltage LED lamp driving.
The output is current-limited by latching off the output voltage when the current reaches a preset level. The current limit is set by selecting the value of the external current shunt that causes the output of the sense amplifier to slightly exceed the 5 V reference level when the current exceeds a maximum level. When the sense amplifier output exceeds 5 V, it trips an internal comparator that causes the driver to latch off the output voltage. The latch is reset during the next PWM cycle. The overcurrent condition can also be detected by a microcontroller or external comparator by measuring the sense amplifier output.
|Title||Content Type||File Type|
|AD8240: LED Driver/Monitor Data Sheet (Rev. 0, 4/2004) (Rev 0, 04/2004) (pdf, 217 kB)||Data Sheets|
|AN-669: Effectively Applying the AD628 Precision Gain Block (pdf, 193 kB)||Application Notes|
|A Designer's Guide to Instrumentation Amplifiers (3rd Edition)||Design Handbooks||HTML|
(doc, 75 kB)
A wide variety of electronic applications, especially those involving very small input signals, require signal paths with very low offset voltage and offset voltage drift over time and temperature.
Current Measurement in Solenoids for Automotive Control Systems
by Scott Beversdorf and Charles Whiting, Analog Devices, Inc. (Analog Dialogue 38-04, April 2004)
High-performance Adder Uses Instrumentation Amplifiers
Make an adder circuit using instrumentation amplifiers to increase input impedance.
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The demand for mobile devices with high-end features has been rapidly increasing. Features like text messaging, gaming, GPS navigation, web browsing and video playback on high resolution color displays are becoming the norm in today’s portable consumer electronics market.
(LED Journal, August/September 2009)
|Applying Instrumentation Amplifiers Effectively: The Importance of an Input Ground Return (pdf, 508 kB)||Technical Documentation|
|Leading Inside Advertorials: Applying Instrumentation Amplifiers EffectivelyThe Importance of an Input Ground Return (pdf, 121 kB)||Overview|
|Power System Management (Spring 2007) (pdf, 1170 kB)||Overview|
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Symbols and Footprints— Analog Devices offers Symbols & Footprints which are compatible with a large set of today’s CAD systems for broader and easier support.
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