Datasheet AD8240 (Analog Devices) - 7

AD8240 CONTROLLING THE LED LAMP
The LEDs are turned on and off depending on the CMOS To determine if the load is correct, the voltage at VSENSE should compatible digital voltage level present at the PWM pin (Pin 3). be as follows during full power operation: This voltage can be continuous for a simple on/off function, or PWM for dimming control. The PWM frequency should be less 500 mA × 0.4 Ω × 24 V/V = 4.8 V than 500 Hz with a range from 5% to 100%. Typical values are If there is a partial LED failure, VSENSE drops in proportion to the 5% for running and 95% for braking. quantity of the failure. For example, if 25% of the LEDs fail, the
USING/EVALUATING THE V
voltage drops by 25%.
SENSE OUTPUT
Important: The output impedance of VSENSE is approximately If there is a short to ground, VSENSE is near 0 V because the 10 kΩ. Because of this, it may be necessary to buffer the output output is latched off and no current is flowing. in order to drive a load of less than 100 kΩ. An oscilloscope, micro-controller A/D converter, or DVM may be used to
Using/Evaluating the Short-Circuit Protection Feature
accurately measure the voltage at the VSENSE pin. If there is a short or an overload condition, the voltage at VSENSE falls close to zero, and the output shuts down The VSENSE output is used to detect a partial LED failure, or an (the transistor driver shuts off). This resets when the PWM overcurrent condition. The voltage present at VSENSE is propor- voltage is brought low and then high again. If the condition tional to the current through the load with the equation persists, the AD8240 attempts to drive the output to 12 V and I then immediately shuts down. If a PWM voltage is used, the LOAD = (VSENSE/24)/RSHUNT AD8240 attempts to start after each PWM cycle. Selection of the shunt resistor can be found by manipulating this equation. For example This can be simulated by increasing the load so the voltage at VSENSE slightly exceeds 5 V. When this happens, the output shuts VSENSE = ILOAD × RSHUNT down, and the VSENSE voltage is close to 0 V. Expected Load = 500 mA RSHUNT = 0.4 Ω Rev. 0 | Page 7 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