Datasheet AD8240 (Analog Devices) - 5

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AD8240 PRODUCT DESCRIPTION
The AD8240 consists of four functional blocks labeled A
HIGH-SIDE CURRENT-SENSE AMPLIFIER WITH
through D, as shown in Figure 2.
OPEN LED DETECTION (BLOCK B) V V PLUS SHUNT BASE O
This amplifier is used to measure the LED current by ampli- fying the voltage across a user-selected shunt resistor. It has a
B
gain of 24 and an overall accuracy of 5%. The output of the
VSENSE
amplifier is typically connected to a microcontroller A/D converter input so that the condition of the LED lamp can be
C
determined. This output can also be tied to other devices such
A
as a latching comparator or output buffer. It is important to note
5V REFERENCE
that the output of this amplifier has a relatively high impedance of approximately 10 kΩ. As a result, a buffer amplifier should be
D PWM
used if the load is less than 100 kΩ.
AD8240 COMPARATOR WITH LATCH-OFF OVERCURRENT PROTECTION (BLOCK C) GND
04824-0-002 This block is used to shut down the output in the case of a Figure 2. Simplified Functional Block Diagram short circuit or an overcurrent condition. When the output of the high-side current-sense amplifier approaches 5 V, the Table 3 briefly describes the blocks, while the sections that comparator output switches, causing the driver to latch off the follow provide more detailed information. output voltage.
Table 3. Block Description INTELLIGENT DRIVER (BLOCK D)
A Linear regulator The intelligent driver provides multiple functions: B High-side current-sense amplifier • Level shifts and conditions the output of the regulator C Comparator amplifier to drive an external user-selected power D Driver transistor.
LINEAR REGULATOR (BLOCK A)
• Accepts PWM input so that LED brightness can be The simplified architecture of the linear regulator block is an controlled by a user-supplied PWM signal. amplifier and resistor divider. One input to the amplifier is • tied to the 5 V reference. The other input is tied to a resistor The PWM input can also be used as a simple on/off divider that sets the ratio of the 5 V reference to V control for applications that do not require variable O. As a result, the accuracy of the voltage output is proportional to brightness. the accuracy of the 5 V reference. For example, if the 5 V • Latch-off input that latches the output off when the reference is 5% high (5.25 V), the output is 5% high (12.6 V). comparator trips during an overcurrent event. The latch is reset by cycling the PWM input. Rev. 0 | Page 5 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