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AD8240YRM
LED Driver/Monitor
LED Driver/MonitorRev. 0
FEATURES
PWM input for LED brightness control
Open LED detection
Latch-off overcurrent protection
Constant voltage regulated output
Supply range: 9 V to 27 V
Regulated voltage range: 12.5 V to 27 V
Operating current: 300 µA
Shutdown current: 10 µA
Temperature range −40°C to +125°C
8-lead MSOP package
GENERAL DESCRIPTION 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.
FUNCTIONAL BLOCK DIAGRAM
VSENSEREFERENCE
PWM
VPLUSSHUNT
BASEFigure 1.
PRODUCT HIGHLIGHTS 1. Partial LED lamp failure detection.
Allows for compliance with automotive regulations
for turn signal functionality detection and minimum
brightness, as well as running/brake light minimum
brightness compliance.
2. Current limiting/latch-off protection.
Limiting and latching off the LED current protects vehicle
wiring and prevents lamp damage
3. PWM input.
Provides brightness control.
4. Constant voltage output.
Saves cost by minimizing wiring and system design
complexity.
5. Drives an external transistor for low power operation.
Providing for an external power transistor allows the
AD8240 to be a low cost solution in a small package. A
more efficient design is made possible when the system
designer can select a power device with specifications
that match the application requirements.
6. Linear regulation.
Minimizes EMI, which allows faster system integration,
qualification, and time to market. Additionally, costs are
reduced by eliminating the inductor required for a
switching design. Because of the power-saving nature of
LED lamps as compared to incandescent lamps, a
switching driver is typically not required.
TABLE OF CONTENTS Specifications.....................................................................................3
Absolute Maximum Ratings............................................................4
Product Description.........................................................................5
Linear Regulator (Block A).........................................................5
High-Side Current-Sense Amplifier with Open LED
Detection (Block B)......................................................................5
Comparator with Latch-off OverCurrent Protection
(Block C)........................................................................................5
Intelligent Driver (Block D)........................................................5
Using/Evaluating the AD8240 LED Driver Monitor....................6
Setup................................................................................................6
Controlling the LED lamp............................................................7
Using/Evaluating the VSense Output.............................................7
Advantages of Driving LED Lamps with Constant Voltage........8
Background....................................................................................8
Monitoring the LEDs....................................................................8
Driving Automotive LEDs............................................................8
Outline Dimensions.......................................................................10
Ordering Guide..........................................................................11
REVISION HISTORY
4/04—Revision 0: Initial Version SPECIFICATIONS TA = operating temperature range, VPLUS = 13.5 V, unless otherwise noted.
Table 1. 1 VO = 12/5 of the applied reference voltage ±1% typical. Minimum VPLUS voltage for regulation depends on the external transistor Vbe and the shunt voltage. VO = 12/5 of the applied reference voltage ±2% typical. Minimum VPLUS voltage for regulation depends on the external transistor Vbe and the shunt voltage.
3 The maximum output current level is set by the selection of the current shunt and power transistor. (VSENSE – 5 V Reference) The latch-off level is determined by the output level of the sense amplifier. When the amplifier output approaches 5 V, the output is latched off.
This allows the maximum current output level to be determined by the shunt resistor value. Latch-off can be restored to on by cycling the PWM input off and back on. Internal delay only. The external delay depends on the external capacitor values, LED string impedance, and wiring inductance.
6 This specifies VPLUS only—ground current includes external transistor base drive.
ABSOLUTE MAXIMUM RATINGS
Table 2. Output short circuits result in a latch-off condition.
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
PRODUCT DESCRIPTIONThe AD8240 consists of four functional blocks labeled A
through D, as shown in Figure 2.
GND
VSENSE
REFERENCE
PWM
VPLUSBASESHUNTFigure 2. Simplified Functional Block Diagram
Table 3 briefly describes the blocks, while the sections that
follow provide more detailed information.
Table 3.
LINEAR REGULATOR (BLOCK A) The simplified architecture of the linear regulator block is an
amplifier and resistor divider. One input to the amplifier is
tied to the 5 V reference. The other input is tied to a resistor
divider that sets the ratio of the 5 V reference to VO. As a
result, the accuracy of the voltage output is proportional to
the accuracy of the 5 V reference. For example, if the 5 V
reference is 5% high (5.25 V), the output is 5% high (12.6 V).
HIGH-SIDE CURRENT-SENSE AMPLIFIER WITH
OPEN LED DETECTION (BLOCK B) This amplifier is used to measure the LED current by ampli-
fying the voltage across a user-selected shunt resistor. It has a
gain of 24 and an overall accuracy of 5%. The output of the
amplifier is typically connected to a microcontroller A/D
converter input so that the condition of the LED lamp can be
determined. This output can also be tied to other devices such
as a latching comparator or output buffer. It is important to note
that the output of this amplifier has a relatively high impedance
of approximately 10 kΩ. As a result, a buffer amplifier should be
used if the load is less than 100 kΩ.
COMPARATOR WITH LATCH-OFF OVERCURRENT
PROTECTION (BLOCK C) This block is used to shut down the output in the case of a
short circuit or an overcurrent condition. When the output
of the high-side current-sense amplifier approaches 5 V, the
comparator output switches, causing the driver to latch off the
output voltage.
INTELLIGENT DRIVER (BLOCK D) The intelligent driver provides multiple functions: Level shifts and conditions the output of the regulator
amplifier to drive an external user-selected power
transistor. Accepts PWM input so that LED brightness can be
controlled by a user-supplied PWM signal. The PWM input can also be used as a simple on/off
control for applications that do not require variable
brightness. Latch-off input that latches the output off when the
comparator trips during an overcurrent event. The latch
is reset by cycling the PWM input.
USING/EVALUATING THE AD8240 LED DRIVER MONITOR Figure 3 shows a connection diagram for a typical application.
SETUP In order to set up and evaluate the AD8240, the following
components and equipment are needed: A shunt resistor (typically 0.1 Ω to 0.5 Ω depending on the
load). See the Shunt Resistor Selection section. A transistor (the type depends on the load) Two capacitors LED load 5 V reference voltage 9 V to 27 V supply Oscilloscope Digital voltmeter (DVM)
Shunt Resistor Selection The shunt resistor is chosen by the equation
LOADSHUNTIRV2.0
For example if the load is expected to be 500 mA, the shunt
value should be equal to or less than 0.4 Ω. This keeps the
output of the current sense amplifier from being greater than
4.8 V in normal operation to prevent noise from causing the
output to latch off.
Circuit Configuration Connect the pass transistor, capacitors, and LED load(s) as
shown in Figure 3. It is important to note that the value of CM
should be at least 22 nF to ensure circuit stability.
The LED lamp should be configured to expect 12 V. This is
the result of selecting the series/parallel combinations of LEDs
and series resistors. The series resistors can be used to adjust
for LED supplier brightness variations from lot to lot.
Connect 5 V to Pin 2 (5 V reference) and at least 9 V to Pin 5
(VPLUS). It may be necessary to raise the VPLUS voltage to more
than 13 V, depending on the drop across the pass transistor, for
the output to be regulated at 12 V. This varies according to the
application and the pass transistor type.
VSENSE
REFERENCE
PWM
VBATT
RSHUNT04824-0-003
Figure 3. Connections for Typical Applications
