Analog Devices AD8240 Datasheet
LED Driver/Monitor
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. V
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.
is turned on when
O
AD8240
FUNCTIONAL BLOCK DIAGRAM
V
P
L
U
S
SHUNT
BASE
6
LATCH-OFF
DRIVER
7
V
SENSE
REFERENCE
PWM
5
R
SENSE
1
10k
Ω
5V
2
3
AD8240
Figure 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.
V
O
8
R1
350k
R2
250k
4
GND
Ω
Ω
04824-0-001
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.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700 www.analog.com
Fax: 781.326.8703 © 2004 Analog Devices, Inc. All rights reserved.
AD8240
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)
Comparator with Latch-off OverCurrent Protection
(Block C)
Intelligent Driver (Block D)........................................................ 5
........................................................................................ 5
REVISION HISTORY
...................................................................... 5
Using/Evaluating the AD8240 LED Driver Monitor....................6
Setup................................................................................................6
Controlling the LED lamp............................................................7
Using/Evaluating the V
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
Output.............................................7
Sense
4/04—Revision 0: Initial Version
Rev. 0 | Page 2 of 12
AD8240
SPECIFICATIONS
TA = operating temperature range, V
Table 1.
Parameter Conditions Min Typ Max Unit
V
O
VO Regulation1 at 25°C V
VO Regulation2 -40°C to 125°C V
3
I
OUT
VO Rise Time I
V
Gain 24
SENSE
V
Accuracy −7 ±3 +7 %
SENSE
V
Output Impedance 8 10 12 kΩ
SENSE
Base Drive 15 mA
PWM Turn-On Threshold 3.5 V
PWM Turn-Off Threshold 1.4 V
V
Latch-Off Threshold4 0 0.1 0.3 V
SENSE
Latch-Off Delay
5
PWM Frequency Range DC 500 Hz
C
M
C
Range CL = 47 nF −30 +30 %
LOAD
Operating Current
V
Operating Range 9 27 V
PLUS
6
Shutdown Current PWM low 10 µA
5 V Reference Current 160 200 240 µA
Operating Temperature Range −40 +125 °C
1
VO = 12/5 of the applied reference voltage ±1% typical. Minimum V
2
VO = 12/5 of the applied reference voltage ±2% typical. Minimum V
3
The maximum output current level is set by the selection of the current shunt and power transistor.
4
(V
– 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.
SENSE
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.
5
Internal delay only. The external delay depends on the external capacitor values, LED string impedance, and wiring inductance.
6
This specifies V
only—ground current includes external transistor base drive.
PLUS
= 13.5 V, unless otherwise noted.
PLUS
5 V Reference = 5 V 12 V
= 13 V to 27 V −3 ±1 +3 %
PLUS
= 13 V to 27 V −6 ±2 +6 %
PLUS
R
= 0.4 Ω 500 mA
SHUNT
= 250 mA, CM = 22 nF, CL = 47 nF 12 µs
OUT
2 µs
22 nF
V
> 13 V 360 µA
PLUS
voltage for regulation depends on the external transistor Vbe and the shunt voltage.
PLUS
voltage for regulation depends on the external transistor Vbe and the shunt voltage.
PLUS
Rev. 0 | Page 3 of 12
AD8240
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameters Rating
Supply Voltage, Continuous 27 V
Supply Voltage, Transient 30 V
Reverse Supply Protection −0.3 V
Operating Temperature 125°C
Storage Temperature −65°C to +150°C
Output Short-Circuit Duration1Indefinite
1
Output short circuits result in a latch-off condition.
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.
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.
Rev. 0 | Page 4 of 12
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