LED Driver/Monitor

AD8240

Rev. 0

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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

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

R1

350k

Ω

R2

250k

Ω

LATCH-OFF

DRIVER

10k

Ω

GND

1

2

3

4

5

7

6

8

5V

REFERENCE

PWM

AD8240

BASE

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.