AD8240

Rev. 0 | Page 5 of 12

PRODUCT DESCRIPTION

The AD8240 consists of four functional blocks labeled A

through D, as shown in Figure 2.

B

GND

5V

REFERENCE

PWM

BASE

SHUNT

C

AD8240

D

A

Figure 2. Simplified Functional Block Diagram

Table 3 briefly describes the blocks, while the sections that

follow provide more detailed information.

Table 3.

Block

Description

A

Linear regulator

B

High-side current-sense amplifier

C

Comparator

D

Driver

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

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.