AD8240
PRODUCT DESCRIPTION
The AD8240 consists of four functional blocks labeled A
through D, as shown in Figure 2.
V
PLUS
SHUNT BASE
V
O
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.
04824-0-002
B
V
SENSE
C
A
5V
REFERENCE
D
PWM
AD8240
GND
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.
Figure 2. Simplified Functional Block Diagram
follow provide more detailed information.
Table 3.
Block
A
B
C
D
Description
Linear regulator
High-side current-sense amplifier
Comparator
Driver
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.
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 V
O
. 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).
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