ADA4431-1 Analog Devices, ADA4431-1 Datasheet - Page 9

• Manufacturer Part Number: ADA4431-1
• Description: Ultralow Power SD Video Filter
• Manufacturer: Analog Devices

THEORY OF OPERATION
OVERVIEW
The ADA4431-1 is designed for exceptional performance as
both a filter and a low power driver for portable video applications.
This performance is achieved by providing third-order filtering
without trading off power consumption or device size. While
consuming only 4.7 mA quiescent supply current, the ADA4431-1
provides video output on a single-supply as low as 2.5 V.
The ADA4431-1 also features a load detect circuit, which senses
current through the external 75 Ω back-termination resistor.
When either no video load or a short circuit is detected, the
ADA4431-1 enters a low power state. In this state, it draws
0.1 mA, continues to monitor the load current, and powers up
automatically when a video load is connected. Optionally, the
ADA4431-1 can be powered down via the disable pin (DIS).
Another external pin (LDO) outputs the load detection state
(for example, to an external system controller). This pin is high
(+V
of a video load or when the output is short-circuited.
The ADA4431-1 is intended for use in applications that have
both ac- and dc-coupled inputs and outputs. The rail-to-rail
buffer on the ADA4431-1 output is able to drive 2 V p-p video
signals into two doubly terminated video loads (150 Ω each) on
a single 2.5 V supply. The ADA4431-1 has a gain of 2 when the
SAG correction pin is tied directly to the output, which makes
up for the 6 dB termination loss (see Figure 17, Output Option 1).
When the SAG feature is used, the ADA4431-1 has a low
frequency gain of 2.5 (≈8 dB) and a high frequency gain of 2
(see Figure 17, Output Option 2). Signal offsets and supply levels
must be considered when using the SAG correction feature to
ensure that there are no headroom issues.
The input range of the ADA4431-1 includes ground, while the
output range is limited by the saturation of the output devices.
Saturation occurs several tens of mV from the positive and
negative supply rails.
S
) when a video load is present, and low (0 V) in the absence
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The high input impedance and low input capacitance of the
ADA4431-1 offer advantages in a number of low power
applications. In reconstruction filter applications, the DAC can
be placed in its lowest power mode, allowing the use of a large-
valued load resistor. Using a large-valued load resistor does not
interfere with the frequency response of the ADA4431-1.
CHARGE PUMP OPERATION
The on-board charge pump creates a virtual negative supply for
the output driver, which allows the output signal to be dc-coupled,
with its black level at 0 V and sync tip at −300 mV. The charge
pump is enabled whenever the disable pin (DIS) is held high.
If DIS is left floating, or placed in a high impedance state, the
ADA4431-1 is powered up, but the charge pump is disabled,
which is typically the case for ac-coupling of the output. When
DIS is driven to 0 V, the entire device is powered down.
Table 6. Disable (DIS) Pin Function Summary
DIS Pin
Low
High
High-Z
LOAD DETECT
The load detect feature provides additional system power
management to keep the power consumption of a portable
device to the absolute minimum. The ADA4431-1 monitors the
output load for three conditions: a normal load, a shorted load,
and an open load. Each of these conditions occurs for a video
load, an audio load, and no load, respectively. Only in the video
load condition will the ADA4431-1 power up from 0.1 mA to
4.7 mA. A single I/O pin, LDO, notifies the system by either
being high for a video load or low for a short and open load.
With this information, it is intended that the system controller
power down any power intensive video processing blocks to
realize substantial power savings. The ADA4431-1 operates
autonomously, requiring no inputs to monitor the condition of
the output load.
Device State
Disabled
Enabled
Enabled
Charge Pump State
Disabled
Enabled
Disabled
ADA4431-1