ADA4431-1

Rev. 0 | Page 9 of 12

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

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.

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

Device State

Charge Pump State

Low

Disabled

Disabled

High

Enabled

Enabled

High-Z

Enabled

Disabled

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.