–9–

AD8568/AD8569/AD8570

REV. A

APPLICATIONS

Theory of Operation

This family of buffers is designed to drive large capacitive loads

in LCD applications. Each has high output current drive, rail-

to-rail input/output operation and can be powered from a single

16 V supply. They are also intended for other applications where

low distortion and high output current drive are needed.

Input Overvoltage Protection

As with any semiconductor device, whenever the input exceeds

either supply voltage, attention needs to be paid to the input

overvoltage characteristics. As an overvoltage occurs, the amplifier

could be damaged, depending on the voltage level and the magnitude

of the fault current. When the input voltage exceeds either supply

by more than 0.6 V, internal pin junctions will allow current to

flow from the input to the supplies.

This input current is not inherently damaging to the device as

long as it is limited to 5 mA or less. If a condition exists using

the buffers where the input exceeds the supply by more than 0.6 V,

a series external resistor should be added. The size of the resistor

can be calculated by using the maximum overvoltage divided by

5 mA. This resistance should be placed in series with the input

exposed to an overvoltage.

Output Phase Reversal

The buffer family is immune to phase reversal. Although the device’s

output will not change phase, large currents due to input overvoltage

could damage the device. In applications where the possibility exists

of an input voltage exceeding the supply voltage, overvoltage protec-

tion should be used as described in the previous section.

Power Dissipation

The maximum allowable internal junction temperature of 150

°C

limits the buffer family Maximum Power Dissipation. As the ambi-

ent temperature increases, the maximum power dissipated by the

buffer family must decrease linearly to maintain the maximum

junction temperature. If this maximum junction temperature is

exceeded momentarily, the part will still operate properly once

the junction temperature is reduced below 150

°C. If the maxi-

mum junction temperature is exceeded for an extended period

of time, overheating could lead to permanent damage of the device.

°C. Using

the following formula, we can obtain the maximum power that

the buffer family can safely dissipate as a function of temperature.

A

where:

°C)

The power dissipated by the device can be calculated as

where:

Figure 1 shows the maximum power dissipation versus temperature.

To achieve proper operation, use the previous equation to calculate

AMBIENT TEMPERATURE – C

1.0

0.75

0

35

85

15

525

45

65

0.5

0.25

10-LEAD MSOP

6-LEAD SOT-23

Figure 1. Maximum Power Dissipation vs. Temperature

for 6- and 10-Lead Packages

THD + N

The buffer family features low total harmonic distortion. Figure 2

shows a graph of THD + N versus frequency. The Total Harmonic

Distortion plus Noise for the buffer over the entire supply range is

below 0.08%. When the device is powered from a 16 V supply, the

THD + N stays below 0.03%. Figure 2 shows the AD8568

THD + N versus frequency performance.

FREQUENCY – Hz

20

30k

100

1k

10k

10

1

0.01

0.1



Figure 2. AD8568 THD + N vs. Frequency

Short Circuit Output Conditions

The buffer family does not have internal short circuit protection

circuitry. As a precautionary measure, do not short the output

directly to the positive power supply or to ground.

It is not recommended to operate the AD856x with more than

35 mA of continuous output current. The output current can be

limited by placing a series resistor at the output of the amplifier

whose value can be derived using the following equation:

R

V

mA

X

S

≥

35

value of 143

Ω.