AD8569_15 Datasheet, PDF (9/12 Pages) Analog Devices – 16 V Rail-to-Rail Buffer Amplifiers

AD8568/AD8569/AD8570
APPLICATIONS
Theory of Operation
This family of buffers is designed to drive large capacitive loads in
LCD applications. Each has high output current drive and 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, the internal pn 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,
an external series resistor should be added. The size of the resis-
tor 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 protection should be used as described in
the previous section.
Power Dissipation
The maximum allowable internal junction temperature of 150°C
limits the device’s maximum power dissipation. As the ambient
temperature increases, the maximum power dissipated by the
device must decrease linearly to maintain the maximum junc-
tion temperature. If this maximum junction temperature is
exceeded momentarily, the device will still operate properly once
the junction temperature is reduced below 150°C. If the maximum
junction temperature is exceeded for an extended period of time,
overheating could lead to permanent damage of the device.
The maximum safe junction temperature, T_JMAX, is 150°C. Using
the following formula, we can obtain the maximum power that
the buffer family can safely dissipate as a function of temperature.
( ) ^PDISS ^{= T}JMAX ^{− T}A ^{/ θ}JA
where:
P_DISS = the power dissipation.
T_JMAX = the maximum allowable junction temperature
(150°C).
T_A = the ambient temperature of the circuit.
θ_JA = the AD856x package thermal resistance,
junction-to-ambient.
The power dissipated by the device can be calculated as
( ) ^PDISS ^{= V}S ^{− V}OUT ^{× I}LOAD
where:
V_S = the supply voltage.
V_OUT = the output voltage.
I_LOAD = the output load current.
Figure 1 shows the maximum power dissipation versus temperature.
To achieve proper operation, use the previous equation to calculate
P_DISS for a specific package at any given temperature, or see Figure 1.
1.00
0.75
10-LEAD MSOP
6-LEAD SOT-23
0.50
0.25
0
؊35
؊15
5
25
45
65
85
AMBIENT TEMPERATURE – ؇C
Figure 1. Maximum Power Dissipation vs. Temperature
for 6- and 10-Lead Packages
Total Harmonic Distortion + Noise (THD+N)
The buffer family features low THD+N. 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.
10
1

0.1
V_S = ؎2.5V
V_S = ؎8V
0.01
20
100
1k
FREQUENCY – Hz
10k 30k
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_X
≥
V_S
35 mA
For a 5 V single-supply operation, R_X should have a minimum
value of 143 Ω.
REV. C
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