AD8531A Analog Devices, AD8531A Datasheet - Page 11

• Manufacturer Part Number: AD8531A
• Description: Low Cost, 250 ma Output Single-supply Amplifiers
• Manufacturer: Analog Devices

REV. 0
A High Output Current, Buffered Reference/Regulator
Many applications require stable voltage outputs relatively close
in potential to an unregulated input source. This “low drop-
out” type of reference/regulator is readily implemented with a
rail-to-rail output op amp, and is particularly useful when using
a higher current device such as the AD8531/AD8532/AD8534.
A typical example is the 3.3 V or 4.5 V reference voltage devel-
oped from a 5 V system source. Generating these voltages re-
quires a three terminal reference, such as the REF196 (3.3 V) or
the REF194 (4.5 V), both which feature low power, with sourc-
ing outputs of 30 mA or less. Figure 38 shows how such a ref-
erence can be outfitted with an AD8531/AD8532/AD8534
buffer for higher currents and/or voltage levels, plus sink and
source load capability.
The low dropout performance of this circuit is provided by
stage U2, an AD8531 connected as a follower/buffer for the basic
reference voltage produced by U1. The low voltage saturation
characteristic of the AD8531/AD8532/AD8534 allows up to
100 mA of load current in the illustrated use, as a 5 V to 3.3 V
converter with good dc accuracy. In fact, the dc output voltage
change for a 100 mA load current delta measured less than
1 mV. This corresponds to an equivalent output impedance of
< 0.01 . In this application, the stable 3.3 V from U1 is ap-
plied to U2 through a noise filter, R1–C1. U2 replicates the U1
voltage within a few millivolts, but at a higher current output at
V
—unlike most IC references. R2 and C2 in the feedback path of
U2 provide additional noise filtering.
Transient performance of the reference/regulator for a 100 mA
step change in load current is also quite good and is determined
largely by the R5–C5 output network. With values as shown,
the transient is about 20 mV peak and settles to within 2 mV in
less than 10 s for either polarity. Although there exists room
for optimizing the transient response, any changes to the R5–C5
network should be verified by experiment to preclude the possi-
bility of excessive ringing with some capacitor types.
ON/OFF
CONTROL
INPUT CMOS HI
(OR OPEN) = ON
COMMON
LO = OFF
OUT1
Figure 38. A High Output Current Reference/Regulator
+V
+5V
V
V
S
S
C
, with the ability to both sink and source output current(s)
0.1µF
C3
3
0.1µF
REF196
C1
U1
2
4
6
V
OUT2
3.3V
=
(SeeText)
R1
10k
1%
C4
1µF
R3
R4
3.3k
10k 1%
U2
AD8531
0.1µF
R2
C2
3.3V @ 100mA
R5
0.2
C5
100µF/16V
TANTALUM
V
COMMON
V
OUT1
OUT
=
–11–
To scale V
resistor R3 (shown dotted) is added causing the new V
become:
The circuit can be used either as shown as a 5 V to 3.3 V
reference/regulator, or it can also be used with ON/OFF con-
trol. By driving Pin 3 of U1 with a logic control signal as noted,
the output is switched ON/OFF. Note that when ON/OFF con-
trol is used, resistor R4 must be used with U1, to speed ON-
OFF switching.
A Single-Supply, Balanced Line Driver
The circuit in Figure 39 is a unique line driver circuit topology
used in professional audio applications and has been modified
for the automotive and multimedia audio applications. On a
single +5 V supply, the line driver exhibits less than 0.7% dis-
tortion into a 600
with an input signal level of 4 V p-p. In fact, the output drive
capability of the AD8531/AD8532/AD8534 maintains this level
for loads as small as 32 . For input signals less than 1 V p-p,
the THD is less than 0.1%, regardless of load. The design is a
transformerless, balanced transmission system where output
common-mode rejection of noise is of paramount importance.
Like the transformer-based system, either output can be shorted
to ground for unbalanced line driver applications without
changing the circuit gain of 1. Other circuit gains can be set ac-
cording to the equation in the diagram. This allows the design
to be easily configured for noninverting, inverting, or differential
operation.
V
IN
Figure 39. A Single-Supply, Balanced Line Driver for
Multimedia and Automotive Applications
A1, A2 = 1/2 AD8532
GAIN =
SET: R7, R10, R11 = R2
SET: R6, R12, R13 = R3
22µF
C1
R3
R2
OUT2
2
3
+5V
A1
to another (higher) output level, the optional
10k
10k
R1
10k
R10
V
1
R2
OUT 1
load from 20 Hz to 15 kHz (not shown)
AD8531/AD8532/AD8534
R7
10k
R11
10k
2
3
6
5
V
OUT 2
10k
A2
A2
R3
7
10k
R12
+12V
10k
R13
1
7
A1
1
100k
6
5
10k
R6
R9
R2
R3
+5V
R14
50
50
R5
R8
100k
1µF
C2
OUT1
47µF
47µF
600
C4
C3
R
L
to
V
V
O2
O1