AD8531/AD8532/AD8534
APPLICATIONS INFORMATION
HIGH OUTPUT CURRENT, BUFFERED
REFERENCE/REGULATOR
for optimizing the transient response, any changes to the R5 to
C5 network should be verified by experiment to preclude the
possibility of excessive ringing with some capacitor types.
Many applications require stable voltage outputs relatively close
in potential to an unregulated input source. This low dropout
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 developed
from a 5 V system source. Generating these voltages requires a
three terminal reference, such as the REF196 (3.3 V) or the
REF194 (4.5 V), both of which feature low power, with sourcing
outputs of 30 mA or less. Figure 42 shows how such a reference
can be outfitted with an AD8531/AD8532/AD8534 buffer for
higher currents and/or voltage levels, plus sink and source load
capability.
To scale VOUT2 to another (higher) output level, the optional
resistor R3 (shown dotted in Figure 42) is added, causing the
new VOUT1 to become
R2
R3
⎛
⎝
⎞
⎟
⎠
VOUT1 = VOUT2 × 1 +
⎜
The circuit can either be used as shown, as a 5 V to 3.3 V
reference/regulator, or with on/off control. By driving Pin 3 of
U1 with a logic control signal as noted, the output is switched
on/off. Note that when on/off control is used, R4 must be used
with U1 to speed on/off switching.
SINGLE-SUPPLY, BALANCED LINE DRIVER
V
S
The circuit in Figure 43 is a unique line driver circuit topology
used in professional audio applications. It was modified for
automotive and multimedia audio applications. On a single 5 V
supply, the line driver exhibits less than 0.7% distortion into a
600 Ω load from 20 Hz to 15 kHz (not shown) 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 Ω. For input signals less than 1 V p-p, the THD is
less than 0.1%, regardless of load. The design is a transformer-
less, balanced transmission system where output common-
mode rejection of noise is of paramount importance. As with
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 according to the
equation in the diagram. This allows the design to be easily
configured for inverting, noninverting, or differential operation.
U2
AD8531
5V
C1
0.1µF
V
=
OUT1
3.3V @ 100mA
R2
10kΩ 1%
R1
10kΩ
1%
C2
0.1µF
C3
0.1µF
R3
2
C5
(See Text)
100µF/16V
TANTALUM
6
3
U1
REF196
V
C
V
=
ON/OFF
CONTROL
INPUT CMOS HI
(OR OPEN) = ON
OUT2
3.3V
R5
4
0.2Ω
C4
1µF
LO = OFF
R4
3.3kΩ
V
S
COMMON
V
OUT
COMMON
Figure 42. High Output Current Reference/Regulator
R3
10kΩ
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
measures less than 1 mV. This corresponds to an equivalent
output impedance of < 0.01 Ω. In this application, the stable
3.3 V from U1 is applied to U2 through a noise filter, R1 to C1.
U2 replicates the U1 voltage within a few millivolts, but at a
higher current output at VOUT1, with the ability to both sink and
source output current(s), unlike most IC references. R2 and C2
in the feedback path of U2 provide additional noise filtering.
C3
47µF
R5
50Ω
2
3
1
A2
V
OUT1
R6
10kΩ
R2
10kΩ
R7
10kΩ
5V
5V
12V
2
3
6
C1
22µF
R8
100kΩ
R
1
7
L
A1
A1
5
600Ω
V
IN
C2
1µF
R9
100kΩ
R1
10kΩ
R11
10kΩ
R12
10kΩ
A1, A2 = 1/2 AD8532
C4
47µF
R10
10kΩ
R14
50Ω
6
R3
GAIN =
R2
7
A2
V
OUT2
5
R13
10kΩ
SET: R7, R10, R11 = R2
SET: R6, R12, R13 = R3
Transient performance of the reference/regulator for a 100 mA
step change in load current is also quite good and is largely
determined by the R5 to C5 output network. With values as
shown, the transient is about 20 mV peak and settles to within
2 mV in less than 10 μs for either polarity. Although room exists
Figure 43. Single-Supply, Balanced Line Driver for Multimedia and
Automotive Applications
Rev. F | Page 14 of 20
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