AD8531/AD8532/AD8534
REV. A
–11–
A High Output Current, Buffered Reference/Regulator
Many applications require stable voltage outputs relatively close
in potential to an unregulated input source. T his “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.
C2
0.1μF
R2
10k
1%
V
OUT1
=
3.3V @ 100mA
R5
0.2
C5
100μF/16V
TANTALUM
R1
10k
1%
C1
0.1μF
+V
S
+5V
V
OUT2
=
3.3V
C4
1μF
6
2
3
4
V
COMMON
C3
0.1μF
V
C
ON/OFF
CONTROL
INPUT CMOS HI
(OR OPEN) = ON
LO = OFF
V
COMMON
R3
(SeeText)
R4
3.3k
U2
AD8531
U1
REF196
Figure 38. A High Output Current Reference/Regulator
T he 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. T he 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. T his 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
OUT 1
, 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.
T ransient 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–C5 output network. With values as
shown, the transient is about 20 mV peak and settles to within
2 mV in less than 10
μ
s for either polarity. Although room exists
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.
T o scale V
OUT 2
to another (higher) output level, the optional
resistor R3 (shown dotted) is added, causing, the new V
OUT 1
to
become:
V
OUT
1
=
V
OUT
2
×
1
+
R
2
R
3
T he 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, resistor R4
must be used with U1 to speed ON-OFF switching.
A Single-Supply, Balanced Line Driver
T he circuit in Figure 39 is a unique line driver circuit topology
used in professional audio applications and has been 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
load from 20 Hz to 15 kHz (not shown) with an in-
put 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 T HD is
less than 0.1%, regardless of load. T he design is a transformerless,
balanced transmission system where output common-mode re-
jection of noise is of paramount importance. As with the trans-
former-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 equa-
tion in the diagram. T his allows the design to be easily config-
ured for inverting, noninverting or differential operation.
R
L
600
C1
22μF
A2
7
6
5
3
1
2
A1
+5V
R1
10k
R2
10k
R11
10k
R7
10k
6
7
5
A1
+12V
+5V
R8
100k
R9
100k
C2
1μF
R12
10k
R14
50
A2
1
2
3
R3
10k
R6
10k
R13
10k
C3
47μF
V
O1
V
O2
C4
47μF
A1, A2 = 1/2 AD8532
R3
R2
GAIN =
SET: R7, R10, R11 = R2
SET: R6, R12, R13 = R3
V
IN
R10
10k
R5
50
Figure 39. A Single-Supply, Balanced Line Driver for
Multimedia and Automotive Applications