AD8531/AD8532/AD8534

Rev. F | Page 14 of 20

APPLICATIONS INFORMATION

HIGH OUTPUT CURRENT, BUFFERED

REFERENCE/REGULATOR

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.

R2

10kΩ 1%

3.3V @ 100mA

R5

0.2Ω

C5

100µF/16V

TANTALUM

R1

10kΩ

1%

C1

0.1µF

5V

3.3V

C4

1µF

6

2

3

4

COMMON

C3

0.1µF

C2

0.1µF

ON/OFF

CONTROL

INPUT CMOS HI

(OR OPEN) = ON

LO = OFF

COMMON

R3

(See Text)

R4

3.3kΩ

U2

AD8531

U1

REF196

Figure 42. High Output Current Reference/Regulator

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

source output current(s), 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 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

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.

resistor R3 (shown dotted in Figure 42) is added, causing the

⎟

⎠

⎞

⎜

⎝

⎛ +

×

=

R3

R2

V

V

OUT2

OUT1

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

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.

600Ω

C1

22µF

A2

7

6

5

3

1

2

A1

5V

R1

10kΩ

R2

10kΩ

R11

10kΩ

R7

10kΩ

6

7

5

A1

12V

5V

R8

100kΩ

R9

100kΩ

C2

1µF

R12

10kΩ

R14

50Ω

A2

1

2

3

R3

10kΩ

R6

10kΩ

R13

10kΩ

C3

47µF

C4

47µF

A1, A2 = 1/2 AD8532

GAIN = R3

R2

SET: R7, R10, R11 = R2

SET: R6, R12, R13 = R3

R10

10kΩ

R5

50Ω

Figure 43. Single-Supply, Balanced Line Driver for Multimedia and

Automotive Applications