AD8531/AD8532/AD8534

REV. D

–11–

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

requires a three terminal reference, such as the REF196 (3.3 V)

or the REF194 (4.5 V), both which feature low power, with

sourcing outputs of 30 mA or less. Figure 38 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.

C2

0.1 F

R2

10k

1%

3.3V @ 100mA

R5

0.2

C5

100 F/16V

TANTALUM

R1

10k

1%

C1

0.1 F

5V

3.3V

C4

1 F

6

2

3

4

COMMON

C3

0.1 F

ON/OFF

CONTROL

INPUT CMOS HI

(OR OPEN) = ON

LO = OFF

COMMON

R3

(SeeText)

R4

3.3k

U2

AD8531

U1

REF196

Figure 38. A 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 measured less than

1 mV. This corresponds to an equivalent output impedance of

< 0.01

W. In this application, the stable 3.3 V from U1 is applied

to U2 through a noise filter, R1–C1. U2 replicates the U1

voltage within a few millivolts, but at a higher current output at

—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–C5 output network. With values as

shown, the transient is about 20 mV peak and settles to within

2 mV in less than 10

ms 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.

become:

R2

R3

Ê

ËÁ

ˆ

¯˜

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, 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 automotive and multimedia audio applications. On a single

5 V supply, the line driver exhibits less than 0.7% distortion into

a 600

W 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

W. 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.

600

C1

22 F

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

1 F

R12

10k

R14

50

A2

1

2

3

R3

10k

R6

10k

R13

10k

C3

47 F

C4

47 F

A1, A2 = 1/2 AD8532

GAIN =

R3

R2

SET: R7, R10, R11 = R2

SET: R6, R12, R13 = R3

R10

10k

R5

50

Figure 39. A Single-Supply, Balanced Line Driver for

Multimedia and Automotive Applications