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AD680JN

Low Power, Low Cost 2.5 V Reference

Analog Devices 

AD680

4.5 V REFERENCE FROM A 5 V SUPPLY

The AD680 can be used to provide a low power, 4.5 V reference,

as shown in Figure 19. In addition to the AD680, the circuit

uses a low power op amp and a transistor in a feedback con-

figuration that provides a regulated 4.5 V output for a power

supply voltage as low as 4.7 V. The high quality tantalum 10 μF

capacitor (C1) in parallel with the ceramic 0.1 μF capacitor (C2)

and the 3.9 Ω resistor (R5) ensure a low output impedance up to

approximately 50 MHz (see Figure 19).

0.1μF

VIN

VOUT

AD680

GND

R3

CC

1kΩ 3.3μF

4.7V TO 15V

2N2907A

+IN

3

7 V+

OUT

OP90 6

–IN

2

4 V–

CF

R4

3.57kΩ

0.1μF

+ C1

10μF

R5

3.9Ω

C2

0.1μF

R2

R1

2.5kΩ

2kΩ

1%

1%

Figure 19. 4.5 V Reference Running from a Single 5 V Supply

VOLTAGE REGULATOR FOR PORTABLE

EQUIPMENT

The AD680 is ideal for providing a stable, low cost, low power

reference voltage in portable equipment power supplies.

Figure 20 shows how the AD680 can be used in a voltage

regulator that not only has low output noise (as compared to a

switch mode design) and low power, but it also has a very fast

recovery after current surges. Some caution should be taken in

the selection of the output capacitors. Too high an ESR (effective

series resistance) could endanger the stability of the circuit. A solid

tantalum capacitor, 16 V or higher, and an aluminum electro-

lytic capacitor, 10 V or higher, are recommended for C1 and C2,

respectively. Also, the path from the ground side of C1 and C2

to the ground side of R1 should be kept as short as possible.

CHARGER

INPUT

0.1μF

6V +

LEAD-ACID

BATTERY

2

VIN

VOUT 6

AD680

TEMP 3

GND

4

R3

510kΩ

2– 7

OP777 6

3+ 4

IRF9530

R2

402kΩ 1%

R1

402kΩ 1%

+

C1

68μF

+

C2

1000μF

TANT ELECT

Figure 20. Voltage Regulator for Portable Equipment

Rev. H | Page 10 of 12

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