AD680

REV. C

–3–

THEORY OF OPERATION

Bandgap references are the high performance solution for low

supply voltage operation. A typical precision bandgap will con-

sist of a reference core and buffer amplifier. Based on a new,

patented bandgap reference design (Figure 2), the AD680

merges the amplifier and the core bandgap function to produce

a compact, complete precision reference. Central to the device

is a high gain amplifier with an intentionally large Proportional

To Absolute Temperature (PTAT) input offset. This offset is

controlled by the area ratio of the amplifier input pair, Q1 and

Q2, and is developed across resistor R1. Transistor Q12’s base

emitter voltage has a Complementary To Absolute Temperature

(CTAT) characteristic. Resistor R2 and the parallel combina-

tion of R3 and R4 “multiply” the PTAT voltage across R1.

Trimming resistors R3 and R4 to the proper ratio produces a

temperature invariant 2.5 V at the output. The result is an

accurate, stable output voltage accomplished with a minimum

number of components.

Q10

R5

TEMP

GND

Q6

Q2

Q9

Q3

Q1

Q8

Q4

Q5

Q11

R1

R2

R4

R3

Q7

C1

R6

R7

8x

1x

Q12

V

OUT

Figure 2. AD680 Schematic Diagram

An additional feature with this approach is the ability to mini-

mize the noise while maintaining very low overall power

dissipation for the entire circuit. Frequently it is difficult to

independently control the dominant noise sources for bandgap

references: bandgap transistor noise and resistor thermal noise.

By properly choosing the operating currents of Q1 and Q2 and

separately sizing R1, low wideband noise is realized while main-

taining 1 mW typical power dissipation.

ABSOLUTE MAXIMUM RATINGS*

Power Dissipation (25

°C) . . . . . . . . . . . . . . . . . . . . . . 500 mW

Storage Temperature . . . . . . . . . . . . . . . . . . . –65

°C to +125°C

Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . . . 300

°C

Package Thermal Resistance

θ

. . . . . . . . . . . . . . . . . . . . . . . . 120

°C/W

Output Protection: Output safe for indefinite short to ground

*Stresses above those listed under “Absolute Maximum Ratings” may cause

permanent damage to the device. This is a stress rating only and functional

operation of the device at these or any other conditions above those indicated in the

operational sections of this specification is not implied. Exposure to absolute

maximum rating conditions for extended periods may affect device reliability.

8-Pin Plastic DIP

and

8-Pin SOIC Packages

1

2

3

4

8

7

6

5

TOP VIEW

(Not to Scale)

AD680

TP*

+V

IN

TEMP

GND

TP*

TP*

NC

NC = NO CONNECT

TP DENOTES FACTORY TEST POINT.

NO CONNECTIONS SHOULD BE MADE

TO THESE PINS.

*

TO-92 Package

3

2

1

+V

IN

V

OUT

GND

AD680

BOTTOM VIEW

(Not to Scale)

Figure 1. Connection Diagrams

ORDERING GUIDE

Initial

Temperature

Error

Coeff.

Temperature

Package

Package

Model

mV

ppm/

°C

Range

Description

Option*

AD680JN

10

25

0

°C to +70°C

Plastic

N-8

AD680JR

10

25

0

°C to +70°C

SOIC

SO-8

AD680JT

10

30

0

°C to +70°C

TO-92

TO-92

AD680AN

5

20

–40

°C to +85°C

Plastic

N-8

AD680AR

5

20

–40

°C to +85°C

SOIC

SO-8

*N = Plastic DIP Package; SO = SOIC Package; T = TO-92 Package.