ADR280

–4–

ADR280

–5–

THEORY OF OPERATION

The ADR280 provides the basic core 1.2 V band gap reference.

It contains two NPN transistors, Q9 and Q17, with their emitter

areas scaled in a fixed ratio.The difference in their Vbes produces

a PTAT (proportional to absolute temperature) voltage that

cancels the CTAT (complementary to absolute temperature) Q9

Vbe voltage. As a result, a core band gap voltage that is almost a

constant 1.2 V over temperature is generated (see Figure 2). Preci-

sion laser trimming of the internal resistors and other proprietary

circuit techniques are used to enhance the initial accuracy, tem-

perature curvature, and temperature drift performance.

R1

R3

R5

R7

R9

Q17

Q9

Q18

R8

Q3

R6

Q5

Q6

R4

Q1

PNP3

R2

R12

Q2

Q10

R13 C1

Q7

R10

R11

V–

V+

I1

Figure 2. Simplified Architecture

1.2

1.0

0.8

0.6

0.4

0.2

0

2

4

6

8

10

12

14

16

18

20

0

TIME (ms)

TPC 7. SettlingTime

APPLICATIONS

The ADR280 should be decoupled with a 0.1 µF ceramic cap at

the output for optimum stability. It is also good practice to

include 0.1 µF ceramic caps at the IC supply pin.These capaci-

tors should be mounted close to their respective pins (see Figure 3).

V+

0.1F

0.1F

V–

ADR280

Figure 3. Basic Configuration

The low supply voltage input pin V– can be elevated above

ground; a 1.2 V differential voltage can therefore be established

above V– (see Figure 4).

V+

0.1F

0.1F

V–

ADR280

5V

1.2V

2.5V

Figure 4. Floating References

REV. A

REV. A