Data Sheet

AD780

Rev. F | Page 5 of 12

THEORY OF OPERATION

Band gap references are the high performance solution for low

supply voltage and low power voltage reference applications. In

this technique, a voltage with a positive temperature coefficient

produce a constant band gap voltage.

In the AD780, the band gap cell contains two NPN transistors

(Q6 and Q7) that differ in emitter area by 12×. The difference in

produces a PTAT voltage across R4 that, when combined with

temperature. Precision laser trimming of the resistors and other

patented circuit techniques are used to further enhance the drift

performance.

R16

R10

R4

R5

Q6

Q7

R11

R14

R13

R15

NC

TRIM

GND

O/P SELECT

2.5V – NC

3.0V – GND

TEMP

NC

NC = NO CONNECT

AD780

7

2

1

3

4

8

5

6

Figure 4. Schematic Diagram

The output voltage of the AD780 is determined by the

configuration of Resistors R13, R14, and R15 in the amplifier’s

feedback loop. This sets the output to either 2.5 V or 3.0 V,

depending on whether R15 (Pin 8) is grounded or not

connected.

A unique feature of the AD780 is the low headroom design of

the high gain amplifier, which produces a precision 3 V output

from an input voltage as low as 4.5 V (or 2.5 V from a 4.0 V

input). The amplifier design also allows the part to work with

This allows the AD780 to work as a 2-terminal shunt regulator,

providing a −2.5 V or −3.0 V reference voltage output without

external components.

The PTAT voltage is also used to provide the user with a

thermometer output voltage (at Pin 3) that increases at a rate of

approximately 2 mV/°C.

solely for production test purposes. Users who are currently

using the LT1019 self-heater pin (Pin 7) must take into account

the different load on the heater supply.