AD780

REV. B

–4–

THEORY OF OPERATION

Bandgap 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 is

combined with the negative coefficient of a transistor’s Vbe to

produce a constant bandgap voltage.

In the AD780, the bandgap cell contains two npn transistors

(Q6 and Q7) which differ in emitter area by 12 . The differ-

ence in their Vbe’s produces a PTAT current in R5. This in

turn produces a PTAT voltage across R4, which when com-

bined with the Vbe of Q7, produces a voltage Vbg that does not

vary with temperature. Precision laser trimming of the resistors

and other patented circuit techniques are used to further enhance

the drift performance.

NC

TEMP

TRIM

GND

O/P SELECT

2.5V - NC

3.0V - GND

Q6

Q7

R10

R11

R5

R4

R14

R15

R16

R13

NC = NO CONNECT

NC

AD780

Figure 1. Schematic Diagram

The output voltage of the AD780 is determined by the configu-

ration 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 two terminal shunt regula-

tor providing a –2.5 V or –3.0 V reference voltage output with-

out external components.

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

mometer output voltage (at Pin 3) which increases at a rate of

approximately 2 mV/

°C.

The AD780’s NC Pin 7 is a 20 k

Ω resistor to V+ which is used

solely for production test purposes. Users who are currently us-

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

the different load on the heater supply.

APPLYING THE AD780

The AD780 can be used without any external components to

achieve specified performance. If power is supplied to Pin 2 and

Pin 4 is grounded, Pin 6 provides a 2.5 V or 3.0 V output de-

pending on whether Pin 8 is left unconnected or grounded.

A bypass capacitor of 1

load capacitance in the application is expected to be greater

than 1 nF. The AD780 in 2.5 V mode typically draws 700

µA of

Iq at 5 V. This increases by ~2

µA/V up to 36 V.

NC

TEMP

TRIM

GND

O/P SELECT

2.5V – NC

3.0V – GND

NC

R POT.

1 F

AD780

NC = NO CONNECT

Figure 2. Optional Fine Trim Circuit

Initial error can be nulled using a single 25 k

Ω potentiometer

with an adjustment range of

±4% and is only included here for

compatibility purposes with other references. A fine trim can be

implemented by inserting a large value resistor (e.g. 1–5 M

Ω) in

series with the wiper of the potentiometer. See Figure 2 above.

The trim range, expressed as a fraction of the output, is simply

greater than or equal to 2.1 k

3.0 V mode.

The external null resistor affects the overall temperature coeffi-

For example a 1 mV (.03%) shift in the output caused by the

trim circuit, with a 100 ppm/

°C null resistor will add less than

0.06 ppm/

°C to the output drift (0.03%

200 ppm/

°C, since

the resistors internal to the AD780 also have temperature coeffi-

cients of less than 100 ppm/

°C).