Data Sheet
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
is combined with the negative coefficient of a transistor’s V
be
to
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
their V
be
s produces a PTAT current in R5. This, in turn,
produces a PTAT voltage across R4 that, when combined with
the V
be
of Q7, produces a voltage (V
bg
) 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.
+V
IN
2
AD780
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
+V
IN
= V
OUT
when current is forced into the output terminal.
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.
The AD780’s NC (Pin 7) is a 20 kΩ resistor to +V
IN
that is used
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.
NC
7
AD780
R10
NC
1
6
R11
V
OUT
R13
Q6
R5
TEMP
3
R14
R15
Q7
R16
5
TRIM
R4
4
8
00841-004
GND
NC = NO CONNECT
O/P SELECT
2.5V – NC
3.0V – GND
Figure 4. Schematic Diagram
Rev. F | Page 5 of 12
AD [ ANALOG DEVICES ]
