Datasheet

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
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.
00841-004
R16
R10
+V
IN
R4
R5
Q6
Q7
R11
R14
R13
R15
NC
V
OUT
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
+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.