Datasheet

Data Sheet AD780
THEORY OF OPERATION
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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.
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 thermome-
ter output voltage (at Pin 3) that increases at a rate of approximate-
ly 2 mV/°C.
The DNC (Pin 7) of the AD780 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.