AD780
THEORY OF OPERATION
APPLYING THE AD780
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.
+V
IN
NC
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
µF
(V
IN
to GND) should be used if the
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.
+V
IN
NC
1 F
NC
V
OUT
AD780
TRIM
TEMP
O/P SELECT
2.5V – NC
3.0V – GND
R
NULL
R POT.
AD780
R10
R11
GND
NC
V
OUT
R13
NC = NO CONNECT
Figure 2. Optional Fine Trim Circuit
Q6
Q7
R5
R16
R14
R4
R15
TRIM
TEMP
GND
NC = NO CONNECT
O/P SELECT
2.5V - NC
3.0V - GND
Initial error can be nulled using a single 25 kΩ potentiometer
connected between V
OUT
, Trim and GND. This is a coarse trim
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 MΩ) 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 kΩ/R
NULL
for either the 2.5 V or
3.0 V mode.
The external null resistor affects the overall temperature coeffi-
cient by a factor equal to the percentage of V
OUT
nulled.
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).
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
V
IN
= V
OUT
when current is forced into the output terminal.
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 kΩ 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.
–4–
REV. B
AD [ ANALOG DEVICES ]
