ADR391A AD [Analog Devices], ADR391A Datasheet - Page 14

• Manufacturer Part Number: ADR391A
• Description: Micropower, Low Noise Precision Voltage References with Shutdown
• Manufacturer: AD [Analog Devices]

ADR391/ADR392/ADR395
THEORY OF OPERATION
Band gap references are the high performance solution for low
supply voltage and low power voltage reference applications,
and the ADR391/ADR392/ADR395 are no exception. The
uniqueness of these devices lies in the architecture. As shown in
Figure 33, the ideal zero TC band gap voltage is referenced to
the output, not to ground. Therefore, if noise exists on the
ground line, it is greatly attenuated on V
consists of the PNP pair, Q51 and Q52, running at unequal
current densities. The difference in V
a positive TC, which is amplified by a ratio of
This PTAT voltage, combined with V
produces a stable band gap voltage.
Reduction in the band gap curvature is performed by the ratio
of Resistors R44 and R59, one of which is linearly temperature
dependent. Precision laser trimming and other patented circuit
techniques are used to further enhance the drift performance.
2 ×
SHDN
R58
R54
Figure 33. Simplified Schematic
R59
R54
R60
Q51
Q1
R53
R58
Q52
R44
R61
R48
R49
BE
BE
s of Q51 and Q52,
results in a voltage with
OUT
. The band gap cell
V
V
V
GND
OUT (FORCE)
OUT (SENSE)
IN
Rev. H | Page 14 of 20
DEVICE POWER DISSIPATION CONSIDERATIONS
The ADR391/ADR392/ADR395 are capable of delivering load
currents to 5 mA, with an input voltage that ranges from 2.8 V
(ADR391 only) to 15 V. When these devices are used in
applications with large input voltages, care should be taken to
avoid exceeding the specified maximum power dissipation or
junction temperature because it could result in premature
device failure. The following formula should be used to
calculate the maximum junction temperature or dissipation of
the device:
where:
T
P
θ
SHUTDOWN MODE OPERATION
The ADR391/ADR392/ADR395 include a shutdown feature
that is TTL/CMOS level compatible. A logic low or a zero volt
condition on the SHDN pin is required to turn the devices off.
During shutdown, the output of the reference becomes a high
impedance state, where its potential would then be determined
by external circuitry. If the shutdown feature is not used, the
SHDN pin should be connected to V
JA
D
J
and T
is the device power dissipation.
is the device package thermal resistance.
P
D
A
=
are, respectively, the junction and ambient temperatures.
T
J
θ
–
JA
T
A
IN
(Pin 2).
(5)