Datasheet

ADR380/ADR381
Rev. C | Page 11 of 16
THEORY OF OPERATION
Band gap references are the high performance solution for low
supply voltage and low power voltage reference applications, and
the ADR380/ADR381 are no exception. However, the uniqueness
of this product lies in its architecture. As shown in Figure 26,
the ideal zero TC band gap voltage is referenced to the output,
not to ground. The band gap cell consists of the PNP pair Q51
and Q52, running at unequal current densities. The difference
in V
BE
results in a voltage with a positive TC that is amplified
by the ratio of 2 × R58/R54. This PTAT voltage, combined with
the V
BE
of Q51 and Q52, produce the stable band gap voltage.
Reduction in the band gap curvature is performed by the ratio
of the two resistors, R44 and R59. Precision laser trimming and
other patented circuit techniques are used to further enhance
the drift performance.
GND
V
OUT
V
IN
Q1
R59
R54
Q51
R60
R61
R48
R49
R44
R58
R53
Q52
–
+
02175-026
Figure 26. Simplified Schematic
DEVICE POWER DISSIPATION CONSIDERATIONS
The ADR380/ADR381 are capable of delivering load currents to
5 mA with an input voltage that ranges from 2.8 V (ADR381 only)
to 15 V. When this device is used in applications with large input
voltages, take care to avoid exceeding the specified maximum
power dissipation or junction temperature that may result in
premature device failure. Use the following formula to calculate
a device’s maximum junction temperature or dissipation:
JA
A
J
D
TT
P
θ
−
=
where:
P
D
is the device power dissipation,
T
J
and T
A
are junction and ambient temperatures, respectively.
θ
JA
is the device package thermal resistance.
INPUT CAPACITOR
An input capacitor is not required on the ADR380/ADR381.
There is no limit for the value of the capacitor used on the input,
but a capacitor on the input improves transient response in
applications where the load current suddenly increases.
OUTPUT CAPACITOR
The ADR380/ADR381 do not need an output capacitor for
stability under any load condition. Using an output capacitor,
typically 0.1 μF, removes any very low level noise voltage and does
not affect the operation of the part. The only parameter that
degrades by applying an output capacitor is turn-on time. (This
varies depending on the size of the capacitor.) Load transient
response is also improved with an output capacitor, which acts
as a source of stored energy for a sudden increase in load current.