Datasheet

Data Sheet ADR3525/ADR3530/ADR3533/ADR3540/ADR3550
Rev. 0 | Page 17 of 20
THEORY OF OPERATION
BAND GAP
VOLTAGE
REFERENCE
ENABLE
GND FORCE
V
OUT
FORCE
V
OUT
SENSE
R
FB2
R
FB1
V
IN
V
BG
GND SENSE
09594-036
Figure 36. Block Diagram
The ADR3525W/ADR3530W/ADR3533W/ADR3540W/
ADR3550W use a patented voltage reference architecture to
achieve high accuracy, low temperature coefficient (TC), and
low noise in a CMOS process. Like all band gap references, the
references combine two voltages of opposite TCs to create an
output voltage that is nearly independent of ambient tempera-
ture. However, unlike traditional band gap voltage references, the
temperature-independent voltage of the references is arranged to
be the base-emitter voltage, V
BE
, of a bipolar transistor at room
temperature rather than the V
BE
extrapolated to 0 K (the V
BE
of
bipolar transistor at 0 K is approximately V
G0
, the band gap
voltage of silicon). A corresponding positive TC voltage is then
added to the V
BE
voltage to compensate for its negative TC.
The key benefit of this technique is that the trimming of the
initial accuracy and TC can be performed without interfering
with one another, thereby increasing overall accuracy across
temperature. Curvature correction techniques further reduce
the temperature variation.
The band gap voltage (V
BG
) is then buffered and amplified to
produce stable output voltages of 2.5 V and 5.0 V. The output
buffer can source up to 10 mA and sink up to −3 mA of load
current.
The ADR35xx references leverage Analog Devices patented
DigiTrim technology to achieve high initial accuracy and low
TC, and precision layout techniques lead to very low long-term
drift and thermal hysteresis.
LONG-TERM OUTPUT VOLTAGE DRIFT
One of the key parameters of the ADR35xx references is long-
term output voltage drift. Independent of the output voltage
model and in a 50°C environment, these devices exhibit a
typical drift of approximately 30 ppm after 1000 hours of
continuous, unloaded operation.
It is important to understand that long-term output voltage drift
is not tested or guaranteed by design and that the output from
the device may shift beyond the typical 30 ppm specification.
Because most of the drift occurs in the first 200 hours of device
operation, burning in the system board with the reference
mounted can reduce subsequent output voltage drift over time.
See the AN-713 Application Note, The Effect of Long-Term Drift
on Voltage References, at www.analog.com for more information
regarding the effects of long-term drift and how it can be
minimized.
POWER DISSIPATION
The ADR35xx voltage references are capable of sourcing up to
10 mA of load current at room temperature across the rated
input voltage range. However, when used in applications subject
to high ambient temperatures, the input voltage and load cur-
rent should be carefully monitored to ensure that the device
does not exceed its maximum power dissipation rating. The
maximum power dissipation of the device can be calculated via
the following equation:
][W
TT
P
JA
A
J
D



where:
P
D
is the device power dissipation.
T
J
is the device junction temperature.
T
A
is the ambient temperature.
θ
JA
is the package (junction-to-air) thermal resistance.
Because of this relationship, the acceptable load current in high
temperature conditions may be less than the maximum current-
sourcing capability of the device. In no case should the part be
operated outside of its maximum power rating because doing so
can result in premature failure or permanent damage to the device.