ADR440 Analog Devices, Inc., ADR440 Datasheet - Page 14

• Manufacturer Part Number: ADR440
• Description: Ultralow Noise, Ldo Xfet Voltage References With Current Sink And Source
• Manufacturer: Analog Devices, Inc.

ADR440/ADR441/ADR443/ADR444/ADR445
THEORY OF OPERATION
The ADR44x series of references uses a new reference generation
technique known as XFET (eXtra implanted junction FET).
This technique yields a reference with low dropout, good
thermal hysteresis, and exceptionally low noise. The core of the
XFET reference consists of two junction field-effect transistors
(JFETs), one of which has an extra channel implant to raise its
pinch-off voltage. By running the two JFETs at the same drain
current, the difference in pinch-off voltage can be amplified
and used to form a highly stable voltage reference.
The intrinsic reference voltage is around 0.5 V with a negative
temperature coefficient of about –120 ppm/°C. This slope is
essentially constant to the dielectric constant of silicon, and it can
be closely compensated for by adding a correction term generated
in the same fashion as the proportional-to-absolute temperature
(PTAT) term used to compensate band gap references. The
advantage of an XFET reference is its correction term, which is
approximately 20 times lower and requires less correction than
that of a band gap reference. Because most of the noise of a band
gap reference comes from the temperature compensation
circuitry, the XFET results in much lower noise.
Figure 33 shows the basic topology of the ADR44x series. The
temperature correction term is provided by a current source
with a value designed to be proportional-to-absolute temperature.
The general equation is
where:
G is the gain of the reciprocal of the divider ratio.
ΔV
I
ADR44x devices are created by on-chip adjustment of R2
and R3 to achieve the different voltage option at the
reference output.
PTAT
P
is the difference in pinch-off voltage between the two JFETs.
is the positive temperature coefficient correction current.
V
I
OUT
PTAT
= G (ΔV
*EXTRA CHANNEL IMPLANT
V
OUT
*
I
1
Figure 33. Simplified Schematic Device
= G (ΔV
P
− R1 × I
I
1
P
– R1 × I
ΔV
PTAT
P
PTAT
)
)
R1
ADR44x
R3
R2
GND
V
V
IN
OUT
Rev. B | Page 14 of 20
(1)
POWER DISSIPATION CONSIDERATIONS
The ADR44x family of references is guaranteed to deliver load
currents to 10 mA with an input voltage that ranges from 3 V to
18 V. When these devices are used in applications at higher
currents, use the following equation to account for the
temperature effects of increases in power dissipation:
where:
T
respectively.
P
θ
BASIC VOLTAGE REFERENCE CONNECTIONS
The ADR44x family requires a 0.1 μF capacitor on the input
and the output for stability. While not required for operation,
a 10 μF capacitor at the input can help with line voltage
transient performance.
NOISE PERFORMANCE
The noise generated by the ADR44x family of references is
typically less than 1.4 μV p-p over the 0.1 Hz to 10.0 Hz band
for ADR440, ADR441, and ADR443. Figure 26 shows the 0.1 Hz
to 10 Hz noise of the ADR441, which is only 1.2 μV p-p. The
noise measurement is made with a band-pass filter made of a
2­pole high-pass filter with a corner frequency at 0.1 Hz and a
2­pole low-pass filter with a corner frequency at 10.0 Hz.
TURN-ON TIME
Upon application of power (cold start), the time required for
the output voltage to reach its final value within a specified
error band is defined as the turn-on settling time. Two compo-
nents normally associated with this are the time for the active
circuits to settle and the time for the thermal gradients on the
chip to stabilize. Figure 20 and Figure 21 show the turn-on and
turn-off settling times for the ADR441.
JA
D
J
and T
is the device power dissipation.
is the device package thermal resistance.
T
V
NOTES
1.
2.
J
IN
= P
NC = NO CONNECT
TP = TEST PIN (DO NOT CONNECT)
10µF
A
are the junction and ambient temperatures,
D
+
Figure 34. Basic Voltage Reference Configuration
× θ
0.1µF
JA
+ T
A
GND
NC
TP
1
2
3
4
(Not to Scale)
ADR440/
ADR441/
ADR443/
ADR444/
ADR445
TOP VIEW
8
7
6
5
TP
NC
V
TRIM
OUT
0.1µF
(2)