ADR293GBC 데이터 시트보기 (PDF) - Analog Devices
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ADR293GBC Datasheet PDF : 11 Pages
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ADR293
THEORY OF OPERATION
The ADR293 uses a new reference generation technique known
as XFET, which yields a reference with low noise, low supply
current and very low thermal hysteresis.
The core of the XFET reference consists of two junction field-
effect transistors 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/K. This slope is
essentially locked to the dielectric constant of silicon and can be
closely compensated by adding a correction term generated in
the same fashion as the proportional-to-temperature (PTAT)
term used to compensate bandgap references. The big advan-
tage over a bandgap reference is that the intrinsic temperature
coefficient is some thirty times lower (therefore less correction is
needed) and this results in much lower noise since most of the
noise of a bandgap reference comes from the temperature com-
pensation circuitry.
The simplified schematic below shows the basic topology of the
ADR293. The temperature correction term is provided by a
current source with value designed to be proportional to abso-
lute temperature. The general equation is:
( )( ) VOUT
=
∆VP
R1+
R2 +
R1
R3
+
I PTAT
R3
where ∆VP is the difference in pinch-off voltage between the two
FETs and IPTAT is the positive temperature coefficient correction
current.
The process used for the XFET reference also features vertical
NPN and PNP transistors, the latter of which are used as output
devices to provide a very low drop-out voltage.
VIN
I1 I1
*
⌬VP
VOUT
R1
IPTAT
R2
R3
*EXTRA CHANNEL IMPLANT
VOUT ؍R1؉R2؉R3 ؋ ⌬VP؉ IPTAT ؋ R3
R1
GND
Figure 18. Simplified Schematic
Device Power Dissipation Considerations
The ADR293 is guaranteed to deliver load currents to 5 mA
with an input voltage that ranges from 5.5 V to 15 V. When this
device is used in applications with large input voltages, care
should be exercised to avoid exceeding the published specifica-
tions for maximum power dissipation or junction temperature
that could result in premature device failure. The following
formula should be used to calculate a device’s maximum junc-
tion temperature or dissipation:
PD = TJ −TA
θJA
In this equation, TJ and TA are the junction and ambient tem-
peratures, respectively, PD is the device power dissipation, and
θJA is the device package thermal resistance.
Basic Voltage Reference Connections
References, in general, require a bypass capacitor connected
from the VOUT pin to the GND pin. The circuit in Figure 19
illustrates the basic configuration for the ADR293. Note that
the decoupling capacitors are not required for circuit stability.
INPUT
NC 1
2
+
10F
NC 3
0.1F
4
ADR293
8 NC
7 NC
OUTPUT
6
5 NC
NC = NO CONNECT
0.1F
Figure 19. Basic Voltage Reference Configuration
Noise Performance
The noise generated by the ADR293 is typically less than
15 µVp-p over the 0.1 Hz to 10 Hz band. The noise measure-
ment is made with a bandpass filter made of a 2-pole high-pass
filter with a corner frequency at 0.1 Hz and a 2-pole low-pass
filter with a corner frequency at 10 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 components
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 13 shows the typical turn-on time for the
ADR293.
–8–
REV. 0
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