RC2951 データシートの表示（PDF） - Fairchild Semiconductor
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RC2951 Datasheet PDF : 11 Pages
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RC2951
.
PRODUCT SPECIFICATION
+VIN
100K
8
Error
Output
+VIN
5 ERROR VOUT 1
RC2951
**Shutdown 3
Input
SD
GND
FB
4
7 1.23V
VOUT
1.2 30V
++
R1
3.3µF
.01
µF
VREF
R2
2950-11
VOUT
=
VREF
1
+
R-R----12-
** Drive with TTL-high to shut down. Ground or leave open if shutdown feature is not to be used.
Figure 2. Adjustable Regulator
The complete equation for the output voltage is
VOUT
=
VREF
•
1
+
R-R----12-
+ IFBR1
V-----R----E---F- = ---V----O----U----T----
R2
R1 + R2
VOUT
=
VRE
F
R-----1--R--+--2---R----2-
VOUT
=
VRE
F
1
+
RR-----12-
Adding the error term,
VOUT
=
VRE
F
1
+
RR-----12-
+ IFBR2
where VREF is the nominal 1.235 reference voltage and IFB
is the feedback pin bias current, nominally -20 nA. The
minimum recommended load current of 1 µA forces an
upper limit of 1.2 MΩ on the value of R2, if the regulator
must work with no load (a condition often found in CMOS in
standby). IFB will produce a 2% typical error in VOUT which
may be eliminated at room temperature by trimming R1.
For better accuracy, choosing R2 = 100k reduces this error
to 0.17% while increasing the resistor program current to
12 µA. Since the RC2951 typically draws 60 µA at no load
with Pin 2 open-circuited, this is a small price to pay.
Reducing Output Noise
In reference applications it may be advantageous to reduce
the AC noise present at the output. One method is to reduce
the regulator bandwidth by increasing the size of the output
capacitor.
Noise can be reduced fourfold by a bypass capacitor across
R1, since it reduces the high frequency gain from 4 to unity.
Pick
CBYPASS ≅ -2---π----R----1----•-1---2---0---0---H-----z-
or about 0.01 µF. When doing this, the output capacitor must
be increased to 3.3 µF to maintain stability. These changes
reduce the output noise from 430 µV to 100 µV rms for a
100 kHz bandwidth at 5V output. With the bypass capacitor
added, noise no longer scales with output voltage so that
improvements are more dramatic at higher output voltages.
6
REV. 1.0.0 2/8/01
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