rc2951mt Fairchild Semiconductor, rc2951mt Datasheet - Page 6

• Manufacturer Part Number: rc2951mt
• Description: Adjustable Micropower Voltage Regulator
• Manufacturer: Fairchild Semiconductor

RC2951
.
** Drive with TTL-high to shut down. Ground or leave open if shutdown feature is not to be used.
The complete equation for the output voltage is
Adding the error term,
where V
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 R
must work with no load (a condition often found in CMOS in
standby). I
may be eliminated at room temperature by trimming R
For better accuracy, choosing R
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.
6
V
V
------------- -
V
V
V
R
OUT
OUT
OUT
OUT
REF
2
=
=
=
=
=
REF
------------------ -
R
V
V
V
V
FB
V
1
REF
REF
REF
REF
is the nominal 1.235 reference voltage and I
OUT
+
will produce a 2% typical error in V
R
**Shutdown






•
2
R
------------------ -
1
1
Input
Error
Output


1
+
+
1
R
+
R
----- -
R
R
----- -
R
+
2
R
1
2
1
2
R
----- -
R




2
1
2


100K
+


5
3
I
+
FB
SD
ERROR
GND
I
FB
2
R
4
= 100k reduces this error
RC2951
2
+V
R
+V
1
IN
IN
8
V
FB
V
OUT
2
7
REF
, if the regulator
1.23V
1
Figure 2. Adjustable Regulator
R
R
OUT
1
2
1.2
V
+ +
.01
µF
which
OUT
1
.
30V
FB
2950-11
3.3µF
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
R
Pick
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.
C
1
, since it reduces the high frequency gain from 4 to unity.
BYPASS
V
OUT
=
≅
----------------------------------- -
2πR
V
REF
1


•
1
1
200Hz
+
R
----- -
R
1
2


PRODUCT SPECIFICATION
REV. 1.0.1 4/8/02