LP2950
Figure 16. Adjustable Regulator
decreasesthenoisefrom430µVto160µVrmsfora100kHz
bandwidth at the 5.0 V output.
V
in
Noise can be reduced fourfold by a bypass capacitor
across R1, since it reduces the high frequency gain from 4
to unity. Pick
100 k
5
8
1
V
in
C
1
Error
Output
Bypass
2 R1 x 200 Hz
V
out
Error
V
out
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 126 µVrms for a
100 kHz bandwidth at 5.0 V output. With bypass
capacitor added, noise no longer scales with output voltage
so that improvements are more dramatic at higher output
voltages.
2
1.23 to 30 V
NC
NC
SNS
R1
Shutdown
Input
3
6
0.01 µF
SD
V T
O
3.3 µF
Gnd FB
4
7
R
2
Figure 17. 1.0 A Regulator with 1.2 V Dropout
Unregulated
The complete equation for the output voltage is:
Input
MTB23P06E
1.0 µF
(
)
V
1
R1 R2
I
R1
out
ref
FB
10 k
0.01 µF
whereV isthenominal1.235VreferencevoltageandI
ref FB
V
out
is the feedback pin bias current, nominally –20 nA. The
minimum recommended load current of 1.0 µA forces an
upper limit of 1.2 MΩ on the value of R2, if the regulator
5.0 V ±1.0%
0 to 1.0 A
8
V
in
1
2
5
3
Error
Output
V
Error
out
must work with no load. I will produce a 2% typical error
FB
in V which may be eliminated at room temperature by
out
SNS
LP2951CN
adjusting R1. For better accuracy, choosing R2 = 100 k
reduces this error to 0.17% while increasing the resistor
program current to 12 µA. Since the LP2951 typically draws
75 µA at no load with Pin 2 open circuited, the extra 12 µA
of current drawn is often a worthwhile tradeoff for
eliminating the need to set output voltage in test.
6
Shutdown
Input
SD
V T
O
220 µF
Gnd FB
4
7
0.002 µF
1.0 M
Output Noise
2.0 k
In many applications it is desirable to reduce the noise
present at the output. Reducing the regulator bandwidth by
increasing the size of the output capacitor is the only method
for reducing noise on the 3 lead LP2950. However,
increasing the capacitor from 1.0 µF to 220 µF only
http://onsemi.com
9
ONSEMI [ ONSEMI ]
