LP2950 Datasheet, PDF (9/16 Pages) National Semiconductor (TI) – Series of Adjustable Micropower Voltage Regulators

LP2950
Figure 16. Adjustable Regulator
Vin
Error
Output
Shutdown
Input
100 k
8
5
Error
Vin
Vout
1
SNS ² NC
3 _SD
VO T ⁶
R1
NC
Gnd FB
4
7
Vout
1.23 to 30 V
0.01 µF _{3.3 µF}
R2
The complete equation for the output voltage is:
+ ) ń ) Vout V_ref (1 R1 R2) IFB R1
where Vref is the nominal 1.235 V reference voltage and IFB
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
must work with no load. IFB will produce a 2% typical error
in Vout which may be eliminated at room temperature by
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.
Output Noise
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
decreases the noise from 430 µV to 160 µVrms for a 100 kHz
bandwidth at the 5.0 V output.
Noise can be reduced fourfold by a bypass capacitor
across R1, since it reduces the high frequency gain from 4
to unity. Pick
[ CBypass
1
2pR1 x 200 Hz
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.
Figure 17. 1.0 A Regulator with 1.2 V Dropout
Unregulated
Input
1.0 µF _{10 k}
MTB23P06E
0.01 µF
Error
Output
Shutdown
Input
8
5 _Error
Vin
Vout
1
SNS ²
LP2951CN
3 _SD
VO T ⁶
Gnd FB
4
7
0.002 µF
Vout
5.0 V ±1.0%
0 to 1.0 A
220 µF
1.0 M
2.0 k
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