LP2951CDG Datasheet, PDF (10/22 Pages) ON Semiconductor – 100 mA, Low Power Low Dropout Voltage Regulator

LP2950, LP2951, NCV2951
V_in
Error
Output
Shutdown
Input
100 k
8
5
Error
V_in
V_out
1
2
SNS
NC
3 _SD
V_O T
6
R1
NC
GND FB
4
7
V_out
1.23 to 30 V
0.01 mF _{3.3 mF}
R₂
Figure 19. Adjustable Regulator
The complete equation for the output voltage is:
Vout + V_ref ⁽1 ) R1ńR2⁾ ) IFB R1
where V_ref is the nominal 1.235 V reference voltage and I_FB
is the feedback pin bias current, nominally − 20 nA. The
minimum recommended load current of 1.0 mA forces an
upper limit of 1.2 MW on the value of R2, if the regulator
must work with no load. I_FB will produce a 2% typical error
in V_out 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 mA. Since the LP2951 typically draws
75 mA at no load with Pin 2 open circuited, the extra 12 mA
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 mF to 220 mF only
decreases the noise from 430 mV to 160 mVrms 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
[
2pR1
1
x 200
Hz
or about 0.01 mF. When doing this, the output capacitor must
be increased to 3.3 mF to maintain stability. These changes
reduce the output noise from 430 mV to 126 mVrms 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.
Unregulated
Input
1.0 mF _{10 k}
MTB23P06E
0.01 mF
Error
Output
Shutdown
Input
8
5 _Error
V_in
V_out
1
SNS ²
LP2951CN
3 _SD
V_O T ⁶
GND FB
4
7
0.002 mF
V_out
5.0 V 1.0%
0 to 1.0 A
220 mF
1.0 M
2.0 k
Figure 20. 1.0 A Regulator with 1.2 V Dropout
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