SPX2954AM3-3.3(2003) Просмотр технического описания (PDF) - Signal Processing Technologies
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SPX2954AM3-3.3 Datasheet PDF : 17 Pages
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APPLICATIONS HINTS (CONT)
upper limit of 1.2MΩ on value of R2. If no load
is presented the IFB produces an error of typically
2% in VOUT which may be eliminated at room
temperature by trimming R1. To improve the
accuracy choose the value of R2 = 100k this
reduces the error by 0.17% and increases the
resistor program current by 12µA. Since the
SPX2954 typically draws 60µA at no load with
Pin 2 open-circuited, this is a small price to pay.
+V IN
470KΩ
8
5 ______
+V IN
ERROR
470kΩ
1
V OUT
SPX2954
3
SD
R1
RESET
GND
FB
4
7
V OUT
+
1µF
REDUCING OUTPUT NOISE
It may be an advantage to reduce the AC noise
present at the output. One way is to reduce the
regulator bandwidth by increasing the size of the
output capacitor. This is the only way that noise
can be reduced on the 3 lead SPX2954 but is
relatively inefficient, as increasing the capacitor
from 1µF to 220 µF only decreases the noise
from 430µV to 160µVRMS for a 100kHz band-
width at 5V output. Noise can be reduced four-
fold by using a bypass capacitor across R , since
1
it reduces the high frequency gain from 4 to
unity. Pick
CBYPASS ≅ (1 / 2πR1 x 200 Hz)
or choose 0.01µF. When doing this, the output
capacitor must be increased to 3.3µF to maintain
stability. These changes reduce the output noise
from 430µV to 100µVRMS for a 100kHz band-
width at 5V output. With the bypass capacitor
added, noise no longer scales with output volt-
age so that improvements are more dramatic at
higher output voltages.
R2
Figure 2. Latch Off when Error Flag Occurs
+V IN
ERROR
OUTPUT
SHUTDOWN
INPUT
8
5 ______
+V IN
ERROR
1
V OUT
SPX2954
3
SD
GND
FB
4
7
*V OUT = V IN
*MINIMUM INPUT-OUTPUT VOLTAGE RANGES FROM 40mV to 400mV.
DEPENDING ON LOAD CURRENT.
Figure 3. Wide Imput Voltage Range, Current Limiter
4.75V
OUTPUT
VOLTAGE
_______
ERROR*
INPUT
VOLTAGE
+5.0V
+1.3V
* See Application Info.
Figure 1. Error Output Timing
+
+
Rev. 3/6/03
SP2954 250mA Low Dropout Voltage Regulator
9
© Copyright 2003 Sipex Corporation
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