spx2954am3-l-5-0-tr Exar Corporation, spx2954am3-l-5-0-tr Datasheet - Page 9

• Manufacturer Part Number: spx2954am3-l-5-0-tr
• Description: 250ma Low Dropout Voltage Regulator
• Manufacturer: Exar Corporation

APPLICATIONS HINTS (
upper limit of 1.2MΩ on value of R
is presented the I
2% in V
temperature by trimming R
accuracy choose the value of R
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.
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
it reduces the high frequency gain from 4 to
unity. Pick
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 100µV
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.
Date: 12/9/05
Figure 1. Error Output Timing
VOLTAGE
ERROR*
VOLTAGE
_______
OUTPUT
* See Application Info.
INPUT
OUT
C
BYPASS
+
4.75V
which may be eliminated at room
1.3V
+
FB
≅ (1 / 2πR
5.0V
produces an error of typically
RMS
for a 100kHz band-
1
CONT
x 200 Hz)
1
. To improve the
SPX2954, 250mA Low Dropout Voltage Regulator
)
+
2
= 100k this
2
. If no load
+
1
, since
9
Figure 2. Latch Off when Error Flag Occurs
Figure 3. Wide Imput Voltage Range, Current Limiter
INPUT
OUTPUT
SHUTDOWN
ERROR
*MINIMUM INPUT-OUTPUT VOLTAGE RANGES FROM 40mV to 400mV.
DEPENDING ON LOAD CURRENT.
470KΩ
RESET
470kΩ
3
3
5
5
SD
______
ERROR
SD
______
ERROR
GND
SPX2954
4
SPX2954
GND
+V
+V
+V
+V
© Copyright 2005 Sipex Corporation
4
IN
IN
8
IN
IN
8
7
FB
FB
V
V
OUT
7
OUT
1
1
R
R
1
2
*V
OUT
V
OUT
= V
+
1µF
IN