spx2950acn-l-5-0-tr Exar Corporation, spx2950acn-l-5-0-tr Datasheet - Page 9

• Manufacturer Part Number: spx2950acn-l-5-0-tr
• Description: Spx2951 -150ma Low Dropout Voltage Regulator
• Manufacturer: Exar Corporation

PROGRAMMING THE OUTPUT
VOLTAGE OF SPX2951
The SPX2951 may be pin-strapped for 5V using
its internal voltage divider by tying Pin 1 (out-
put) to Pin 2 (sense) and Pin 7 (feedback) to Pin
6 (5V Tap). Also, it may be programmed for any
output voltage between its 1.235V reference
and its 29V maximum rating . As seen in Figure
2, an external pair of resistors is required. Refer
to the below equation for the programming of
the output voltage:
The V
current, nominally -20nA. The minimum rec-
ommended load current of 1 µA forces an upper
limit of 1.2MΩ on value of R
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
SPX2951 typically draws 60µA at no load with
Pin 2 open-circuited this is a small price to pay.
TYPICAL APPLICATIONS
Figure 2. Adjustable Regulator
DATE: 12/01/04
V
REF
OUT
OUT
is 1.235 and I
= V
which may be eliminated at room
FB
REF
produces an error of typically
× (1 + R
FB
ERROR
OUTPUT
SHUTDOWN
INPUT
is the feedback bias
1
1
/R
. To improve the
SPX2950/SPX2951 150mA Low Dropout Voltage Regulator
100K
2
2
)+ I
. If no load is
2
= 100k this
FB
3
R
5
1
SD
______
ERROR
GND
4
SPX2951
+V
+V
9
IN
IN
8
7
FB
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 SPX2950 but
is relatively inefficient, as increasing the ca-
pacitor from 1µF to 220µF only decreases the
noise from 430µV to 160µV
bandwidth at 5V output. Noise could also be
reduced fourfold by a bypass capacitor across
R
4 to unity. Pick
or choose 0.01µF. When doing this, the output
capacitor must be increased to 3.3µF to main-
tain stability. These changes reduce the output
noise from 430µV to 100µV
bandwidth at 5V output. With the bypass ca-
pacitor added, noise no longer scales with out-
put voltage so that improvements are more dra-
matic at higher output voltages.
1.23V
V
1
V
OUT
, since it reduces the high frequency gain from
REF
1
C
R
R
BYPASS
1
2
1.24 to 29V
.01µF
≅ (1 / 2πR
+
3.3µF
©Copyright 2004 Sipex Corporation
1
× 200 Hz)
RMS
RMS
for a 100kHz
for a 100kHz