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SPX2955 Datasheet(PDF) 5 Page - Sipex Corporation

Part No. SPX2955
Description  350mA Low Drop Out Voltage Regulator with Shutdown
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Maker  SIPEX [Sipex Corporation]
Homepage  http://www.sipex.com
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SPX2955 Datasheet(HTML) 5 Page - Sipex Corporation

   
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Rev. 11/21/00
SPX2955/56/57
APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the SPX2955/56/57 requires a 2.2
µF or greater
capacitor between output and ground. Oscillation could occur
without this capacitor. Most types of tantalum or aluminum
electrolytic works fine here. For operations of below -25
°C solid
tantalum is recommended since the many aluminum types have
electrolytes the freeze at about –30
°C. The ESR of about 5Ω or
less and resonant frequency above 500kHz are the most
important parameters in the value of the capacitor. The capacitor
value can be increased without limit.
At lower values of output current, less output capacitance is
required for stability. For the currents below 10mA the value of
the capacitor can be reduced to 0.5
µF and 0.15µF for 1mA. More
output capacitance needed for the 8-pin version at voltages below
5V since it runs the error amplifier at lower gain. At worst case
5
µF or greater must be used for the condition of 250mA load at
1.23V output.
The SPX2955/56/57, unlike other low dropout regulators will
remain stable and in regulation with no load in addition to the
internal voltage divider. This feature is especially important in
application like CMOS RAM keep-alive. When setting the output
voltage of the SPX2955/56/57, a minimum load of 10mA is
recommended.
If there is more than 10 inches of wire between the input and the
AC filter capacitor or if a battery is used as the input then a 0.1
µF
tantalum or aluminum electrolytic capacitor should be placed
from the input to the ground.
Instability can occur if there is stray capacitance to the
SPX2955/56/57 feedback terminal (pin 7). This could cause
more problems when using a higher value of external resistors to
set the output voltage.
This problem can be fixed by adding a 100pF capacitor between
output and feedback and increasing the output capacitor to at least
3.3
µF.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the
SPX2955/56/57 output falls out of regulation by more than around
5%. This is around 60mV offset divided by the 1.235 reference
voltage. This trip level remains 5% below normal regardless of the
programmed output voltage of the regulator. Figure 1 shows the
timing diagram depicting the ERROR signal and the regulator output
voltage as the SPX2955/56/57 input is ramped up and down. The
ERROR signal becomes low at around 1.3V input, and goes high
around 5V input (input voltage at which Vout = 4.75). Since the
SPX2955/56/57’s dropout voltage is load dependent, the input voltage
trip point (around 5V) will vary with the load current. The output
voltage trip point (approx. 4.75V) does not vary with load.
The error comparator has an open-collector output, which requires an
external pull-up resistor. Depending on the system requirements the
resistor may be returned to 5V output or other supply voltage. In
determining the value of this resistor, note that the output is rated to
sink 400
µA, this value adds to battery drain in a low battery
condition. Suggested values range from 100K to 1M
Ω. If the output
is unused this resistor is not required.
PROGRAMMING THE OUTPUT VOLTAGE OF
SPX2955/56/57
The SPX2956/57 may be pin-strapped for 5V using its internal
voltage divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7
(feedback) to Pin 6 (5V Tap).
+
+
+
+
4.75V
OU TPU T
VO LT AG E
E RRO R*
__ ____ _
IN P U T
VO LT AG E
1 .3 V
5.0V
F igu r e 1. E R R O R O u tp u t T im in g
___ ___ _
* S ee A p p licatio n In fo .
100K
OUTPUT
ERROR
ERROR
______
SD
VOUT
GND
FB
+VIN
+VIN
1.23V
1.2 to 29V
R1
.01uF
10uF
R2
V
REF
3
8
5
1
4
7
SHUTDOWN
INPUT
+
Figure 2. Adjustable Regulator
SPX2955/57


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