spx2950acn-l-5-0-tr Exar Corporation, spx2950acn-l-5-0-tr Datasheet - Page 8

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

APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the SPX2950/SPX2951 requires
a 1µF 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 be-
low -25°C solid tantalum is recommended since
the many aluminum types have electrolytes that
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 capacitors value may be in-
creased 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 0.33µF and 0.1µF for 1mA.
More output capacitance is needed for the 8-pin
version at voltages below 5V since it runs the
error amplifier at lower gain. At worst case 3.3µF
or greater must be used for the condition of
150mA load at 1.23V output.
The SPX2950/51 unlike other low dropout regu-
lators will remain stable and in regulation with
no load in addition to the internal voltage di-
vider. This feature is especially important in
applications like CMOS RAM keep-alive. When
setting the output voltage of the SPX2950/51
version with external resistors, a minimum load
of 1µA 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 1µF tantalum
or aluminum electrolytic capacitor should be
placed from the input to the ground.
Instability can occur if there is stray capacitance
to the SPX2951 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
3.3µF.
DATE: 12/01/04
SPX2950/SPX2951 150mA Low Dropout Voltage Regulator
8
Fig. 1 ERROR Output Timing
ERROR DETECTION COMPARATOR
OUTPUT
The comparator produces a logic low output
whenever the SPX2951 output falls out of regu-
lation by more than around 5%. This is around
60mV offset divided by the 1.235 reference
voltage. This trip level remains 5% below nor-
mal regardless of the programmed output volt-
age of the regulator.
Figure 1 shows the timing diagram depicting the
ERROR signal and the regulator output voltage
as the SPX2951 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 V
SPX2951’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 out-
put, which requires an external pull-up resistor.
Depending on the system requirements the re-
sistor 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
400µA, this value adds to battery drain in a low
battery condition. Suggested values range from
100K to 1MΩ. If the output is unused this
resistor is not required.
VOLTAGE
ERROR*
VOLTAGE
_______
OUTPUT
* See Application Info.
INPUT
+
4.75V
1.3V
+
5.0V
OUT
©Copyright 2004 Sipex Corporation
= 4.75). Since the
+
+