SPX2954 Datasheet, PDF (8/17 Pages) Sipex Corporation – 250 mA Low Drop Out Voltage Regulator

APPLICATIONS HINTS
EXTERNAL CAPACITORS
The stability of the SPX2954 requires a 1µF or
greater capacitor between output and ground.
Oscillation could occur without this capacitor.
Most types of tantalum or aluminum electro-
lytic works fine here. For operations below
-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 3.3
µF or greater must be used for the condition of
150mA load at 1.23V output.
The SPX2954 unlike other low dropout regula-
tors will remain stable and in regulation with no
load in addition to the internal voltage divider.
This feature is especially important in applica-
tions like CMOS RAM keep-alive. When set-
ting the output voltage of the SPX2954 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 SPX2954 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.
ERROR DETECTION COMPARATOR
OUTPUT
The comparator produces a logic low output
whenever the SPX2954 output falls out of regu-
lation by more than around 5%. This is around
60 mV 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 SPX2954 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_OUT = 4.75). Since the
SPX2954’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
400mA, this value adds to battery drain in a low
battery condition. Suggested values range from
100k to 1MΩ. If the output is unused this resis-
tor is not required.
PROGRAMMING THE OUTPUT
VOLTAGE OF SPX2954
The SPX2954 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 30V maximum rating using an external
pair of resistors. Refer to the below equation for
the programming of the output voltage:
V_OUT = V_REF x (1 + R₁/R₂)+ I_FBR₁
The V_REF is 1.235 and I_FB is the feedback bias
current, nominally -20nA. The minimum
recommended load current of 1 µA forces an
Rev. 3/6/03
SP2954 250mA Low Dropout Voltage Regulator
8
© Copyright 2003 Sipex Corporation