PJ2950/PJ2951
150mA Low Dropout Voltage Regulators
4-7
2002/01.rev.A
External Capacitors
For
the
stability
of
the
PJ2950/PJ2951 requires a 1.0 μF or
greater capacitor between output and
ground.
Oscillation
could
occur
without this capacitor. The tantalum
or aluminum electrolytic works fine;
could use the fiLP type but are not
cost effi cient. For the operation of
below -25 ℃
solid tantalum
is
recommend ed
since
the
many
aluminum type have electrolytes the
freeze at about
-30℃. The ESR of
about 5 Ω
or less and resonant
frequ ency above 500KHz are most
important parameter in the value of
Capacitor. The Capacitors value can
be increas ed without any limit.
At lower values of output current,
less output capacitance is required
fo r stability. For the currents below
10 mA the value of capacitor can be
reduce to 0.33 μ F and 0.1 μ F for
1mA.
More
output
capacitance
needed
for the 8-pin
version at
voltage below 5V since it runs the
error amplifi er at lower gain. At
worst case 3.3μF or greater must be
used for condition of 100mA load at
1.250V output .
The PJ2950 not like other low
dropout regulators will remain stable
and
regulation
with
no
load in
addition
to
the
internal
voltage
divider. This feature very important
in
application like CMOS RAM
keep-alive.
The
PJ2951
output
voltage setting with minimum load
of 1 μA and external resistors.
If at the input of PJ2950/LP
2951 connected to battery or between
AC filter capacitor and input is 10
inches wire then 1 μ F tantalum or
aluminum
electrolytic
capacitor
should be connect ed between input
and ground .
Instability can occur
if stray
capacitor to feedb ack terminal pin 7
of the PJ2951. This could cues more
problem more when using
higher
value of external to set the output
voltage . To fix this problem the 100
pF capacitor between output and
feedb ack
and
increasing
output
capacitance to 3.3μF.
The Compactor produces a logic
low output whenever the PJ2951
output falls of regulation by more
than around 5% . This is around 60
mV offs et divided by the 1.250
referen ce voltag e. Thus trip remains
5% below nominal regardl ess of the
programmed output voltage o f the
regulator . the figure 1 shows the
timing
diagram
depicting
the
ERROR signal and the regulator
output voltage as the PJ2951 input is
ramped up and down . the ERROR
signal becomes low at around 1.3 V
input , it goes high around 5Volt
input , (input voltage at which Vout
= 4.75). The PJ2951 dropout voltage
depending on the load , the input
voltage trip point around 5 volt will
vary with load current. The output
voltage trip point is around 4.75 volt
it does not change with load.
The error comparator h as an open-
collector output which requires an
external
pull-up
resistor.
Depending
on
the
system
requirem ent
the resistor maybe
returned to 5 or 3.3 volt output or
other supply voltage depending to
the
system
requirements.
For
determining the size of the resistor,
note that the output is sinking 400μ
A , this value adds to battery drain in
a low better condition, recommended
values 100K to 1MΩ. If the output
is unused this resistor
is not required.
ERROR DETECTION COMPARATOR OUTPUT
PROGRAMMING THE OUTPUT VOLTAGE OF PJ2951
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