PJ2950/PJ2951
150mA Low Dropout Voltage Regulators
5-7
2002/01.rev.A
The
PJ2951
output
voltage
is
programmable for any valu e from it
reference voltage of 1.250 volt
and it maximum rating of 30 volt .
For example for 5 volt needs to pin-
strapped
and
using
the
internal
voltage divider by tying pin 1 to 2
and pin 7 to pin
6. Refer to Figure
2, there are two ext ernal resistor
required for this programming .
Refer to the below equation fo r the
programming the output voltage:
Volt = Vref x (1+R1\R2)+IFBR1
The Vref is 1.250 and IFB
is the
feedb ack bias current , nominally -20
nA. The minimum recommended
load current o f 1μA fo rces an upper
limit of 1.2MΩ on value of R2 , If
no load presented the IFB produces
error o f typically 2% in Vout which
maybe
eliminated
at
room
temperature by trimming R1 . To
improve the accuracy chooses the
value of
R2 =100K
and this
reduces the error by 0.17% and
increas es
the
resistor
program
current by 12 μ A . The PJ2951
typically draw 60 μ A at no load
with
pin
2 open-circuit,and this
value does not look much .
It could be an advantag e to reduce
the AC noise present at the output
One way is to reduce the regulator
bandwidth by increasing the value o f
the output capacitor . this is the only
method that noise could reduce on
the
PJ2950
but
is
relatively
inefficient
, as increasing capacitor
from 1μF to 220μF only decreas es
the noise from 430μV to 160 μV
rms . for a 100KHz bandwidth at 5
volt output .
Noise could
be also
reduce by
fou rfold by a
bypass capacitor
across R1 since it reduces the high
frequ ency gain from 4 to unity . pick
CBYPASS = 1 / 2πR1 x 200Hz or
choose 0.01 μF . When doing this,
the
output
capacitor
must
be
increas ed
to 3.3 μ F to maintain
stability . These changes the output
noise from 430μV rms . for a 100
kHz bandwidth 5 & 3.3 volt output.
With the bypass capacitor added,
noise no longer scales with output
voltage so that improvements
are
more
dramatic
higher
output
voltage .
REDUCING OUTPUT NOISE
FIGURE 1.ERROR OUTPUT TIMING
FIGURE 2.ADJUSTABLE REGULATOR
PJ2951
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