oltag Regulator
Volta ge Re gulator
Austin Semiconductor, Inc.
APPLICATIONS INFORMATION
External Capacitors
A 10µF (or greater) capacitor is required between the output
and ground to prevent oscillations due to instability. Most
types of tantalum or aluminum electrolytics will be adequate;
film types will work, but are costly and therefore not
recommended. Many aluminum electrolytics have
electrolytes that freeze at about -30°C, so solid tantalums are
recommended for operation below -25°C. The important
parameters of the capacitor are an effective series resistance
of about 5Ω or less and a resonant frequency above 500kHz.
The value of this capacitor may be increased without limit.
At lower values of output current, less output capacitance is
required for output stability. The capacitor can be reduced to
0.5µF for current below 10mA or 0.15µF for currents below
1mA. Adjusting the AS29372 to voltages below 5V runs the
error amplifier at lower gains so that more output capacitance
is needed. For the worst-case situation of 750mA load at
1.23V output (Output shorted to Adjust) a 22µF (or greater)
capacitor should be used.
AS29372
Reducing Output Noise
In reference applications it may be advantageous to reduce
the AC noise present at the output. One method is to reduce
the regulator bandwidth by increasing the size of the output
capacitor. This is relatively inefficient, as increasing the
capacitor from 1µF to 220µF only decreases the noise from
430µV to 160µV
RMS
for a 100kHz bandwidth at 5V output.
Noise can be reduced by a factor of four with the adjustable
regulators with a bypass capacitor across R
1
, since it reduces
the high frequency gain from 4 to unity. Pick
C
BYPASS
z
1
2πR
1
H
200 Hz
or about 0.01µF. When doing this, the output capacitor must
be increased to 10µF to maintain stability. These changes
reduce the output noise from 430µV to 100µV
RMS
for a
100kHz bandwidth at 5V output. With the bypass capacitor
added, noise no longer scales with output voltage so that
improvements are more dramatic at higher output voltages.
Programming the Output Voltage
The AS29372 may be programmed for any output voltage
between its 1.235V reference and its 26V maximum rating.
An external pair of resistors is required, as show in Figure 1.
The complete equation for the output voltage is
V
OUT
= V
REF
x {1 + R
1
/R
2
} - |I
FB
|R
1
where V
REF
Is the nominal 1.235 reference voltage and I
FB
is
the Adjust pin bias current, nominally 20nA. The minimum
recommended load current of 1µA forces an upper limit of 1.2MΩ
on the value R
2
, if the regulator must work with no load (a
condition often found in CMOS in standby), I
FB
will produce a
-2% typical error in V
OUT
which may be eliminated at room
temperature by trimming R
1
. For better accuracy, choosing
R
2
= 100k reduces this error to 0.17% while increasing the
resistor program current to 12µA. Since the AS29372 typically
draws 100µA at no load with SHUTDOWN open-circuited, this
is a negligible addition.
AS29372
Rev. 2.0 3/03
Austin Semiconductor, Inc. reserves the right to change products or specifications without notice.
7
AUSTIN [ AUSTIN SEMICONDUCTOR ]
