AAT3232 Datasheet, PDF (7/10 Pages) Advanced Analogic Technologies – 300mA CMOS Low Drop Out Linear Regulator

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AAT3232
300mA CMOS Low Drop Out Linear Regulator
slows down the rate at which the reference turns
on. The result is that output voltage climbs slowly
when the LDO is turned on either by applying
power to the input terminal or by activating
Enable. (See graph titled Turn On / Turn Off.) As
a result, Cnoise can also be used as a soft start
adjustment.
Input Capacitor
The input capacitor has two purposes: The first is
to prevent load current surges from injecting noise
onto the line. The second is to prevent line noise
from affecting the operation of the regulator. In
most circuits, the same value capacitor is used for
input and output capacitors. If cost and space are
an issue, a ceramic capacitor of a few microfarads
works well.
Additional PSRR
In cases where greater PSRR than normally
provided by the AAT3232 is required, additional
filtering may be placed on the input line. This
allows the filter to drop voltage without affecting
the load voltage. For example, if the maximum
load required is 100mA, and a drop out voltage of
1.5 volts is available, then a 10 ohm resistor and a
6.8uF capacitor can be used as an input filter. For
optimal results, the input filter should be designed
to reject signals above one kilohertz, which is
where the AAT3232’s PSRR rolls off.
Layout
Since the AAT3232 is designed to operate with
series resistance, it is not necessary to have the
regulator close to the output capacitor. However,
if a ceramic output capacitor is used, a small
amount of ESR should be placed between the
output capacitor and the regulator; 10 squares of
trace on 1 oz. copper (about 5 mΩ ) is enough,
but 10 mΩ gives better results. The load should
be connected to the output capacitor’s side of the
resistive trace rather than the regulator’s side of
the resistive trace.
PC board layout guidelines (in order of
importance):
1. Cnoise capacitor to BYP and GND: Traces
should be kept short.
2. Input capacitor to Vin and GND: Traces
should be wide and short.
3. Filter resistor to Input capacitor (for the case
of additional PSRR filtering): Connection
need not be short but other loads should not
be connected to the LDO side of the resistor.
4. Enable is not sensitive to position.
Advanced Analogic Technologies, Inc.
1250 Oakmead Pkwy, Suite 310, Sunnyvale, CA 94086
[phone] Fax [phone]
1-49
3232.2000.05.0.98