AAT3221/2
150mA NanoPower™ LDO Linear Regulator
Applications Information
To ensure that the maximum possible performance
is obtained from the AAT3221/2, please refer to the
following application recommendations.
The total output capacitance required can be cal-
culated using the following formula:
C
OUT
=
Where:
∆I
= maximum step in output current
∆V
= maximum excursion in voltage that the load
can tolerate
Note that use of this equation results in capacitor
values approximately two to four times the typical
value needed for an AAT3221/2 at room tempera-
ture. The increased capacitor value is recommend-
ed if tight output tolerances must be maintained over
extreme operating conditions and maximum opera-
tional temperature excursions. If tantalum or alu-
minum electrolytic capacitors are used, the capacitor
value should be increased to compensate for the
substantial ESR inherent to these capacitor types.
∆I
× 15µF
∆V
Input Capacitor
A 1µF or larger capacitor is typically recommended
for C
IN
in most applications. A C
IN
capacitor is not
required for basic LDO regulator operation.
However, if the AAT3221/2 is physically located any
distance more than one or two centimeters from the
input power source, a C
IN
capacitor will be needed
for stable operation. C
IN
should be located as
closely to the device V
IN
pin as practically possible.
C
IN
values greater than 1µF will offer superior input
line transient response and will assist in maximizing
the power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capac-
itors may be selected for C
IN
, as there is no specif-
ic capacitor ESR requirement. For 150mA LDO
regulator output operation, ceramic capacitors are
recommended for C
IN
due to their inherent capabil-
ity over tantalum capacitors to withstand input cur-
rent surges from low impedance sources such as
batteries in portable devices.
Capacitor Characteristics
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors for use
with the AAT3221/2. Ceramic capacitors offer
many advantages over their tantalum and alu-
minum electrolytic counterparts. A ceramic capac-
itor typically has very low ESR, is lower cost, has a
smaller PCB footprint, and is non-polarized. Line
and load transient response of the LDO regulator is
improved by using low-ESR ceramic capacitors.
Since ceramic capacitors are non-polarized, they
are less prone to damage if incorrectly connected.
Equivalent Series Resistance (ESR):
ESR is a
very important characteristic to consider when
selecting a capacitor. ESR is the internal series
resistance associated with a capacitor, which
includes lead resistance, internal connections,
capacitor size and area, material composition, and
ambient temperature. Typically, capacitor ESR is
measured in milliohms for ceramic capacitors and
can range to more than several ohms for tantalum
or aluminum electrolytic capacitors.
Ceramic Capacitor Materials:
Ceramic capacitors
less than 0.1µF are typically made from NPO or C0G
materials. NPO and C0G materials are typically tight
tolerance and very stable over temperature. Larger
capacitor values are typically composed of X7R,
X5R, Z5U, and Y5V dielectric materials. Large
Output Capacitor
For proper load voltage regulation and operational
stability, a capacitor is required between pins V
OUT
and GND. The C
OUT
capacitor connection to the
LDO regulator ground pin should be made as direct
as practically possible for maximum device per-
formance. The AAT3221/2 has been specifically
designed to function with very low ESR ceramic
capacitors. Although the device is intended to oper-
ate with these low ESR capacitors, it is stable over
a wide range of capacitor ESR, thus it will also work
with some higher ESR tantalum or aluminum elec-
trolytic capacitors. However, for best performance,
ceramic capacitors are recommended.
The value of C
OUT
typically ranges from 0.47µF to
10µF; however, 1µF is sufficient for most operating
conditions.
If large output current steps are required by an
application, then an increased value for C
OUT
should be considered. The amount of capacitance
needed can be calculated from the step size of the
change in output load current expected and the
voltage excursion that the load can tolerate.
10
3221.2005.12.1.11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
