PRODUCT DATASHEET
AAT3221/2
PowerLinearTM
150mA NanoPower™ LDO Linear Regulator
The total output capacitance required can be calculated
using the following formula:
Applications Information
To ensure that the maximum possible performance is
obtained from the AAT3221/2, please refer to the follow-
ing application recommendations.
ΔI
COUT
=
· 15µF
ΔV
Where:
Input Capacitor
ΔI = maximum step in output current
ΔV = maximum excursion in voltage that the load can
tolerate
A 1μF or larger capacitor is typically recommended for
CIN in most applications. A CIN 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
CIN capacitor will be needed for stable operation. CIN
should be located as closely to the device VIN pin as
practically possible. CIN values greater than 1μF will
offer superior input line transient response and will
assist in maximizing the power supply ripple rejection.
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 temperature. The increased
capacitor value is recommended if tight output toler-
ances must be maintained over extreme operating con-
ditions and maximum operational temperature excur-
sions. If tantalum or aluminum electrolytic capacitors
are used, the capacitor value should be increased to
compensate for the substantial ESR inherent to these
capacitor types.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for CIN, as there is no specific capacitor
ESR requirement. For 150mA LDO regulator output
operation, ceramic capacitors are recommended for CIN
due to their inherent capability over tantalum capacitors
to withstand input current surges from low impedance
sources such as batteries in portable devices.
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT3221/2. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor 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 capaci-
tors. Since ceramic capacitors are non-polarized, they
are less prone to damage if incorrectly connected.
Output Capacitor
For proper load voltage regulation and operational sta-
bility, a capacitor is required between pins VOUT and GND.
The COUT capacitor connection to the LDO regulator
ground pin should be made as direct as practically pos-
sible for maximum device performance. The AAT3221/2
has been specifically designed to function with very low
ESR ceramic capacitors. Although the device is intended
to operate 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 electrolytic
capacitors. However, for best performance, ceramic
capacitors are recommended.
Equivalent Series Resistance (ESR)
ESR is a very important characteristic to consider when
selecting a capacitor. ESR is the internal series resis-
tance 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.
The value of COUT typically ranges from 0.47μF to 10μF;
however, 1μF is sufficient for most operating conditions.
If large output current steps are required by an applica-
tion, then an increased value for COUT should be consid-
ered. The amount of capacitance needed can be calcu-
lated from the step size of the change in output load
current expected and the voltage excursion that the load
can tolerate.
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 tempera-
ture. Larger capacitor values are typically composed of
w w w . a n a l o g i c t e c h . c o m
3221.2007.11.1.12
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