DATA SHEET • AAT3221/AAT3222 150 mA NANOPOWERTM LDO LINEAR REGULATOR
Output Capacitor
Equivalent Series Resistance (ESR)
For proper load voltage regulation and operational stability, 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 possible for maximum device
performance.
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.
The AAT3221/3222 have been specifically designed to function
with very low ESR ceramic capacitors. Although the devices are
intended to operate with these low ESR capacitors, they are
stable over a wide range of capacitor ESRs. Therefore, they can
also work with some higher ESR tantalum or aluminum
electrolytic capacitors. However, for best performance, ceramic
capacitors are recommended.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1 µF are typically made from NPO
or C0G materials. NPO and C0G materials have a typically tight
tolerance and are very stable over temperature ranges. Larger
capacitor values are typically composed of X7R, X5R, Z5U, and
Y5V dielectric materials. Large ceramic capacitors, typically
greater than 2.2 µF, are often available in low-cost Y5V and Z5U
dielectrics. These two material types are not recommended for
use with LDO regulators since the capacitor tolerance can vary
more than ±50% over the operating temperature range of the
device.
The value of COUT typically ranges from 0.47 µF to 10 µF;
however, 1 µF is sufficient for most operating conditions.
If large output current steps are required by an application, then
an increased value for COUT 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.
A 2.2 µF, Y5V capacitor could be reduced to 1 µF over the full
operating temperature range. This can cause problems for circuit
operation and stability. X7R and X5R dielectrics are much more
desirable. The temperature tolerance of X7R dielectric is better
than ±15%.
The total output capacitance required can be calculated using the
following formula:
Capacitor area is another contributor to ESR. Capacitors that are
physically large in size have a lower ESR when compared to a
smaller sized capacitor of equivalent material and capacitance
value. These larger devices can also improve circuit transient
response when compared to an equal value capacitor in a smaller
package size.
Where:
∆I = maximum step of output current
∆V = maximum excursion 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 or AAT3222 at room temperature. The increased
capacitor value is recommended if tight output tolerances must
be maintained over extreme operating conditions and maximum
operational temperature excursions. 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.
Consult capacitor vendor Data Sheets carefully when selecting
capacitors for use with LDO regulators.
Enable Function
The AAT3221/3222 devices feature an LDO regulator
enable/disable function. This pin (EN) is compatible with CMOS
logic. Active high or active low options are available (see Ordering
Information).
Capacitor Characteristics
For a logic high signal, the EN control level must be greater than
2.4 V. A logic low signal is asserted when the voltage on the EN
pin falls below 0.8 V. For example, the active high versions of the
AAT3221 and AAT3222 turns on when a logic high is applied to
the EN pin. If the enable function is not needed in a specific
application, it may be tied to the respective voltage level to keep
the LDO regulator in a continuously “on” state (e.g., the active
high version AAT3221/3222 can tie VIN to EN to remain on).
Ceramic composition capacitors are highly recommended over all
other types of capacitors for use with the AAT3221/3222.
Ceramic capacitors offer many advantages over their tantalum
and aluminum electrolytic counterparts. A ceramic capacitor
typically has a very low ESR, a lower cost, 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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
10
May 17, 2013 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • 202251B
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