AAT3258
300mA LDO Linear Regulator with µP Reset
Equivalent Series Resistance: ESR is a very
important characteristic to consider when selecting a
capacitor. ESR is the internal series resistance
associated with a capacitor that includes lead resist-
ance, internal connections, 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.
Bypass Capacitor and Low Noise
Applications
A bypass capacitor pin is provided to enhance the
low noise characteristics of the AAT3258 LDO reg-
ulator. The bypass capacitor is not necessary for
operation. However, for best device performance, a
small ceramic capacitor should be placed between
the bypass pin (BYP) and the device ground pin
(GND). The value of CBYP may range from 470pF
to 10nF. For lowest noise and best possible power
supply ripple rejection performance, a 10nF capac-
itor should be used. To practically realize the high-
est power supply ripple rejection and lowest output
noise performance, it is critical that the capacitor
connection between the BYP pin and GND pin be
direct and PCB traces should be as short as possi-
ble. Refer to the Evaluation Board Layout section
of this document for examples.
Ceramic Capacitor Materials: Ceramic capaci-
tors less than 0.1µF are typically made from NPO
or C0G materials. NPO and C0G materials gener-
ally have tight tolerance and are very stable over
temperature. Larger capacitor values are usually
composed of X7R, X5R, Z5U, or Y5V dielectric
materials. These two material types are not recom-
mended for use with LDO regulators since the
capacitor tolerance can vary more than ±50% over
the operating temperature range of the device. A
2.2µF Y5V capacitor could be reduced to 1µF over
temperature; this could cause problems for circuit
operation. X7R and X5R dielectrics are much more
desirable. The temperature tolerance of X7R
dielectric is better than ±15%.
There is a relationship between the bypass capac-
itor value and the LDO regulator turn-on and turn-
off time. In applications where fast device turn-on
and turn-off time are desired, the value of CBYP
should be reduced.
In applications where low noise performance and/
or ripple rejection are less of a concern, the bypass
capacitor may be omitted. The fastest device turn-
on time will be realized when no bypass capacitor
is used.
Capacitor area is another contributor to ESR.
Capacitors that are physically large in size will have
a lower ESR when compared to a smaller sized
capacitor of an equivalent material and capaci-
tance value. These larger devices can improve cir-
cuit transient response when compared to an equal
value capacitor in a smaller package size.
DC leakage on this pin can affect the LDO regula-
tor output noise and voltage regulation perform-
ance. For this reason, the use of a low leakage,
high quality, ceramic (NPO or C0G type) or film
capacitor is highly recommended.
Consult capacitor vendor datasheets carefully
when selecting capacitors for LDO regulators.
Shutdown Function
Capacitor Characteristics
The shutdown pin is designed to turn off the LDO
regulator when the device is not in use. This pin is
active high and is compatible with CMOS logic. To
assure the LDO regulator will switch on, the SHDN
turn-on control level must be greater than 1.5V.
The LDO regulator will go into the disable shut-
down mode when the voltage falls below 0.6V. If
the shutdown function is not needed in a specific
application, it may be tied to VIN to keep the LDO
regulator in a continuously on state.
Ceramic composition capacitors are highly recom-
mended over all other types of capacitors.
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 capacitors. Since ceramic
capacitors are non-polarized, they are not prone to
incorrect connection damage.
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3258.2006.03.1.6
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
