AAT3242_08 Datasheet, PDF (10/15 Pages) Advanced Analogic Technologies – 300mA/150mA Dual CMOS LDO Linear Regulator

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PRODUCT DATASHEET
AAT3242
300mA/150mA Dual CMOS LDO Linear Regulator
lator output operation, ceramic capacitors are recom-
mended for C_IN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources such as batteries in porta-
ble devices.
Output Capacitor
For proper load voltage regulation and operational stabil-
ity, 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 pos-
sible for maximum device performance. The AAT3242
has been specifically designed to function with very low
ESR ceramic capacitors. For best performance, ceramic
capacitors are recommended.
Typical output capacitor values for maximum output cur-
rent conditions range from 1μF to 10μF.
Applications utilizing the exceptionally low output noise
and optimum power supply ripple rejection characteris-
tics of the AAT3242 should use 2.2μF or greater for C_OUT.
If desired, C_OUT may be increased without limit. In low
output current applications where output load is less
than 10mA, the minimum value for C_OUT can be as low as
0.47μF.
Capacitor Characteristics
Ceramic composition capacitors are highly recommended
over all other types of capacitors for use with the
AAT3242. 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 not prone to incorrect connection damage.
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 resistance,
internal connections, size and area, material composi-
tion, and ambient temperature. Typically, capacitor ESR
is measured in milliohms for ceramic capacitors and can
range to more than several ohms for tantalum or alumi-
num electrolytic capacitors.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1μF are typically made
from NPO or C0G materials. NPO and C0G materials gen-
erally have tight tolerance and are very stable over tem-
perature. Larger capacitor values are usually composed
of X7R, X5R, Z5U, or Y5V dielectric materials. 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. 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 desir-
able. The temperature tolerance of X7R dielectric is bet-
ter than ±15%. Capacitor area is another contributor to
ESR. Capacitors which are physically large in size will
have a lower ESR when compared to a smaller sized
capacitor of an equivalent material and capacitance
value. These larger devices can improve circuit transient
response when compared to an equal value capacitor in
a smaller package size. Consult capacitor vendor
datasheets carefully when selecting capacitors for LDO
regulators.
POK Output
The AAT3242 features integrated Power OK comparators
which can be used as an error flag. The POK open drain
output goes low when output voltage is 6% (typ) below
its nominal regulation voltage. Additionally, any time one
of the regulators is in shutdown, the respective POK out-
put is pulled low. Connect a pull-up resistor from POKA
to OUTA, and POKB to OUTB.
Enable Function
The AAT3242 features an LDO regulator enable/disable
function. Each LDO has its own dedicated enable pin.
These pins (EN) are active high and are compatible with
CMOS logic. To assure the LDO regulators will switch on,
ENA/B must be greater than 1.6V. The LDO regulators
will shut down when the voltage on the ENA/B pins falls
below 0.6V. In shutdown, the AAT3242 will consume less
than 1.0μA of current. If the enable function is not
needed in a specific application, it may be tied to V_IN to
keep the LDO regulator in a continuously on state.
When the LDO regulators are in shutdown mode, an
internal 20Ω resistor is connected between V_OUT and
GND. This is intended to discharge C_OUT when the LDO
regulators are disabled. The internal 20Ω has no adverse
effects on device turn-on time.
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3242.2008.08.1.11