AAT3221 Datasheet, PDF (10/16 Pages) Advanced Analogic Technologies – 150mA NanoPower™ LDO Linear Regulator

AAT3221/2
150mA NanoPower™ LDO Linear Regulator
Large ceramic capacitors, typically greater than
2.2µF are often available in the low cost Y5V and Z5U
dielectrics. These two material types are not recom-
mended for use with LDO regulators since the capac-
itor 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 the full
operating temperature range. This can cause prob-
lems for circuit operation and stability. X7R and X5R
dielectrics are much more desirable. The tempera-
ture tolerance of X7R dielectric is better 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 equivalent material and capaci-
tance value. These larger devices can also improve
circuit transient response when compared to an
equal value capacitor in a smaller package size.
Consult capacitor vendor data sheets carefully when
selecting capacitors for use with LDO regulators.
Enable Function
The AAT3221/2 features 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). For a logic high
signal, the EN control level must be greater then 2.4
volts. A logic low signal is asserted when the voltage
on the EN pin falls below 0.6 volts. For example, the
active high version 3221/2 will turn 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 regula-
tor in a continuously on state; e.g. the active high ver-
sion 3221/2 will tie V_IN to EN to remain on.
Short Circuit Protection and Thermal
Protection
The AAT3221/2 is protected by both current limit
and over temperature protection circuitry. The
internal short circuit current limit is designed to acti-
vate when the output load demand exceeds the
maximum rated output. If a short circuit condition
were to continually draw more than the current limit
threshold, the LDO regulator's output voltage will
drop to a level necessary to supply the current
demanded by the load. Under short circuit or other
over current operating conditions, the output volt-
age will drop and the AAT3221/2's die temperature
will increase rapidly. Once the regulator's power
10
dissipation capacity has been exceeded and the
internal die temperature reaches approximately
140°C the system thermal protection circuit will
become active. The internal thermal protection cir-
cuit will actively turn off the LDO regulator output
pass device to prevent the possibility of over tem-
perature damage. The LDO regulator output will
remain in a shutdown state until the internal die
temperature falls back below the 140°C trip point.
The interaction between the short circuit and ther-
mal protection systems allow the LDO regulator to
withstand indefinite short circuit conditions without
sustaining permanent damage.
No-Load Stability
The AAT3221/2 is designed to maintain output volt-
age regulation and stability under operational no-
load conditions. This is an important characteristic
for applications where the output current may drop
to zero. An output capacitor is required for stability
under no load operating conditions. Refer to the
output capacitor considerations section for recom-
mended typical output capacitor values.
Thermal Considerations and High
Output Current Applications
The AAT3221/2 is designed to deliver a continuous
output load current of 150mA under normal operat-
ing conditions. The limiting characteristic for the
maximum output load safe operating area is essen-
tially package power dissipation and the internal pre-
set thermal limit of the device. In order to obtain high
operating currents, careful device layout and circuit
operating conditions need to be taken into account.
The following discussions will assume the LDO reg-
ulator is mounted on a printed circuit board utilizing
the minimum recommended footprint and the print-
ed circuit board is 0.062inch thick FR4 material with
one ounce copper.
At any given ambient temperature (T_A) the maxi-
mum package power dissipation can be deter-
mined by the following equation:
P_D(MAX) = [T_J(MAX) - T_A] / Θ _JA
Constants for the AAT3221/2 are T_J(MAX), the maxi-
mum junction temperature for the device which is
125°C and Θ_JA = 150°C/W, the package thermal
resistance. Typically, maximum conditions are cal-
culated at the maximum operating temperature
where T_A = 85°C, under normal ambient conditions
3221.2002.03.0.94