AAT3221/2
150mA NanoPower™ LDO Linear Regulator
ceramic capacitors, typically greater than 2.2µF, 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 temper-
ature range of the device. A 2.2µF Y5V capacitor
could be reduced to 1µF over the full operating tem-
perature 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%.
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 equivalent material and capacitance
value. These larger devices can also improve cir-
cuit transient response when compared to an equal
value capacitor in a smaller package size.
Consult capacitor vendor datasheets carefully when
selecting capacitors for use with LDO regulators.
will rapidly increase. Once the regulator's power
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 allows 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 of this doc-
ument for recommended typical output capacitor
values.
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
than 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 AAT3221/2 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 regulator in a continuously
on state; e.g., the active high version AAT3221/2
will tie V
IN
to EN to remain on.
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 0.062-inch 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-
11
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 die temperature
3221.2005.12.1.11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
