AAT3223
250mA NanoPower™ LDO
Linear Regulator with Power-OK
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. Large ceramic capacitors (i.e., 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 temperature range of the
device. A 2.2µF Y5V capacitor could be reduced to
1µF 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%.
resistor (100kΩ is a good resistor value for this pur-
pose). An internal comparator has a reference
threshold set to trigger at 10% of the nominal
AAT3223 output voltage. If the output voltage level
drops below this preset threshold, the POK func-
tion will become active and turn on an open-drain
N-channel MOSFET to pull the POK output pin to
ground. There is a fixed 1ms delay circuit between
the POK comparator output and the N-channel
MOSFET gate. The purpose of the delay is to pre-
vent a false triggering of the POK output during
device turn-on or during very short duration load
transient events. If necessary, additional POK flag
delay can be added by placing a capacitor in paral-
lel with the POK pull-up resistor. The additional
delay time will be set by the RC time constant, the
pull-up resistor, and parallel capacitor values.
When the AAT3223 is in the shutdown state with
the EN pin low, the POK pin becomes low imped-
ance. The LDO output will be discharged through
the high value POK pull-up resistor. When entering
the shutdown state, there is no delay associated
with the POK output; the open-drain device turns
on immediately.
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.
This offers the added advantage of having a hard
application turn-off when the LDO regulator is
turned off. This additional function has no adverse
effect on regulator turn-on time.
Consult capacitor vendor datasheets carefully when
selecting capacitors for use with LDO regulators.
Enable Function
Short-Circuit and Thermal Protection
The AAT3223 features an LDO regulator enable /
disable function. This pin (EN) is compatible with
CMOS logic. For a logic high signal, the EN control
level must be greater than 2.0V. A logic low signal
is asserted when the voltage on the EN pin falls
below 0.5V. For example, the active high version
AAT3223 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 respec-
tive voltage level to keep the LDO regulator in a
continuously on state; e.g., the active high version
AAT3223 will tie VIN to EN to remain on.
The AAT3223 is protected by both current limit and
over-temperature protection circuitry. The internal
short-circuit current limit is designed to activate
when the output load demand exceeds the maxi-
mum rated output. If a short-circuit condition were
to continually draw more than the current limit
threshold, the LDO regulator's output voltage
would drop to a level necessary to supply the cur-
rent demanded by the load. Under short-circuit or
other over-current operating conditions, the output
voltage would drop and the AAT3223's die temper-
ature would rapidly increase. Once the regulator's
power dissipation capacity has been exceeded and
the internal die temperature reaches approximate-
ly 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
Power-OK Function
The Power-OK (POK) function is a very useful
basic active low error flag. When the AAT3223
output voltage level is within regulation limits, the
POK output pin is a high impedance and should be
tied high to the LDO output through a high value
3223.2006.03.1.5
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