AAT3215
150mA CMOS High Performance LDO
When the LDO regulator is in the shutdown mode,
an internal 1.5kΩ resistor is connected between
VOUT and GND. This is intended to discharge COUT
when the LDO regulator is disabled. The internal
1.5kΩ has no adverse effect on device turn on time.
Applications Information
Equivalent Series Resistance (ESR): ESR is a very
important characteristic to consider when selecting a
capacitor. ESR is the internal series resistance asso-
ciated with a capacitor, which 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.
Short Circuit Protection
The AAT3215 contains an internal short circuit pro-
tection circuit that will trigger when the output load
current exceeds the internal threshold limit. Under
short circuit conditions the output of the LDO regu-
lator will be current limited until the short circuit
condition is removed from the output or LDO regu-
lator package power dissipation exceeds the
device thermal limit.
Ceramic Capacitor Materials: Ceramic capacitors
less than 0.1µF are typically made from NPO or
COG materials. NPO and COG materials are typi-
cally tight tolerance very stable over temperature.
Larger capacitor values are typically composed of
X7R, X5R, Z5U and Y5V dielectric materials. Large
ceramic capacitors, typically greater then 2.2µF are
often available in the low cost Y5V and Z5U
dielectrics. These two material types are not rec-
ommended for use with LDO regulators since the
capacitor tolerance can vary more than 50ꢀ 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 15ꢀ.
Thermal Protection
The AAT3215 has an internal thermal protection cir-
cuit which will turn on when the device die temper-
ature exceeds 150°C. The internal thermal protec-
tion circuit will actively turn off the LDO regulator
output pass device to prevent the possibility of over
temperature damage. The LDO regulator output
will remain in a shutdown state until the internal die
temperature falls back below the 150°C trip point.
The combination and interaction between the short
circuit and thermal protection systems allow the
LDO regulator to withstand indefinite short circuit
conditions without sustaining permanent damage.
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 tran-
sient response when compared to an equal value
capacitor in a smaller package size.
No-Load Stability
The AAT3215 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.
Reverse Output to Input Voltage
Conditions and Protection
Consult capacitor vendor data sheets carefully
when selecting capacitors for LDO regulators.
Under normal operating conditions a parasitic
diode exists between the output and input of the
LDO regulator. The input voltage should always
remain greater then the output load voltage main-
taining a reverse bias on the internal parasitic
diode. Conditions where VOUT might exceed VIN
should be avoided since this would forward bias
the internal parasitic diode and allow excessive
current flow into the VOUT pin possibly damaging
the LDO regulator.
Enable Function
The AAT3215 features an LDO regulator enable/
disable function. This pin (EN) is active high and is
compatible with CMOS logic. To assure the LDO
regulator will switch on, the EN turn on control level
must be greater than 2.0 volts. The LDO regulator
will go into the disable shutdown mode when the
voltage on the EN pin falls below 0.6 volts. If the
enable function is not needed in a specific applica-
tion, it may be tied to VIN to keep the LDO regula-
tor in a continuously on state.
3215.2002.03.0.91
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