AAT3242_08 Datasheet, PDF (11/15 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
Short-Circuit Protection
The AAT3242 contains internal short-circuit protection
that will trigger when the output load current exceeds
the internal threshold limit. Under short-circuit condi-
tions, the output of the LDO regulator will be current
limited until the short-circuit condition is removed from
the output or LDO regulator package power dissipation
exceeds the device thermal limit.
Thermal Protection
The AAT3242 has an internal thermal protection circuit
which will turn on when the device die temperature
exceeds 145°C. The LDO regulator output will remain in
a shutdown state until the internal die temperature falls
back below the 145°C trip point. The combination and
interaction between the short-circuit and thermal pro-
tection systems allows the LDO regulators to withstand
indefinite short-circuit conditions without sustaining per-
manent damage.
No-Load Stability
The AAT3242 is designed to maintain output voltage
regulation and stability under operational no-load condi-
tions. This is an important characteristic for applications
where the output current may drop to zero.
Reverse Output-to-Input
Voltage Conditions and Protection
Under normal operating conditions, a parasitic diode
exists between the output and input of the LDO regula-
tor. The input voltage should always remain greater than
the output load voltage maintaining a reverse bias on
the internal parasitic diode. Conditions where V_OUT might
exceed V_IN should be avoided since this would forward
bias the internal parasitic diode and allow excessive cur-
rent flow into the V_OUT pin, possibly damaging the LDO
regulator. In applications where there is a possibility of
V_OUT exceeding V_IN for brief amounts of time during nor-
mal operation, the use of a larger value C_IN capacitor is
highly recommended. A larger value of C_IN with respect
to C_OUT will effect a slower C_IN decay rate during shut-
down, thus preventing V_OUT from exceeding V_IN. In appli-
cations where there is a greater danger of V_OUT exceed-
ing V_IN for extended periods of time, it is recommended
to place a Schottky diode across V_IN to V_OUT (connecting
the cathode to V_IN and anode to V_OUT). The Schottky
diode forward voltage should be less than 0.45V.
Thermal Considerations and
High Output Current Applications
The AAT3242 is designed to deliver continuous output
load currents of 300mA and 150mA under normal oper-
ations, and can supply up to 500mA during circuit start-
up conditions. This is desirable for circuit applications
where there might be a brief high in-rush current during
a power-on event.
The limiting characteristic for the maximum output load
current safe operating area is essentially package power
dissipation and the internal preset thermal limit of the
device. In order to obtain high operating currents, care-
ful device layout and circuit operating conditions need to
be taken into account.
The following discussions will assume the LDO regulator
is mounted on a printed circuit board utilizing the mini-
mum recommended footprint as stated in the layout
considerations section of this document. At any given
ambient temperature (T_A), the maximum package power
dissipation can be determined by the following equa-
tion:
P_D(MAX)
=
T_J(MAX) -
θ_JA
T_A
Constants for the AAT3242 are T_J(MAX) (the maximum
junction temperature for the device, which is 125°C) and
θ_JA = 110°C/W (the package thermal resistance).
Typically, maximum conditions are calculated at the
maximum operating temperature of T_A = 85°C and
under normal ambient conditions where T_A = 25°C.
Given T_A = 85°C, the maximum package power dissipa-
tion is 364mW. At T_A = 25°C, the maximum package
power dissipation is 909mW.
The maximum continuous output current for the AAT3242
is a function of the package power dissipation and the
input-to-output voltage drop across the LDO regulator.
To determine the maximum output current for a given
output voltage, refer to the following equation. This cal-
culation accounts for the total power dissipation of the
LDO regulator, including that caused by ground current.
P_D(MAX) = [(V_IN - V_OUTA)I_OUTA + (V_IN x I_GND)] + [(V_IN - V_OUTB)I_OUTB + (V_IN x I_GND)]
3242.2008.08.1.11
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