AAT2500B Datasheet, PDF (16/21 Pages) Advanced Analogic Technologies – Adjustable 3-Channel Regulator

SystemPower^TM
POK Output
LDOA of the AAT2500B features an integrated Power OK
comparator which can be used as an error flag. The POK
open drain output goes low when output voltage is 10%
(typ) below its nominal regulation voltage. Additionally,
any time LDOA is in shutdown, the POK output is pulled
low. Connect a pull-up resistor from POK to OUTA.
Enable Function
The AAT2500B features an LDO regulator enable/disable
function. Each LDO has its own dedicated enable pin.
These pins (ENA, ENB) are active high and are compatible
with CMOS logic. To assure the LDO regulators will switch
on, ENA/B must be greater than 1.4V. The LDO regulators
will shut down when the voltage on the ENA/B pins falls
below 0.6V. In shutdown, the LDO regulators will con-
sume less than 1.0μA of current. If the enable function is
not needed in a specific application, it may be tied to V_IN
to keep the LDO regulator in a continuously on state.
Thermal Protection
Each of the two LDOs of the AAT2500B has an internal
thermal protection circuit which will turn on when the
device die temperature exceeds 140°C. The LDO regulator
outputs will remain in a shutdown state until the internal
die temperature falls back below the 125°C trip point.
No-Load Stability
The LDOs in the AAT2500B are designed to maintain out-
put voltage 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
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
PRODUCT DATASHEET
AAT2500B
Adjustable 3-Channel Regulator
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 exceeding
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 LDOs of the AAT2500B are designed to deliver con-
tinuous output load currents of 300mA each under nor-
mal operation. 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 minimum
recommended footprint as stated in the layout consider-
ations section of this document. At any given ambient
temperature (T_A), the maximum package power dissipa-
tion can be determined by the following equation:
P_D(MAX)
=
T_J(MAX) -
θ_JA
T_A
Constants for the AAT2500B are T_J(MAX) (the maximum
junction temperature for the device, which is 125°C) and
θ_JA = 50°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 dissipation is 800mW. At T_A
= 25°C, the maximum package power dissipation is 2W.
The maximum continuous output current for the AAT2500B
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 · I_GND)] + [(V_IN - V_OUTB)I_OUTB + (V_IN · I_GND)]
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