AAT3244
300mA Adjustable Dual CMOS
Low Voltage LDO Linear Regulator
The following is an example for a 2.5V output:
VOUTA = 2.5V
Thermal Considerations and High
Output Current Applications
The AAT3244 is designed to deliver continuous out-
put load currents of 300mA under normal operating
conditions and can supply up to 600mA during circuit
start-up conditions. This is desirable for applications
where there might be a brief high inrush current dur-
ing 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, careful
device layout and circuit operating conditions need
to be taken into account. The following discussions
will assume the LDO regulator is mounted on a print-
ed circuit board utilizing the minimum recommended
footprint as stated in the layout considerations sec-
tion of this document. At any given ambient temper-
ature (TA), the maximum package power dissipation
can be determined by the following equation:
VOUTB = 1.5V
IOUTB = 150mA
VIN = 4.2V
IGND = 125µA
625mW - (2 · 4.2V · 125µA) - (4.2 - 1.5) · 150mA
IOUTA(MAX)
=
4.2 - 2.5
IOUTA(MAX) = 129mA
From the discussion above, PD(MAX) was deter-
mined to equal 625mW at TA = 25°C.
Therefore, with Regulator B delivering 150mA at
1.5V, Regulator A can sustain a constant 2.5V out-
put at a 129mA load current at an ambient temper-
ature of 25°C. Higher input-to-output voltage differ-
entials can be obtained with the AAT3244, while
maintaining device functions within the thermal
safe operating area. To accomplish this, the device
thermal resistance must be reduced by increasing
the heat sink area or by operating the LDO regula-
tor in a duty-cycled mode.
T
J(MAX) - TA
PD(MAX)
=
θ
JA
Constants for the AAT3244 are TJ(MAX) (the maxi-
mum junction temperature for the device, which is
125°C) and θJA = 160°C/W (the package thermal
resistance). Typically, maximum conditions are cal-
culated at the maximum operating temperature of
TA = 85°C and under normal ambient conditions
where TA = 25°C. Given TA = 85°C, the maximum
For example, an application requires VIN = 4.2V
while VOUTA = 1.5V at a 300mA load, VOUTB = 1.5V
at a 200mA load, and TA = 25°C. To maintain this
high input voltage and output current level, the
LDO regulator must be operated in a duty-cycled
mode.
package power dissipation is 250mW. At TA
=
25°C, the maximum package power dissipation is
625mW. The maximum continuous output current
for the AAT3244 is a function of the package power
dissipation and the input-to-output voltage drop
across the LDO regulator. To determine the maxi-
mum output current for a given output voltage, refer
to the following equation. This calculation accounts
for the total power dissipation of the LDO regulator,
including that caused by ground current.
Refer to the following calculation for duty-cycle
operation:
IGND = 125μA
IOUTA = 300mA
IOUTB = 200mA
VIN = 4.2V
VOUT = 1.5V
PD(MAX) = [(VIN - VOUTA)IOUTA + (VIN · IGND)] + [(VIN - VOUTB)IOUTB + (VIN · IGND)]
PD(MAX) is assumed to be 625mW
100(PD(MAX)
)
%DC =
[(VIN - VOUTA)IOUTA + (VIN · IGND)] + [(VIN - VOUTB)IOUTB + (VIN · IGND)]
This formula can be solved for IOUTA to determine
the maximum output current for LDOA:
100 · 625mW
%DC =
[(4.2V - 1.5V)300mA + (4.2V · 125µA)] + [(4.2V - 1.5V)200mA + (4.2V · 125µA)]
P
D(MAX) - (2 · VIN · IGND) - (VIN - VOUTB) · IOUTB
ꢀDC = 46.3ꢀ
IOUTA(MAX)
=
VIN - VOUTA
12
(ADVANCED INFORMATION) 3244.200708.0.66
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