AAT3218IJS-1.9-T1 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
'AAT3218IJS-1.9-T1' PDF : 18 Pages View PDF
AAT3218
150mA MicroPower™ High Performance LDO
This formula can be solved for VIN to determine the
maximum input voltage.
VIN(MAX)
=
PD(MAX) + (VOUT ·
IOUT · IGND
IOUT)
The following is an example for an AAT3218 set for
a 2.5V output:
VOUT = 2.5V
IOUT = 150mA
IGND = 150µA
526mW + (2.5V × 150mA)
VIN(MAX) =
150mA + 150µA
VIN(MAX) = 6.00V
From the discussion above, PD(MAX) was deter-
mined to equal 526mW at TA = 25°C.
Thus, the AAT3218 can sustain a constant 2.5V
output at a 150mA load current as long as VIN is
≤ 6.0V at an ambient temperature of 25°C. 6.0V is
the absolute maximum voltage where an AAT3218
would never be operated, thus at 25°C, the device
would not have any thermal concerns or opera-
tional VIN(MAX) limits.
This situation can be different at 85°C. The follow-
ing is an example for an AAT3218 set for a 2.5V
output at 85°C:
VOUT = 2.5V
IOUT = 150mA
IGND = 150µA
211mW + (2.5V × 150mA)
VIN(MAX) =
150mA + 150µA
VIN(MAX) = 3.90V
From the discussion above, PD(MAX) was deter-
mined to equal 211mW at TA = 85°C.
Higher input-to-output voltage differentials can be
obtained with the AAT3218, 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 regulator in a
duty-cycled mode.
For example, an application requires VIN = 4.2V
while VOUT = 2.5V at a 150mA load and TA = 85°C.
VIN is greater than 3.90V, which is the maximum
safe continuous input level for VOUT = 2.5V at
150mA for TA = 85°C. To maintain this high input
voltage and output current level, the LDO regulator
must be operated in a duty-cycled mode. Refer to
the following calculation for duty-cycle operation
(PD(MAX) is assumed to be 211mW):
IGND = 150μA
IOUT = 150mA
VIN = 4.2V
VOUT = 2.5V
%DC =
100
PD(MAX)
(VIN - VOUT)IOUT + (VIN × IGND)
211mW
%DC = 100 (4.2V - 2.5V)150mA + (4.2V × 150µA)
%DC = 85.54%
For a 150mA output current and a 2.7V drop across
the AAT3218 at an ambient temperature of 85°C,
the maximum on-time duty cycle for the device
would be 85.54%.
The following family of curves show the safe oper-
ating area for duty-cycled operation from ambient
room temperature to the maximum operating level.
12
3218.2006.04.1.8
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