AAT3223
250mA NanoPower™ LDO
Linear Regulator with Power-OK
The following is an example for an AAT3223 set for
a 2.8V output:
Higher input-to-output voltage differentials can be
obtained with the AAT3223, while maintaining
device functions in 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.
VOUT
IOUT
IGND
= 2.9V
= 250mA
= 1.1µA
For example, an application requires VIN = 5.0V
while VOUT = 2.8V at a 150mA load and TA = 85°C.
VIN is greater than 4.58V, which is the maximum
safe continuous input level for VOUT = 2.8V at
150mA for TA = 85°C. To maintain this high input
voltage and output current level, the LDO regula-
tor must be operated in a duty-cycled mode.
Refer to the following calculation for duty-cycle
operation:
667mW + (2.8V × 150mA)
VIN(MAX)
=
150mA + 1.1µA
VIN(MAX) = 9.11V
From the discussion above, PD(MAX) was deter-
mined to equal 667mW at TA = 25°C.
Thus, the AAT3223 can sustain a constant 2.8V
output at a 150mA load current as long as VIN is
≤9.11V at an ambient temperature of 25°C. 5.5V
is the maximum input operating voltage for the
AAT3223, thus at 25°C, the device would not
have any thermal concerns or operational VIN(MAX)
limits.
IGND = 1.1µA
IOUT = 150mA
VIN = 5.0V
VOUT = 2.8V
This situation can be different at 85°C. The follow-
ing is an example for an AAT3223 set for a 2.8V
output at 85°C:
PD(MAX)
(VIN - VOUT)IOUT + (VIN × IGND
%DC = 100
)
267mW
(5.0V - 2.8V)150mA + (5.0V × 1.1µA)
%DC = 100
VOUT
IOUT
IGND
= 2.9V
%DC = 80.9%
= 150mA
= 1.1µA
PD(MAX) was assumed to be 267mW.
267mW + (2.8V × 150mA)
For a 150mA output current and a 2.2V drop across
the AAT3223 at an ambient temperature of 85°C,
the maximum on-time duty cycle for the device
would be 80.9%.
VIN(MAX)
=
150mA + 1.1µA
VIN(MAX) = 4.58V
The following family of curves shows the safe
operating area for duty-cycled operation from
ambient room temperature to the maximum oper-
ating level.
From the discussion above, PD(MAX) was deter-
mined to equal 267mW at TA = 85°C.
3223.2006.03.1.5
13
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
