AAT3221/2
150mA NanoPower™ LDO Linear RegulatorPowerLinear
TM
PRODUCT DATASHEET
3221.2007.11.1.12 11
www.analogictech.com
maximum conditions are calculated at the maximum
operating temperature where T
A
= 85°C, under normal
ambient conditions T
A
= 25°C. Given T
A
= 85°C, the
maximum package power dissipation is 267mW. At T
A
=
25°C, the maximum package power dissipation is
667mW.
The maximum continuous output current for the
AAT3221/2 is a function of the package power dissipa-
tion and the input-to-output voltage drop across the
LDO regulator. Refer to the following simple equation:
I
OUT(MAX)
=
P
D(MAX)
(V
IN
- V
OUT
)
For example, if V
IN
= 5V, V
OUT
= 2.5V and T
A
= 25°C,
I
OUT(MAX)
< 267mA. The output short-circuit protection
threshold is set between 150mA and 300mA. If the out-
put load current were to exceed 267mA or if the ambient
temperature were to increase, the internal die tempera-
ture would increase. If the condition remained constant
and the short-circuit protection did not activate, there
would be a potential damage hazard to the LDO regula-
tor since the thermal protection circuit would only acti-
vate after a short-circuit event occured on the LDO
regulator output.
To determine the maximum input voltage for a given
load current, refer to the following equation. This calcu-
lation accounts for the total power dissipation of the LDO
regulator, including that caused by ground current.
P
D(MAX)
= (V
IN
- V
OUT
)I
OUT
+ (V
IN
· I
GND
)
This formula can be solved for V
IN
to determine the
maximum input voltage.
V
IN(MAX)
=
(P
D(MAX)
+ [V
OUT
· I
OUT
])
(I
OUT
+ I
GND
)
The following is an example for an AAT3221/2 set for a
2.5 volt output:
V
OUT
= 2.5 volts
I
OUT
= 150mA
I
GND
= 1.1μA
V
IN(MAX)
=
V
IN(MAX)
= 6.95V
(667mW + [2.5V · 150mA])
(150mA + 1.1µA)
From the discussion above, P
D(MAX)
was determined to
equal 667mW at T
A
= 25°C. Thus, the AAT3221/2 can
sustain a constant 2.5V output at a 150mA load current
as long as V
IN
is ≤6.95V at an ambient temperature of
25°C. 5.5V is the maximum input operating voltage for
the AAT3221/2, thus at 25°C the device would not have
any thermal concerns or operational V
IN(MAX)
limits.
This situation can be different at 85°C. The following is
an example for an AAT3221/2 set for a 2.5 volt output
at 85°C:
V
OUT
= 2.5 volts
I
OUT
= 150mA
I
GND
= 1.1μA
V
IN(MAX)
=
V
IN(MAX)
= 4.28V
(267mW + [2.5V · 150mA])
(150mA + 1.1µA)
From the discussion above, P
D(MAX)
was determined to
equal 267mW at T
A
= 85°C.
Higher input-to-output voltage differentials can be
obtained with the AAT3221/2, while maintaining device
functions in the thermal safe operating area. To accom-
plish 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 V
IN
= 5.0V while
V
OUT
= 2.5V at a 150mA load and T
A
= 85°C. V
IN
is
greater than 4.28V, which is the maximum safe continu-
ous input level for V
OUT
= 2.5V at 150mA for T
A
= 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:
I
GND
= 1.1μA
I
OUT
= 150mA
V
IN
= 5.0 volts
V
OUT
= 2.5 volts
%DC = 100
P
D(MAX)
([V
IN
- V
OUT
]I
OUT
+ [V
IN
· I
GND
])
%DC = 100
%DC = 71.2%
267mW
([5.0V - 2.5V]150mA + [5.0V · 1.1µA]
P
D(MAX)
is assumed to be 267mW.
