AAT3220
150mA NanoPower™ LDO Linear Regulator
At any given ambient temperature (T
A
), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
T
J(MAX)
- T
A
θ
JA
P
D(MAX)
= (V
IN
- V
OUT
)I
OUT
+ (V
IN
×
I
GND
)
P
D(MAX)
=
This formula can be solved for V
IN
to determine the
maximum input voltage.
P
D(MAX)
+ (V
OUT
×
I
OUT
)
I
OUT
+ I
GND
Constants for the AAT3220 are T
J(MAX)
, the maxi-
mum junction temperature for the device which is
125°C and
Θ
JA
= 200°C/W, the package thermal
resistance. Typically, maximum conditions are cal-
culated 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 pack-
age power dissipation is 200mW. At T
A
= 25°C, the
maximum package power dissipation is 500mW.
The maximum continuous output current for the
AAT3220 is a function of the package power dissi-
pation and the input-to-output voltage drop across
the LDO regulator. Refer to the following simple
equation:
P
D(MAX)
V
IN
- V
OUT
V
IN(MAX)
=
The following is an example for an AAT3220 set for
a 3.0V output:
V
OUT
I
OUT
I
GND
= 3.0V
= 150mA
= 1.1µA
500mW + (3.0V
×
150mA)
150mA + 1.1µA
V
IN(MAX)
=
V
IN(MAX)
= > 5.5V
From the discussion above, P
D(MAX)
was deter-
mined to equal 417mW at T
A
= 25°C.
Thus, the AAT3220 can sustain a constant 3.0V
output at a 150mA load current as long as V
IN
is
≤
5.5V at an ambient temperature of 25°C. 5.5V is
the maximum input operating voltage for the
AAT3220, 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 follow-
ing is an example for an AAT3220 set for a 3.0V
output at 85°C:
V
OUT
I
OUT
I
GND
= 3.0V
= 150mA
= 1.1µA
200mW + (3.0V
×
150mA)
150mA + 1.1µA
I
OUT(MAX)
<
For example, if V
IN
= 5V, V
OUT
= 3V and T
A
= 25°C,
I
OUT(MAX)
< 250mA. The output short-circuit protec-
tion threshold is set between 150mA and 300mA. If
the output load current were to exceed 250mA or if
the ambient temperature were to increase, the inter-
nal die temperature 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 regulator since the thermal pro-
tection circuit will only activate after a short-circuit
event occurs on the LDO regulator output.
To determine the maximum input voltage for a
given load current, refer to the following equation.
This calculation accounts for the total power dissi-
pation of the LDO regulator, including that caused
by ground current.
V
IN(MAX)
=
V
IN(MAX)
= 4.33V
3220.2006.01.1.4
11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
