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At any given ambient temperature (T
A
), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
Constants for the AAT3200 are T
J(MAX)
, the maxi-
mum junction temperature for the device which is
125°C and
Θ
JA
= 200°C/W, the SOT23 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
AAT3200 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:
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
internal die temperature will increase. If the condi-
tion remained constant and the short-circuit protec-
tion were not to activate, there would be a potential
damage hazard to LDO regulator since the thermal
protection circuit will only activate after a short-cir-
cuit 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.
This formula can be solved for V
IN
to determine the
maximum input voltage.
The following is an example for an AAT3200 set for
a 3.0 volt output:
From the discussion above, P
D(MAX)
was deter-
mined to equal 417mW at T
A
= 25°C.
Thus, the AAT3200 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
AAT3200, 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 AAT3200 set for a 3.0 volt
output at 85°C:
From the discussion above, P
D(MAX)
was deter-
mined to equal 200mW at T
A
= 85°C.
V
IN(MAX)
=
V
OUT
= 3.0V
I
OUT
= 150mA
I
GND
= 20
μ
A
V
IN(MAX)
= 4.33V
200mW + (3.0V
×
150mA)
150mA + 20
μ
A
V
IN(MAX)
=
V
OUT
= 3.0V
I
OUT
= 150mA
I
GND
= 20
μ
A
V
IN(MAX)
> 5.5V
500mW + (3.0V
×
150mA)
150mA + 20
μ
A
V
IN(MAX)
= P
D(MAX)
+ (V
OUT
×
I
OUT
)
I
OUT
+ I
GND
P
D(MAX)
= (V
IN
- V
OUT
)I
OUT
+ (V
IN
×
I
GND
)
I
OUT(MAX)
<
P
D(MAX)
V
IN
- V
OUT
P
D(MAX)
= T
J(MAX)
- T
A
JA
AAT3200
OmniPower LDO Linear Regulator
3200.2005.04.1.1
11