AAT3220
150mA NanoPower™ LDO Linear Regulator
At any given ambient temperature (TA), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
PD(MAX) = (VIN - VOUT)IOUT + (VIN × IGND
)
This formula can be solved for VIN to determine the
maximum input voltage.
TJ(MAX) - TA
θJA
PD(MAX)
=
PD(MAX) + (VOUT × IOUT
)
VIN(MAX)
=
Constants for the AAT3220 are TJ(MAX), the maxi-
mum junction temperature for the device which is
125°C and ΘJA = 200°C/W, the package thermal
resistance. Typically, maximum conditions are cal-
culated at the maximum operating temperature
where TA = 85°C; under normal ambient conditions
TA = 25°C. Given TA = 85°C, the maximum pack-
age power dissipation is 200mW. At TA = 25°C, the
maximum package power dissipation is 500mW.
IOUT + IGND
The following is an example for an AAT3220 set for
a 3.0V output:
VOUT
IOUT
IGND
= 3.0V
= 150mA
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:
= 1.1µA
500mW + (3.0V × 150mA)
150mA + 1.1µA
VIN(MAX)
=
VIN(MAX) = > 5.5V
PD(MAX)
VIN - VOUT
IOUT(MAX)
<
From the discussion above, PD(MAX) was deter-
mined to equal 417mW at TA = 25°C.
Thus, the AAT3220 can sustain a constant 3.0V
output at a 150mA load current as long as VIN 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 VIN(MAX) limits.
For example, if VIN = 5V, VOUT = 3V and TA = 25°C,
IOUT(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.
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:
VOUT
IOUT
IGND
= 3.0V
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.
= 150mA
= 1.1µA
200mW + (3.0V × 150mA)
150mA + 1.1µA
VIN(MAX)
=
VIN(MAX) = 4.33V
3220.2006.01.1.4
11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
