AAT3215
150mA CMOS High Performance LDO
pation and the input to output voltage drop across
the LDO regulator. Refer to the following simple
equation:
Applications Information
In applications where there is a possibility of VOUT
exceeding VIN for brief amounts of time during nor-
mal operation, the use of a larger value CIN capaci-
tor is highly recommended. A larger value of CIN
with respect to COUT will effect a slower CIN decay
rate during shutdown, thus preventing VOUT from
exceeding VIN. In applications where there is a
greater danger of VOUT exceeding VIN for extended
periods of time, it is recommended to place a schot-
tky diode across VIN to VOUT (connecting the cath-
ode to VIN and anode to VOUT). The Schottky diode
forward voltage should be less than 0.45 volts.
IOUT(MAX) < PD(MAX) / (VIN - VOUT
)
For example, if VIN = 5V, VOUT = 3V and TA = 25°,
IOUT(MAX) < 264mA. If the output load current were to
exceed 264mA or if the ambient temperature were to
increase, the internal die temperature will increase.
If the condition remained constant, the LDO regula-
tor thermal protection circuit will activate.
To figure what the maximum input voltage would be
for a given load current refer to the following equa-
tion. This calculation accounts for the total power
dissipation of the LDO Regulator, including that
caused by ground current.
Thermal Considerations and High
Output Current Applications
The AAT3215 is designed to deliver a continuous
output load current of 150mA under normal operat-
ing conditions.
P
D(MAX) = (VIN - VOUT)IOUT + (VIN x IGND)
This formula can be solved for VIN to determine the
maximum input voltage.
The limiting characteristic for the maximum output
load current safe operating area is essentially
package power dissipation and the internal preset
thermal limit of the device. In order to obtain high
operating currents, careful device layout and circuit
operating conditions need to be taken into account.
VIN(MAX) = (PD(MAX) + (VOUT x IOUT)) / (IOUT + IGND
)
The following is an example for an AAT3215 set for
a 2.5 volt output:
From the discussion above, PD(MAX) was deter-
mined to equal 526mW at TA = 25°C.
The following discussions will assume the LDO reg-
ulator is mounted on a printed circuit board utilizing
the minimum recommended footprint as stated in
the layout considerations section of the document.
VOUT = 2.5 volts
IOUT = 150mA
IGND = 150µA
At any given ambient temperature (TA) the maxi-
mum package power dissipation can be deter-
mined by the following equation:
VIN(MAX)=(526mW+(2.5Vx150mA))/(150mA +150µA)
VIN(MAX) = 6.00V
Thus, the AAT3215 can sustain a constant 2.5V
output at a 150mA load current as long as VIN is ≤
6.00V at an ambient temperature of 25°C. 6.0V is
the absolute maximum voltage where an AAT3215
would never be operated, thus at 25°C, the device
would not have any thermal concerns or opera-
tional VIN(MAX) limits.
PD(MAX) = [TJ(MAX) - TA] / ΘJA
Constants for the AAT3215 are TJ(MAX), the maxi-
mum junction temperature for the device which is
125°C and ΘJA = 190°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°, the maximum package
power dissipation is 211mW. At TA = 25°C°, the
maximum package power dissipation is 526mW.
This situation can be different at 85°C. The follow-
ing is an example for an AAT3215 set for a 2.5 volt
output at 85°C:
From the discussion above, PD(MAX) was deter-
mined to equal 211mW at TA = 85°C.
The maximum continuous output current for the
AAT3215 is a function of the package power dissi-
10
3215.2002.03.0.91
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