150mA CMOS High Performance LDO
taining a reverse bias on the internal parasitic
diode. Conditions where V
OUT
might exceed V
IN
should be avoided since this would forward bias
the internal parasitic diode and allow excessive
current flow into the V
OUT
pin, possibly damaging
the LDO regulator.
In applications where there is a possibility of V
OUT
exceeding V
IN
for brief amounts of time during nor-
mal operation, the use of a larger value C
IN
capaci-
tor is highly recommended. A larger value of C
IN
with respect to C
OUT
will effect a slower C
IN
decay
rate during shutdown, thus preventing V
OUT
from
exceeding V
IN
. In applications where there is a
greater danger of V
OUT
exceeding V
IN
for extended
periods of time, it is recommended to place a
Schottky diode across V
IN
to V
OUT
(connecting the
cathode to V
IN
and anode to V
OUT
). The Schottky
diode forward voltage should be less than 0.45V.
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 211mW. At T
A
= 25°C, the
maximum package power dissipation is 526mW.
The maximum continuous output current for the
AAT3215 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:
AAT3215
I
OUT(MAX)
<
P
D(MAX)
(V
IN
- V
OUT
)
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.
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.
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 this datasheet.
At any given ambient temperature (T
A
), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
For example, if V
IN
= 5V, V
OUT
= 3V, and T
A
= 25°C,
I
OUT(MAX)
< 264mA. If the output load current were
to exceed 264mA or if the ambient temperature
were to increase, the internal die temperature
would increase. If the condition remained con-
stant, the LDO regulator thermal protection circuit
would activate.
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.
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 AAT3215 set for
a 2.5 volt output:
V
OUT
I
OUT
I
GND
= 2.5V
= 150mA
= 150µA
P
D(MAX)
=
T
J(MAX)
- T
A
θ
JA
V
IN(MAX)
=
Constants for the AAT3215 are T
J(MAX)
, the maxi-
mum junction temperature for the device which is
125°C, and
Θ
JA
= 190°C/W, the package thermal
resistance. Typically, maximum conditions are cal-
culated at the maximum operating temperature
526mW + (2.5V
·
150mA)
150mA + 150μA
V
IN(MAX)
= 6.00V
From the discussion above, P
D(MAX)
was deter-
mined to equal 526mW at T
A
= 25°C.
3215.2006.05.1.6
11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
