AAT3236
300mA CMOS High Performance LDO
Reverse Output-to-Input Voltage
Conditions and Protection
TJ(MAX) - TA
θJA
PD(MAX)
=
Under normal operating conditions, a parasitic diode
exists between the output and input of the LDO reg-
ulator. The input voltage should always remain
greater than the output load voltage, maintaining a
reverse bias on the internal parasitic diode.
Conditions where VOUT might exceed VIN should be
avoided since this would forward bias the internal
parasitic diode and allow excessive current flow into
the VOUT pin, possibly damaging the LDO regulator.
Constants for the AAT3236 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°C, the maximum pack-
age power dissipation is 211mW. At TA = 25°C, the
maximum package power dissipation is 526mW.
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 capac-
itor 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
Schottky diode across VIN to VOUT (connecting the
cathode to VIN and anode to VOUT). The Schottky
diode forward voltage should be less than 0.45V.
The maximum continuous output current for the
AAT3236 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:
PD(MAX)
VIN - VOUT
IOUT(MAX)
<
For example, if VIN = 4.2V, VOUT = 3.3V, and TA =
25°C, IOUT(MAX) < 584mA. If the output load current
were to exceed 584mA or if the ambient tempera-
ture were to increase, the internal die temperature
would increase. If the condition remained con-
stant, the LDO regulator thermal protection circuit
would activate.
Thermal Considerations and High
Output Current Applications
The AAT3236 is designed to deliver a continuous
output load current of 300mA under normal opera-
tions and can supply up to 500mA during circuit
start-up conditions. This is desirable for circuit
applications where there might be a brief high in-
rush current during a power-on event.
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.
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.
PD(MAX) = (VIN - VOUT)IOUT + (VIN x IGND
)
This formula can be solved for VIN to determine the
maximum input voltage.
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.
PD(MAX) + (VOUT × IOUT
)
VIN(MAX)
=
IOUT + IGND
At any given ambient temperature (TA), the maxi-
mum package power dissipation can be deter-
mined by the following equation:
The following is an example for an AAT3236 set for
a 3.0V output:
3236.2007.03.1.4
11
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
