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AAT3236

300mA CMOS High Performance LDO

Analog Technology Inc 

AAT3236

300mA CMOS High Performance LDO

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.

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.

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 the document.

At any given ambient temperature (TA) the maxi-

mum package power dissipation can be deter-

mined by the following equation:

PD(MAX) = [TJ(MAX) - TA] / Θ JA

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°, the maximum package

power dissipation is 211mW. At TA = 25°C, the

maximum package power dissipation is 526mW

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:

IOUT(MAX) < PD(MAX) / (VIN - VOUT)

For example, if VIN = 4.2V, VOUT = 3.3V and TA =

25°, 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

will increase. If the condition remained constant,

the LDO regulator 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.

PD(MAX) = (VIN - VOUT)IOUT + (VIN x IGND)

This formula can be solved for VIN to determine the

maximum input voltage.

VIN(MAX) = (PD(MAX) + (VOUT x IOUT)) / (IOUT + IGND)

The following is an example for an AAT3236 set for

a 3.0 volt output:

From the discussion above, PD(MAX) was deter-

mined to equal 526mW at TA = 25°C°.

VOUT = 3.0 volts

IOUT = 500mA

IGND = 150uA

VIN(MAX)=(526mW+(3.0Vx500mA))/(500mA +150µA)

VIN(MAX) = 4.05V

Thus, the AAT3236 can sustain a constant 3V out-

put at a 500mA load current as long as VIN is ≤

4.05V at an ambient temperature of 25°C.

Higher input to output voltage differentials can be

obtained with the AAT3236, while maintaining

device functions within the thermal safe operating

area. To accomplish this, the device thermal resist-

ance must be reduced by increasing the heat sink

area or by operating the LDO regulator in a duty

cycled mode.

For example, an application requires VIN = 4.2V

while VOUT = 3.0V at a 500mA load and TA = 25°C.

VIN is greater then 4.05V, which is the maximum

safe continuous input level for VOUT = 3.0V at

500mA for TA = 25°C. To maintain this high input

voltage and output current level, the LDO regulator

must be operated in a duty cycled mode. Refer to

the following calculation for duty cycle operation:

PD(MAX) is assumed to be 526mW

IGND = 150µA

IOUT = 500mA

10

3236.2001.11.0.9

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