AAT3236IGV-3.5-T1 datasheet(11/18 Pages) AAT

Part #	AAT3236IGV-3.5-T1

Description	300mA CMOS High Performance LDO

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Manufacturer	AAT [Advanced Analog Technology, Inc.]

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AAT3236

300mA CMOS High Performance LDO

3236.2007.03.1.4

11

Reverse Output-to-Input Voltage

Conditions and Protection

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 V

OUT might exceed VIN 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 capac-

itor is highly recommended. A larger value of C

IN

with respect to C

OUT will effect a slower CIN decay

rate during shutdown, thus preventing V

OUT from

exceeding V

IN.

In applications where there is a

greater danger of V

OUT exceeding VIN for extended

periods of time, it is recommended to place a

Schottky diode across V

IN to VOUT (connecting the

cathode to V

IN and anode to VOUT).

The Schottky

diode forward voltage should be less than 0.45V.

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 this datasheet.

At any given ambient temperature (T

A), the maxi-

mum package power dissipation can be deter-

mined by the following equation:

Constants for the AAT3236 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

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

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:

For example, if V

IN = 4.2V, VOUT = 3.3V, and TA =

25°C, I

OUT(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.

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

This formula can be solved for V

IN to determine the

maximum input voltage.

The following is an example for an AAT3236 set for

a 3.0V output:

VIN(MAX) =

PD(MAX) + (VOUT × IOUT)

IOUT + IGND

IOUT(MAX) <

PD(MAX)

VIN - VOUT

PD(MAX) =

TJ(MAX) - TA

θJA

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