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AAT4252A

Dual Slew Rate Controlled Load Switch

Analog Technology Inc 

AAT4252A

Dual Slew Rate Controlled Load Switch

the VOUT pin, possibly damaging the load switch. In

applications where there is a possibility of VOUT

exceeding VIN for brief periods 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

Schottky diode from VIN to VOUT (connecting the

cathode to VIN and anode to VOUT). The Schottky

diode forward voltage should be less than 0.45V.

Thermal Considerations and High

Output Current Applications

The AAT4252A is designed to deliver a continuous

output load current. The limiting characteristic for

maximum safe operating output load current is

package power dissipation. In order to obtain high

operating currents, careful device layout and circuit

operating conditions must be taken into account.

The following discussions will assume the load

switch is mounted on a printed circuit board utilizing

the minimum recommended footprint as stated in

the Printed Circuit Board Layout Recommendations

section of this datasheet.

At any given ambient temperature (TA), the maxi-

mum package power dissipation can be deter-

mined by the following equation:

PD(MAX)

=

TJ(MAX) -

θJA

TA

Constants for the AAT4252A are maximum junction

temperature (TJ(MAX) = 125°C1) and package ther-

mal resistance (θJA = 160°C/W). Worst case condi-

tions are calculated at the maximum operating tem-

perature, TA = 85°C. Typical conditions are calcu-

lated under normal ambient conditions where TA =

25°C. At TA = 85°C, PD(MAX) = 250mW. At TA =

25°C, PD(MAX) = 625mW.

The maximum continuous output current for the

AAT4252A is a function of the package power dis-

sipation and the RDS of the MOSFET at TJ(MAX). The

maximum RDS of the MOSFET at TJ(MAX) is calcu-

lated by increasing the maximum room tempera-

ture RDS by the RDS temperature coefficient. The

temperature coefficient (TC) is 2800ppm/°C.

Therefore, at 125°C:

RDS(MAX) = RDS(25°C) · (1 + TC ⋅ ΔT)

RDS(MAX) = 155mΩ · (1 + 0.002800 · (125°C - 25°C))

RDS(MAX) = 198mΩ

For maximum current, refer to the following equation:

IOUT(MAX) <

PD(MAX)

RDS

For example, if VIN = 5V, RDS(MAX) = 198mΩ, and TA

= 25°C, IOUT(MAX) = 1.8A. If the output load current

were to exceed 1.8A or if the ambient temperature

were to increase, the internal die temperature would

increase and the device would be damaged. Higher

peak currents can be obtained with the AAT4252A.

To accomplish this, the device thermal resistance

must be reduced by increasing the heat sink area or

by operating the load switch in a duty cycle manner.

Duty cycles with peaks less than 2ms in duration

can be considered using the method below.

High Peak Output Current Applications

Some applications require the load switch to operate

at a continuous nominal current level with short dura-

tion, high-current peaks. Refer to the IDM specifica-

tion in the Absolute Maximum Ratings table to

ensure the AAT4252A’s maximum pulsed current rat-

ing is not exceeded. The duty cycle for both output

current levels must be taken into account. To do so,

first calculate the power dissipation at the nominal

continuous current level, and then add the additional

power dissipation due to the short duration, high-cur-

rent peak scaled by the duty factor. For example, a

4V system using an AAT4252A operates at a contin-

uous 100mA load current level and has short 2A cur-

rent peaks, as in a GSM application. The current

peak occurs for 576μs out of a 4.61ms period.

1. The actual maximum junction temperature of AAT4252A is 150°C. However,good design practice is to derate the maximum die tem-

perature down to 125°C to prevent the possibility of over temperature damage.

10

4252A.2007.06.1.0

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