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AAT4282A

Slew Rate Controlled Load Switch

Advanced Analogic Technologies 

SmartSwitchTM

The maximum continuous output current for the

AAT4282A is a function of the package power dissipation

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

of the MOSFET at TJ(MAX) is calculated by increasing the

maximum room temperature 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)Ω

RDS(MAX) = 130mΩ · + 0.002800 · (125°C - 25°C))

RDS(MAX) = 166.4mΩ

For maximum current, refer to the following equation:

IOUT(MAX) <

PD(MAX)

RDS

For example, if VIN = 5V, RDS(MAX) = 166.4mΩ, and TA =

25°C, IOUT(MAX) = 2.93A. If the output load current were

to exceed 2.93A 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 AAT4282A. 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 described in the High Peak Current Applications

section of this datasheet.

High Peak Output Current Applications

Some applications require the load switch to operate at a

continuous nominal current level with short duration,

high-current peaks. Refer to the IDM specification in the

Absolute Maximum Ratings table to ensure the AAT4282A’s

maximum pulsed current rating 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-current peak scaled by the duty factor. For example,

a 4V system using an AAT4282A which has channel A

operates at a continuous 1A load current level, and chan-

nel B operates at a continuous 100mA load current level

PRODUCT DATASHEET

AAT4282A

Slew Rate Controlled Load Switch

and has short 3A current peaks, as in a GSM application.

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

First, the current duty cycle is calculated:

⎛ x ⎞ ⎛ 576μs ⎞

% Peak Duty Cycle = ⎝ 100⎠ = ⎝ 4.61ms⎠

% Peak Duty Cycle = 12.5%

The load current is 100mA for 87.5% of the 4.61ms

period and 3A for 12.5% of the period. Since the

Electrical Characteristics do not report RDS(MAX) for 4V

operation, it must be approximated by consulting the

chart of RDS(ON) vs. VIN. The RDS reported for 5V at 100mA

and 3A can be scaled by the ratio seen in the chart to

derive the RDS for 4V VIN at 25°C: 130mΩ · 63mΩ/60mΩ

= 136.5mΩ. De-rated for temperature: 136.5mΩ · (1 +

0.002800 · (125°C - 25°C)) = 174.7mΩ.

For channel A, the power dissipation for a continuous 1A

load is calculated as follows:

PD(CHA) = IOUT2 · RDS = (1A)2 · 174.7mΩ = 174.7mW

For channel B, the power dissipation for 100mA load is

calculated as follows:

PD(MAX) = IOUT2 · RDS

PD(100mA) = (100mA)2 · 174.7mΩ

PD(100mA) = 1.75mW

PD(87.5%D/C) = %DC · PD(100mA)

PD(87.5%D/C) = 0.875 · 1.75mW

PD(87.5%D/C) = 1.53mW

The power dissipation for 100mA load at 87.5% duty

cycle is 1.53mW. Now the power dissipation for the

remaining 12.5% of the duty cycle at 3A is calculated:

PD(MAX) = IOUT2 · RDS

PD(3A) = (3A)2 · 174.7mΩ

PD(3A) = 1572mW

PD(12.5%D/C) = %DC · PD(3A)

PD(12.5%D/C) = 0.125 · 1572mW

PD(12.5%D/C) = 196.7mW

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4282A.2008.02.1.2

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