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AAT4250

Slew Rate Controlled Load Switch

Advanced Analogic Technologies 

AAT4250

Slew Rate Controlled Load Switch

High Peak Output Current Applications

Some applications require the load switch to oper-

ate 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 AAT4250’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 dissi-

pation 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 AAT4250 oper-

ates at a continuous 100mA load current level and

has short 2A current peaks, as in a GSM applica-

tion. The current peak occurs for 576µs out of a

4.61ms period.

First, the current duty cycle is calculated:

% Peak Duty Cycle: X/100 = 576µs/4.61ms

% Peak Duty Cycle = 12.5%

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

period and 2A 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

RDS can be scaled by the ratio seen in the chart to

derive the RDS for 4V VIN: 175mΩ x 120mΩ/115mΩ

= 183mΩ. Derated for temperature: 183mΩ x (1 +

0.002800 x (125°C -25°C)) = 235mΩ. The power

dissipation for a 100mA load is calculated as follows:

PD(MAX) = I2OUT x RDS

PD(100mA) = (100mA)2 x 235mΩ

PD(100mA) = 2.35mW

PD(87.5%D/C) = %DC x PD(100mA)

PD(87.5%D/C) = 0.875 x 2.35mW

PD(87.5%D/C) = 2.1mW

The power dissipation for 100mA load at 87.5%

duty cycle is 2.1mW. Now the power dissipation for

the remaining 12.5% of the duty cycle at 2A is cal-

culated:

PD(MAX) = I2OUT x RDS

PD(2A) = (2A)2 x 235mΩ

PD(2A) = 940mW

PD(12.5%D/C) = %DC x PD(2A)

PD(12.5%D/C) = 0.125 x 940mW

PD(12.5%D/C) = 117.5mW

The power dissipation for 2A load at 12.5% duty

cycle is 117mW. Finally, the two power figures are

summed to determine the total true power dissipa-

tion under the varied load.

PD(total) = PD(100mA) + PD(2A)

PD(total) = 2.1mW + 117.5mW

PD(total) = 120mW

The maximum power dissipation for the AAT4250

operating at an ambient temperature of 85°C is

267mW. The device in this example will have a total

power dissipation of 120mW. This is well within the

thermal limits for safe operation of the device; in fact,

at 85°C, the AAT4250 will handle a 2A pulse for up

to 28% duty cycle. At lower ambient temperatures,

the duty cycle can be further increased.

Printed Circuit Board Layout

Recommendations

For proper thermal management, and to take

advantage of the low RDS(ON) of the AAT4250, a few

circuit board layout rules should be followed: VIN

and VOUT should be routed using wider than normal

traces, and GND should be connected to a ground

plane. For best performance, CIN and COUT should

be placed close to the package pins.

10

4250.2006.03.1.3

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