AAT4612 datasheet(8/11 Pages) AAT

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AAT4612

Current Limited Load Switch

4612.2006.09.1.3

Applications Information

Input Capacitor

The input capacitor, CIN, protects the power supply

from current transients generated by the load

attached to the AAT4612. When a short circuit is

suddenly applied to the output of the AAT4612, a

large current, limited only by the RDS(ON) of the

MOSFET, will flow for less than 1µs before the cur-

rent limit circuitry activates. (See characteristic

curve "Short Circuit Through 0.3

Ω.") In this event,

a moderately sized CIN will dramatically reduce the

voltage transient seen by the power supply and by

other circuitry upstream from the AAT4612. The

extremely fast short-circuit response time of the

AAT4612 reduces the size requirement of CIN.

CIN should be located as close to the device IN pin

as practically possible. Ceramic, tantalum, or alu-

minum electrolytic capacitors are appropriate for

CIN. There is no specific capacitor ESR requirement

for CIN. However, for higher current operation,

ceramic capacitors are recommended for CIN due to

their inherent capability over tantalum capacitors to

withstand input current surges from low impedance

sources, such as batteries in portable devices.

Output Capacitor

In order to insure stability while current limit is

active, a small capacitance of approximately

0.47µF is required. No matter how large the output

capacitor, however, output current is limited to the

value set by the AAT4612 current limiting circuitry,

so very large output capacitors can be used.

For example, USB ports are specified to have at

least 120µF of capacitance downstream from their

controlling power switch. The current limiting circuit

will allow an output capacitance of 1000µF or more

without disturbing the upstream power supply.

In many systems, power planes are controlled by

integrated circuits which run at lower voltages than

the power plane itself. The enable input (ON) of

the AAT4612 has low and high threshold voltages

that accommodate this condition. The threshold

voltages are compatible with 5V TTL and 2.5V to

5V CMOS systems.

When switching the AAT4612 on into a capacitive

load, the AAT4612 will charge the output capacitive

load at a rate no greater than the current limit setting.

Attaching Loads

Capacitive loads attached to an enabled AAT4612

will charge at a rate no greater than the current limit

setting.

Thermal Considerations

Since the AAT4612 has internal current limit and

over-temperature protection, junction temperature

is rarely a concern. However, if the application

requires large currents in a hot environment, it is

possible that temperature, rather than current limit,

will be the dominant regulating condition. In these

applications, the maximum current available with-

out risk of an over-temperature condition must be

calculated.

The maximum internal temperature

while current limit is not active can be calculated

using Equation 1.

Eq. 1:

In Equation 1, IMAX is the maximum current

required by the load. RDS(ON)(MAX) is the maximum

rated RDS(ON) of the AAT4612 at high temperature.

RθJA is the thermal resistance between the

AAT4612 die and the board onto which it is mount-

ed. TA(MAX) is the maximum temperature that the

PCB under the AAT4612 would be if the AAT4612

were not dissipating power. Equation 1 can be

rearranged to solve for IMAX:

Eq. 2:

TSD(MIN) is the minimum temperature required to

activate the AAT4612 over-temperature protection.

MAX

SD(MIN) - TA(MAX)

DS(ON)(MAX) · R

ΘJA

J(MAX) = IMAX

2 · R

DS(ON)(MAX) · R

ΘJA + TA(MAX)

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