AAT4618 Datasheet, PDF(8/11 Page) Advanced Analogic Technologies

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AAT4618 Datasheet, PDF (8/11 Pages) Advanced Analogic Technologies – Current Limited Load Switch

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Applications Information

Input Capacitor

The input capacitor CIN protects the power supply

from current transients generated by the load

attached to the AAT4618. When a short circuit is

suddenly applied to the output of the AAT4618, a

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

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

rent limit circuitry activates. (See characteristic

curve "Short Circuit Through 0.3Î©.") 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 AAT4618. The

extremely fast short-circuit response time of the

AAT4618 reduces the size requirement of CIN.

CIN should be located as close to the device VIN 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 low capacitance (approximately 0.47ÂµF) is

required. No matter how large the output capaci-

tor, output current is limited to the value set by the

AAT4618 current limiting circuitry, so very large

output capacitors can be used.

For example, USB ports are specified to have at

least 120ÂµF of capacitance downstream from their

controlling power switch. The current limiting circuit

will allow an output capacitance of 1000ÂµF or more

without disturbing the upstream power supply.

ON (Enable Input)

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 AAT4618 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.

AAT4618

Current Limited Load Switch

When switching the AAT4618 on into a capacitive

load, the AAT4618 will charge the output capacitive

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

Attaching Loads

Capacitive loads attached to an enabled AAT4618 will

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

FAULT Output

The FAULT flag is provided to alert the system if an

AAT4618 load is not receiving sufficient voltage to

operate properly. If current limit or over-tempera-

ture circuits in any combination are active for more

than approximately 4ms, the FAULT flag is pulled

to ground through approximately 100Î©. The filter-

ing of voltage or current transients of less than 4ms

prevents capacitive loads connected to the

AAT4618 output from activating the FAULT flag

when they are initially attached. Pull-up resistanc-

FAULT is an open drain terminal, it may be pulled

up to any unrelated voltage less than the maximum

operating voltage of 5.5V, allowing for level shifting

between circuits.

Thermal Considerations

Since the AAT4618 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: TJ(MAX) = IMAX2 Â· RDS(ON)(MAX) Â· RÎJA + TA(MAX)

In Equation 1, IMAX is the maximum current required

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

RDS(ON) of the AAT4618 at high temperature. RÎ¸JA is

the thermal resistance between the AAT4618 die

and the board onto which it is mounted. TA(MAX) is

the maximum temperature that the PCB under the

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