AAT4682 Datasheet, PDF(8/10 Page) Advanced Analogic Technologies

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AAT4682 Datasheet, PDF (8/10 Pages) Advanced Analogic Technologies – Dual Electronic Resettable Switches

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Applications Information

Input Capacitor

The input capacitor protects the power supply from

current transients generated by the loads attached

to the AAT4682. If a short circuit is suddenly

applied to a AAT4682 output, there is a 500

nanosecond long period during which a large cur-

rent can flow before current limit circuitry activates.

(See characteristic curve "Short Circuit Through

0.3Î©.") In this event, a properly sized input capac-

itor can dramatically reduce the voltage transient

seen by the power supply and other circuitry

upstream from the AAT4682.

CIN should be located as close to the device VIN

pin as practically possible. Ceramic, tantalum, or

aluminum electrolytic capacitors may be selected

for CIN. There is no specific capacitor equivalent

series resistance (ESR) requirement for CIN.

However, for higher current operation, ceramic

capacitors are recommended for CIN due to their

inherent capability over tantalum capacitors to with-

stand input current surges from low impedance

sources such as batteries in portable devices.

Output Capacitors

In order to insure stability while current limit is

active, a small output capacitance of approximately

1ÂµF is required at each output. No matter how big

the output capacitor, output current is limited to the

value set by the AAT4682 current limiting circuitry,

allowing very large output capacitors to 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.

Attaching Loads

Capacitive loads attached to the AAT4682 will charge

at a rate no greater than the current limit setting.

AAT4682

Dual Electronic Resettable Switch

FAULT Outputs

FAULT flags are provided to alert a system if a

AAT4682 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 two milliseconds, the corre-

sponding FAULT output is pulled to ground through

approximately 100Î©. Removal of voltage or cur-

rent transients of less than two milliseconds pre-

vents capacitive loads connected to the AAT4682

output from activating the FAULT flag when they

are initially attached. Pull-up resistances of 10kÎ©

to 100kÎ© are recommended. Since the FAULT out-

puts are open drain terminals, they may be pulled

up to any voltage rail less than the maximum oper-

ating voltage of 5.5V, allowing for level shifting

between circuits.

Thermal Considerations

Since the AAT4682 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 combined current avail-

able without risk of an over-temperature condition

can be calculated. The maximum internal temper-

ature while current limit is not active can be calcu-

lated 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 AAT4682 at high temperature.

RÎ¸JA is the thermal resistance between the

AAT4682 die and the board onto which it is mount-

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

PCB under the AAT4682 would be if the AAT4682

were not dissipating power. Equation 1 can be

rearranged to solve for IMAX; Equation 2.

Eq. 2:

IMAX =

TSD(MIN) - TA(MAX)

R Â· R DS(ON)(MAX)

ÎJA

4682.2006.05.1.2

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