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AAT4682IGU-T1 Datasheet(PDF) 8 Page - Advanced Analog Technology, Inc. |
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AAT4682IGU-T1 Datasheet(HTML) 8 Page - Advanced Analog Technology, Inc. |
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8 / 10 page  AAT4682 Dual Electronic Resettable Switch 8 4682.2006.05.1.2 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 0.3 Ω.") 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 1µF 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 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. Attaching Loads Capacitive loads attached to the AAT4682 will charge at a rate no greater than the current limit setting. 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 100 Ω. 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 10k Ω to 100k Ω 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: 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. RθJA 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) RDS(ON)(MAX) · RΘJA T J(MAX) = IMAX 2 · R DS(ON)(MAX) · R ΘJA + TA(MAX) |
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