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AAT4625IHS-1-T1 Datasheet(PDF) 8 Page - Advanced Analogic Technologies |
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AAT4625IHS-1-T1 Datasheet(HTML) 8 Page - Advanced Analogic Technologies |
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8 / 15 page  AAT4625 USB Single-Channel Power Switch 8 4625.2006.04.1.2 Applications Information Operation in Current Limit If an excessive load is applied to the output of an AAT4625, the load current will be limited by the device’s current limit circuitry. Refer to the "Current Limit" curve in the Typical Characteristics section of this datasheet. If a short circuit were to occur on the load, there would be a demand for more current than what is allowed by the internal current limiting circuit and the voltage at the device output will drop. This causes the AAT4625 to dissipate more power than in normal operation, causing the die temperature to increase. When die temperature exceeds the internal over-temperature threshold, the AAT4625 will shut down. After shutting down, the AAT4625 cools to a level below the over-tem- perature threshold, at which point it will start up again. The AAT4625 will continue to cycle off and on until one of the following events occurs: the load current is reduced to a level below the AAT4625's current limit setting; the input power is removed; or the output is turned off by a logic high level applied to the EN pin. Thermal Considerations Since the AAT4625 has internal current limit and over-temperature protection, junction temperature is rarely a concern. If an application requires a large load current in a high-temperature operating environment, there is the possibility that the over- temperature protection circuit (rather than the cur- rent limit circuit) will regulate the current available to the load. In these applications, the maximum current available without risk of activation of the over-temperature circuit can 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 AAT4625 at high temperature. RθJA is the thermal resistance between the device die and the board onto which it is mounted. TA(MAX) is the maximum ambient temperature for the print- ed circuit board assembly under the AAT4625 when the load switch is not dissipating power. Equation 1 can be transformed to provide IMAX; Refer to Equation 2. Eq. 2: TSD(MIN) is the minimum temperature required to activate the device over-temperature protection. The typical thermal limit temperature specification is 125°C for the AAT4625; for calculations, 115°C is a safe minimum value to use. For example, a portable device is specified to oper- ate in a 50°C environment. The printed circuit board assembly will operate at temperatures as high as 85°C. This portable device has a sealed case and the area of the printed board assembly is relatively small, causing RθJA to be approximately 120°C/W. Using Equation 2, If this system requires less than 1.4A, the thermal limit will not activate during normal operation. Input Capacitor The input capacitor serves two purposes. First, it protects the source power supply from transient current effects generated by the application load circuit. If a short circuit is suddenly applied to the output of an AAT4625, there is a microsecond long period during which a large current can flow before the current limit circuit becomes active. Refer to the characteristic curve "Short Circuit Through 0.3 Ω." A properly sized input capacitor can dra- matically reduce the load switch input transient response effects seen by the power supply and other circuitry upstream from the AAT4625. The second purpose of the input capacitor is to pre- vent transient events generated by the load circuit from affecting the operation of the AAT4625. For example, if an AAT4625 is used in a circuit that oper- ates from a 5V power supply with poor step load response, turning on the load switch could cause the = IMAX = 1.4A 115 - 85 130 · 120 = IMAX TSD(MIN) - TA(MAX) RDS(ON)(MAX) · RΘJA T J(MAX) = IMAX 2 × RDS(ON)(MAX) × RθJA + TA(MAX) |
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