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AAT4601A Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT4601A Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 16 page  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 circuit board assembly is relatively small, causing RθJA to be approximately 120°C/W. Using Equation 2: Eq. 3: 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 AAT4601A, there is a microsecond- long period during which a large current can flow before the current limit circuitry activates; refer to the characteristic curve, "Short-Circuit Through 0.3 Ω." A properly sized input capacitor can dramat- ically reduce the load switch input transient response effects seen by the power supply and other circuitry upstream from the AAT4601A. The second purpose of the input capacitor is to prevent transient events generated by the load cir- cuit from affecting operation of the AAT4601A. For example, if an AAT4601A is used in a circuit that operates from a 3V power supply with poor step load response, it is possible that turning on the load switch could cause the input power supply to droop below the AAT4601A's under-voltage lockout threshold. This drop in voltage would cause the AAT4601A to turn off until the input power supply's voltage recovers. Since this cycle would be self- perpetuating, the entire circuit could be seen to be unstable. In the very rare case where capacitor cost is prohibitive, the output load circuit should be slew rate limited when turned on. Output Capacitor In order to insure stability while the device current limit is active, a small capacitance of approximately 1µF should be used. When the AAT4601A is acti- vated using the ON function, there are no momen- tary current transients, as in the case when a short circuit is suddenly applied to a device that is already on; refer to the characteristic curve, "Switch Turn-On Time." No matter how big the output capacitor, out- put current is limited to the value allowed by the threshold determined by RSET and the internal cur- rent limiting circuitry. This permits very large output capacitors to be used. For example, USB ports are specified to have at least 120µF of downstream capacitance from their control- ling power switch. An output capacitance as large as 1000µF would not disturb the input power supply to the AAT4601A used to control the USB port. ON Input When the AAT4601A is in the off state, the output is an open circuit and the device quiecent current consumption is reduced to less than 1µA. The ON threshold voltage is set to allow the AAT4601A to be controlled by 5V TTL levels as well as CMOS power from 2.5V to 5V. The ON function control voltage level should not exceed the input supply level applied to the IN pin. FAULT Output A FAULT flag is provided to alert a system if the load switch is not receiving a sufficient voltage level to properly operate. If either the current limit or over- temperature circuits in any combination are contin- uously active for more than approximately 2ms, the FAULT pin is pulled to ground internally through a 100 Ω resistance. The 2ms delay on the FAULT function is intended to prevent capacitive loads con- nected to the load switch output from activating FAULT when the device is turned on. The place- ment of a pull-up resistor between the FAULT pin and the IN pin is recommended. Reasonable val- ues for the pull-up resistor should range from 10k Ω to 100k Ω. Since FAULT is an open drain terminal, = IMAX = 1.4A 115 - 85 130 · 120 AAT4601A 1.8A Current Limited P-Channel Switch 4601A.2006.05.1.5 11 |
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