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AAT4670IHS-B1 Datasheet(PDF) 7 Page - Advanced Analogic Technologies |
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AAT4670IHS-B1 Datasheet(HTML) 7 Page - Advanced Analogic Technologies |
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7 / 12 page  AAT4670 Dual-Input, Dual-Output Load Switches 4670.2002.1.0.92 7 Applications Information Input Capacitor The input capacitors, CINA and CINB, protect the input power supplies from current transients generated by loads attached to the AAT4670. If a short circuit is suddenly applied to an output of the AAT4670, there is a 750 nanosecond period during which a large current flows before current limit circuitry activates. (See characteristic curve "Short Circuit Through 0.3 Ω.") In this event, a properly sized input capaci- tor can dramatically reduce the voltage transient seen by the power supply and other circuitry upstream from the AAT4670. CIN should be located as close to the device VIN pin as practically possible. Ceramic, tantalum or aluminum electrolytic capaci- tors may be selected for CIN. There is no specific capacitor ESR requirement for CIN. However, for higher current operation, ceramic capacitors are rec- ommended for CIN due to their inherent capability over tantalum capacitors to withstand input current surges from low impedance sources such as batter- ies in portable devices. Output Capacitor In order to insure stability while the current limit is active, a small capacitance of approximately 1µF is required on each output. No matter how big the output capacitor, output current is limited to the value set by the AAT4670 current limiting circuitry, allowing very large output capacitors to be used. For example, USB ports are specified to have at least 120µF of capacitance down stream 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 AAT4670 will charge at a rate no greater than the current limit setting. FAULT Output FAULT flags are provided to alert the system if an AAT4670 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 three milliseconds, the associ- ated FAULT flag is pulled to ground through approximately 100 Ω. Removal of voltage or cur- rent transients of less than three milliseconds pre- vents capacitive loads connected to either AAT4670 output from activating the associated FAULT flag when they are initially attached. Pull up resistances of 1k Ω to 100kΩ are recommended. Since 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 AAT4670 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 appli- cations, the maximum current available without 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. TJ(MAX) = IMAX 2 × R DS(ON)(MAX) × R θJA + TA(MAX) In Equation 1, IMAX is the maximum current required by the load. RDS(ON)(MAX) is the maxi- mum rated RDS(ON) of the AAT4670 at high temper- ature. RθJA is the thermal resistance between the AAT4670 die and the board onto which it is mount- ed. TA(MAX) is the maximum temperature that the PCB under the AAT4670 would be if the AAT4670 were not dissipating power. Equation 1 can be rearranged to solve for IMAX; Equation 2. IMAX= TSD(MIN) - TA(MAX) RDS(ON)(MAX) × RθJA TSD(MIN) is the minimum temperature required to activate the AAT4670 over temperature protection. With typical specification of 125°C, 115°C is a safe minimum value to use. For example, if an application is specified to oper- ate in 50°C environments, the PCB operates at temperatures as high as 85°C. The application is sealed and its PCB is small, causing RθJA to be approximately 120°C/W. Using Equation 2, IMAX= 115 - 85 = 1.25 A 160m × 120 To prevent thermal limiting, the operating load cur- rent in the application must be less than 1.25A which lies in the current limiting range, so in this application, any operating current below the cur- rent limit threshold is allowed. |
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