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AAT3239ITS-1.85-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3239ITS-1.85-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 14 page  AAT3239 500mA MicroPower™ LDO 3239.2006.03.1.2 11 diode. Conditions where V OUT might exceed VIN should be avoided since this would forward bias the internal parasitic diode and allow excessive current flow into the V OUT pin, possibly damaging the LDO regulator. In applications where there is a possibility of V OUT exceeding V IN for brief amounts of time during nor- mal operation, the use of a larger value C IN capaci- tor is highly recommended. A larger value of C IN with respect to C OUT will effect a slower CIN decay rate during shutdown, thus preventing V OUT from exceeding V IN. In applications where there is a greater danger of V OUT exceeding VIN for extended periods of time, it is recommended to place a Schottky diode across V IN to VOUT (connecting the cathode to V IN and anode to VOUT). The Schottky diode forward voltage should be less than 0.45 volts. Thermal Considerations and High Output Current Applications The AAT3239 is designed to deliver a continuous output load current of 500mA under normal opera- tions. The short-circuit current limit is greater than 500mA, typically active at 600mA. The limiting characteristics for the maximum output load current safe operating area is essentially pack- age power dissipation, the internal preset thermal limit of the device, and the input-to-output voltage drop across the AAT3239. In order to obtain high operating currents, careful device layout and circuit operating conditions need to be taken into account. The following discussions will assume the LDO reg- ulator is mounted on a printed circuit board utilizing the minimum recommended footprint as stated in the layout considerations section of this document. At any given ambient temperature (T A), the maxi- mum package power dissipation can be determined by the following equation: Constants for the AAT3239 are T J(MAX), the maxi- mum junction temperature for the device which is 125°C, and T JA = 90°C/W, the package thermal resistance. Typically, maximum conditions are calcu- lated at the maximum operating temperature of T A = 85°C and under normal ambient conditions where T A = 25°C. Given TA = 85°C, the maximum package power dissipation is 444mW. At T A = 25°C, the max- imum package power dissipation is 1.11W. The maximum continuous output current for the AAT3239 is a function of the package power dissi- pation and the input-to-output voltage drop across the LDO regulator. Refer to the following simple equation: For example, if V IN = 4.2V, VOUT = 1.8V, and TA = 25°C, I OUT(MAX) < 463mA. If the output load current were to exceed 463mA or if the ambient tempera- ture were to increase, the internal die temperature would increase. If the condition remained constant, the LDO regulator thermal protection circuit would activate. To determine the maximum output current for a given output voltage, refer to the following equa- tion. This calculation accounts for the total power dissipation of the LDO regulator, including that caused by ground current. This formula can be solved for I OUT to determine the maximum output current. IOUT = PD(MAX) - (VIN × IGND) VIN - VOUT PD(MAX) = (VIN - VOUT)IOUT + (VIN x IGND) IOUT(MAX) < PD(MAX) VIN - VOUT PD(MAX) = TJ(MAX) - TA θ JA |
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