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AAT3218IGV-2.8-T1 Datasheet(PDF) 11 Page - List of Unclassifed Manufacturers |
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AAT3218IGV-2.8-T1 Datasheet(HTML) 11 Page - List of Unclassifed Manufacturers |
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11 / 18 page  Applications Information In applications where there is a possibility of VOUT exceeding VIN for brief amounts of time during nor- mal operation, the use of a larger value CIN capaci- tor is highly recommended. A larger value of CIN with respect to COUT will effect a slower CIN decay rate during shutdown, thus preventing VOUT from exceeding VIN. In applications where there is a greater danger of VOUT exceeding VIN for extended periods of time, it is recommended to place a schot- tky diode across VIN to VOUT (connecting the cath- ode to VIN and anode to VOUT). The Schottky diode forward voltage should be less than 0.45 volts. Thermal Considerations and High Output Current Applications The AAT3218 is designed to deliver a continuous output load current of 150mA under normal operat- ing conditions. The limiting characteristic for the maximum output load current safe operating area is essentially package power dissipation and the internal preset thermal limit of the device. 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 the document. At any given ambient temperature (TA) the maxi- mum package power dissipation can be deter- mined by the following equation: PD(MAX) = [TJ(MAX) - TA] / ΘJA Constants for the AAT3218 are TJ(MAX), the maxi- mum junction temperature for the device which is 125°C and ΘJA = 190°C/W, the package thermal resistance. Typically, maximum conditions are cal- culated at the maximum operating temperature where TA = 85°C, under normal ambient conditions TA = 25°C. Given TA = 85°, the maximum package power dissipation is 211mW. At TA = 25°C°, the maximum package power dissipation is 526mW. The maximum continuous output current for the AAT3218 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: IOUT(MAX) < PD(MAX) / (VIN - VOUT) For example, if VIN = 5V, VOUT = 3V and TA = 25°, IOUT(MAX) < 264mA. If the output load current were to exceed 264mA or if the ambient temperature were to increase, the internal die temperature will increase. If the condition remained constant, the LDO regula- tor thermal protection circuit will activate. To figure what the maximum input voltage would be for a given load current refer to the following equa- tion. This calculation accounts for the total power dissipation of the LDO Regulator, including that caused by ground current. PD(MAX) = (VIN - VOUT)IOUT + (VIN x IGND) This formula can be solved for VIN to determine the maximum input voltage. VIN(MAX) = (PD(MAX) + (VOUT x IOUT)) / (IOUT + IGND) The following is an example for an AAT3218 set for a 2.5 volt output: From the discussion above, PD(MAX) was deter- mined to equal 526mW at TA = 25°C. VOUT = 2.5 volts IOUT = 150mA IGND = 150µA VIN(MAX)=(526mW+(2.5Vx150mA))/(150mA +150µA) VIN(MAX) = 6.00V Thus, the AAT3218 can sustain a constant 2.5V output at a 150mA load current as long as VIN is ≤ 6.00V at an ambient temperature of 25°C. 6.0V is the absolute maximum voltage where an AAT3218 would never be operated, thus at 25°C, the device would not have any thermal concerns or opera- tional VIN(MAX) limits. This situation can be different at 85°C. The follow- ing is an example for an AAT3218 set for a 2.5 volt output at 85°C: From the discussion above, PD(MAX) was deter- mined to equal 211mW at TA = 85°C. AAT3218 150mA MicroPower™ High Performance LDO 3218.2004.02.1.0 11 |
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