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AAT3218IGV-3.0-T1 Datasheet(PDF) 11 Page - Advanced Analogic Technologies |
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AAT3218IGV-3.0-T1 Datasheet(HTML) 11 Page - Advanced Analogic Technologies |
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11 / 18 page  No-Load Stability The AAT3218 is designed to maintain output volt- age regulation and stability under operational no- load conditions. This is an important characteristic for applications where the output current may drop to zero. Reverse Output-to-Input Voltage Conditions and Protection Under normal operating conditions, a parasitic diode exists between the output and input of the LDO regulator. The input voltage should always remain greater than the output load voltage, main- taining a reverse bias on the internal parasitic diode. Conditions where VOUT might exceed VIN should be avoided since this would forward bias the internal parasitic diode and allow excessive current flow into the VOUT pin, possibly damaging the LDO regulator. 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 Schottky diode across VIN to VOUT (connecting the cathode to VIN and anode to VOUT). The Schottky diode forward voltage should be less than 0.45V. 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 must 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 datasheet. At any given ambient temperature (TA), the maxi- mum package power dissipation can be deter- mined by the following equation: 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°C, the maximum pack- age 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: For example, if VIN = 5V, VOUT = 3V, and TA = 25°C, IOUT(MAX) < 264mA. If the output load current were to exceed 264mA or if the ambient temperature were to increase, the internal die temperature would increase. If the condition remained con- stant, the LDO regulator thermal protection circuit would activate. To determine the maximum input voltage for a given load current, refer to the following equation. This calculation accounts for the total power dissi- pation of the LDO regulator, including that caused by ground current. PD(MAX) = (VIN - VOUT)IOUT + (VIN x IGND) P D(MAX) I OUT(MAX) < (V IN - VOUT) [T J(MAX) - TA] P D(MAX) = θJA AAT3218 150mA MicroPower™ High Performance LDO 3218.2006.04.1.8 11 |
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