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LT1521IST-3.3 Datasheet(PDF) 11 Page - Linear Dimensions Semiconductor |
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LT1521IST-3.3 Datasheet(HTML) 11 Page - Linear Dimensions Semiconductor |
11 / 16 page 11 LT1521/LT1521-3 LT1521-3.3/LT1521-5 1521335fb APPLICATIONS INFORMATION is required to prevent oscillations. The LT1521 is a micropower device and output transient response will be a function of output capacitance. See the Transient Response curves in the Typical Performance Characteris- tics. Larger values of output capacitance will decrease the peak deviations and provide improved output transient response for larger load current deltas. Bypass capaci- tors, used to decouple individual components powered by the LT1521, will increase the effective value of the output capacitor. Protection Features The LT1521 incorporates several protection features which make it ideal for use in battery-powered circuits. In addition to the normal protection features associated with monolithic regulators, such as current limiting and ther- mal limiting, the device is protected against reverse input voltages, reverse output voltages and reverse voltages from output to input. Current limit protection and thermal overload protection are intended to protect the device against current overload conditions at the output of the device. For normal opera- tion, the junction temperatures should not exceed 125 °C. The input of the device will withstand reverse voltages of 20V. Current flow into the device will be limited to less than 1mA (typically less than 100 µA) and no negative voltage will appear at the output. The device will protect both itself and the load. This provides protection against batteries that can be plugged in backward. For fixed voltage versions of the device, the output can be pulled below ground without damaging the device. If the input is left open circuit or grounded, the output can be pulled below ground by 20V. The output will act like an open circuit, no current will flow out of the pin. If the input is powered by voltage source, the output will source the short-circuit current of the device and will protect itself by thermal limiting. For the adjustable version of the device, the output pin is internally clamped at one diode drop below ground. Reverse current for the adjustable device must be limited to 5mA. Table 3. SOT-223 Package (Thermal Resistance Junction-to-Tab 20 °C/W) COPPER AREA THERMAL RESISTANCE TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500mm2 2500mm2 2500mm2 50 °C/W 1000mm2 2500mm2 2500mm2 50 °C/W 225mm2 2500mm2 2500mm2 58 °C/W 100mm2 2500mm2 2500mm2 64 °C/W 1000mm2 1000mm2 1000mm2 57 °C/W 1000mm2 0 1000mm2 60 °C/W * Tab of device attached to topside copper. Calculating Junction Temperature Example: Given an output voltage of 3.3V, an input voltage range of 4.5V to 7V, an output current range of 0mA to 150mA and a maximum ambient temperature of 50 °C, what will the maximum junction temperature be? The power dissipated by the device will be equal to: IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX)) Where, IOUT(MAX) = 150mA VIN(MAX) = 7V IGND at (IOUT = 150mA, VIN = 7V) = 2.1mA So, P = 150mA(7V – 3.3V) + (2.1mA)(7V) = 0.57W If we use a SOT-223 package, then the thermal resistance will be in the range of 50 °C/W to 65°C/W depending on the copper area. So the junction temperature rise above ambient will be approximately equal to: 0.57W(60 °C/W) = 34.2°C The maximum junction temperature will then be equal to the maximum junction temperature rise above ambient plus the maximum ambient temperature or: TJMAX = 50°C + 34.2°C = 84.2°C Output Capacitance and Transient Performance The LT1521 is designed to be stable with a wide range of output capacitors. A minimum output capacitor of 1.5 µF |
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