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LT1129IQ-5 Datasheet(PDF) 11 Page - Linear Technology |
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LT1129IQ-5 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LT1129/LT1129-3.3/LT1129-5 11 112935ff OPERATION copper. This data can be used as a rough guideline in estimating thermal resistance. The thermal resistance for each application will be affected by thermal interactions with other components as well as board size and shape. Some experimentation will be necessary to determine the actual value. Table 1. Q Package, 5-Lead DD COPPER AREA THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) TOPSIDE* BACKSIDE BOARD AREA 2500 sq. mm 2500 sq. mm 2500 sq. mm 25°C/W 1000 sq. mm 2500 sq. mm 2500 sq. mm 27°C/W 125 sq. mm 2500 sq. mm 2500 sq. mm 35°C/W * Tab of device attached to topside copper Table 2. ST Package, 3-Lead SOT-223 COPPER AREA THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) TOPSIDE* BACKSIDE BOARD AREA 2500 sq. mm 2500 sq. mm 2500 sq. mm 45°C/W 1000 sq. mm 2500 sq. mm 2500 sq. mm 45°C/W 225 sq. mm 2500 sq. mm 2500 sq. mm 53°C/W 100 sq. mm 2500 sq. mm 2500 sq. mm 59°C/W * Tab of device attached to topside copper Table 3. S8 Package, 8-Lead Plastic SOIC COPPER AREA THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) TOPSIDE* BACKSIDE BOARD AREA 2500 sq. mm 2500 sq. mm 2500 sq. mm 55°C/W 1000 sq. mm 2500 sq. mm 2500 sq. mm 55°C/W 225 sq. mm 2500 sq. mm 2500 sq. mm 63°C/W 100 sq. mm 2500 sq. mm 2500 sq. mm 69°C/W * Device attached to topside copper T Package, 5-Lead TO-220 Thermal Resistance (Junction-to-Case) = 5°C/W Calculating Junction Temperature Example: Given an output voltage of 3.3V, an input voltage range of 4.5V to 5.5V, an output current range of 0mA to 500mA, 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 = 500mA VIN MAX = 5.5V IGND at (IOUT = 500mA, VIN = 5.5V) = 25mA so, P = 500mA • (5.5V – 3.3V) + (25mA • 5.5V) = 1.24W If we use a DD package, then the thermal resistance will be in the range of 25°C/W to 35°C/W depending on copper area. So the junction temperature rise above ambient will be approximately equal to: 1.24W • 30°C/W = 37.2°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 + 37.2°C = 87.2°C Output Capacitance and Transient Performance The LT1129 is designed to be stable with a wide range of output capacitors. The minimum recommended value is 3.3μF with an ESR of 2Ω or less. The LT1129 is a micropower device and output transient response will be a function of output capacitance. See the Transient Response curves in the Typical Performance Character- istics. Larger values of output capacitance will decrease the peak deviations and provide improved output transient response. Bypass capacitors, used to decouple individual components powered by the LT1129, will increase the effective value of the output capacitor. Protection Features The LT1129 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 thermal limiting, the device is protected against reverse input voltages, and reverse voltages from output to input. For fixed voltage devices the output and sense pins are tied together at the output. |
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