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LT1528 Datasheet(PDF) 8 Page - Linear Technology |
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LT1528 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LT1528 8 1528fb The LT1528 is specified with the SENSE pin tied to the OUTPUT pin. This sets the output voltage to 3.3V. Specifications for output voltage greater than 3.3V will be proportional to the ratio of the desired output voltage to 3.3V (VOUT/3.3V). For example, load regulation for an output current change of 1mA to 1.5A is – 5mV (typical) at VOUT = 3.3V. At VOUT = 12V, load regulation would be: (12V/3.3V) • (–5mV) = (–18mV) Thermal Considerations The power handling capability of the device will be limited by the maximum rated junction temperature (125°C). The power dissipated by the device will be made up of two components: 1. Output current multiplied by the input/output voltage differential, IOUT • (VIN – VOUT), and 2. GND pin current multiplied by the input voltage, IGND • VIN. The GND pin current can be found by examining the GND Pin Current curves in the Typical Performance Character- istics. Power dissipation will be equal to the sum of the two components listed above. The LT1528 has internal thermal limiting designed to pro- tect the device during overload conditions. For continuous normal load conditions the maximum junction temperature rating of 125°C must not be exceeded. It is important to give careful consideration to all sources of thermal resis- tance from junction-to-ambient. Additional heat sources mounted nearby must also be considered. For surface mount devices heat sinking is accomplished by using the heat spreading capabilities of the PC board and its copper traces. Experiments have shown that the heat spreading copper layer does not have to be electri- cally connected to the tab of the device. The PC material can be very effective at transmitting heat between the pad area, attached to the tab of the device, and a ground or power plane either inside or on the opposite side of the board. Although the actual thermal resistance of the PC material is high, the length/area ratio of the thermal resistor between layers is small. Copper board stiffeners and plated through holes can also be used to spread the heat generated by power devices. Table 1a lists thermal resistance for the DD package. For the TO-220 package (Table 1b) thermal resistance is given for junction-to-case only since this package is usu- ally mounted to a heat sink. Measured values of thermal resistance for several different copper areas are listed for the DD package. All measurements were taken in still air on 3/32" FR-4 board with one ounce 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 1a. Q-Package, 5-Lead DD COPPER AREA BOARD AREA THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) TOPSIDE* BACKSIDE 2500 sq mm 2500 sq mm 2500 sq mm 23°C/W 1000 sq mm 2500 sq mm 2500 sq mm 25°C/W 125 sq mm 2500 sq mm 2500 sq mm 33°C/W *Device is mounted on topside. Table 1b. T Package, 5-Lead TO-220 Thermal Resistance (Junction-to-Case) 2.5°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) = 4mA so, P = 500mA • (5.5V – 3.3V) + (4mA • 5.5V) = 1.12W If we use a DD package, the thermal resistance will be in the range of 23°C/W to 33°C/W depending on the copper area. So the junction temperature rise above ambient will be approximately equal to: 1.12W • 28°C/W = 31.4°C APPLICATIONS INFORMATION |
Similar Part No. - LT1528_15 |
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Similar Description - LT1528_15 |
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