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LT1521-3.3 Datasheet(PDF) 10 Page - Linear Technology |
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LT1521-3.3 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LT1521/LT1521-3 LT1521-3.3/LT1521-5 output is turned off and the divider current will be zero. Curves of Adjust Pin Voltage vs Temperature and Adjust Pin Bias Current vs Temperature appear in the Typical Performance Characteristics. The reference voltage at the adjust pin has a positive temperature coefficient of ap- proximately 15ppm/ °C. The adjust pin bias current has a negative temperature coefficient. These effects will tend to cancel each other. The adjustable device is specified with the adjust pin tied to the output pin. This sets the output voltage to 3.75V. Specifications for output voltages greater than 3.75V will be proportional to the ratio of the desired output voltage to 3.75V; (VOUT/3.75V). For example: load regulation for an output current change of 1mA to 300mA is –20mV typical at VOUT = 3.75V. At VOUT = 12V, load regulation would be: (12V/3.75V)(– 20mV) = – 64mV 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. Ground pin current multiplied by the input voltage: (IGND)(VIN) APPLICATIONS INFORMATION Figure 2. Adjustable Operation + VIN VOUT LT1521 GND IN SHDN OUT ADJ LT1521 • F01 VOUT = 3.75V 1 + + (IADJ + R2) VADJ = 3.75V IADJ = 50nA AT 25°C OUTPUT RANGE = 3.75V TO 20V R2 R1 ) ) R1 R2 The ground pin current can be found by examining the Ground Pin Current curves in the Typical Performance Characteristics. Power dissipation will be equal to the sum of the two components listed above. The LT1521 series regulators have internal thermal limit- ing designed to protect the device during overload condi- tions. For continuous normal load conditions the maxi- mum junction temperature rating of 125 °C must not be exceeded. It is important to give careful consideration to all sources of thermal resistance 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. Copper board stiffeners and plated through-holes can also be used to spread the heat gener- ated by power devices. The following tables list thermal resistance for each pack- age. Measured values of thermal resistance for several different board sizes and copper areas are listed for each package. All measurements were taken in still air on 3/32" FR-4 board with one ounce copper. All NC leads were connected to the ground plane. Table 1. MS8 Package COPPER AREA THERMAL RESISTANCE TOPSIDE** BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500mm2 2500mm2 2500mm2 110 °C/W 1000mm2 2500mm2 2500mm2 115 °C/W 225mm2 2500mm2 2500mm2 120 °C/W 100mm2 2500mm2 2500mm2 130 °C/W * Pin 4 is ground. ** Device is mounted on topside. Table 2. S8 Package* COPPER AREA THERMAL RESISTANCE TOPSIDE** BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500mm2 2500mm2 2500mm2 60 °C/W 1000mm2 2500mm2 2500mm2 60 °C/W 225mm2 2500mm2 2500mm2 68 °C/W 100mm2 2500mm2 2500mm2 74 °C/W * Pins 3, 6, 7 are ground. ** Device is mounted on topside. |
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