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LT3011EMSETR Datasheet(PDF) 11 Page - Linear Technology |
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LT3011EMSETR Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LT3011 11 3011f During normal operation, an internal glitch filter will ignore short transients (<15μs). Longer transients below the 90% threshold will reset the JK flip-flop. This flip-flop ensures that the capacitor on the CT pin is quickly discharged all the way to the VCT(LOW) threshold before restarting the time delay. This provides a consistent time delay after the ADJ pin is within 10% of the regulated voltage before the PWRGD pin switches to high impedance. Thermal Considerations The power handling capability of the device will be limited by the maximum rated junction temperature (125°C, LT3011E/ LT3011I or 150°C, LT3011H). 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 is found by examining the GND pin current curves in the Typical Performance Characteristics section. Power dissipation will be equal to the sum of the two components listed above. The LT3011 series regulators have internal thermal limiting designed to protect the device during overload conditions. For continuous normal conditions, the maximum junction temperature rating of 125°C (LT3011E/ LT3011I) or 150°C (LT3011H) 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. Q J K VREF • 90% ADJ VCT(LOW) 0.1V VCT(HIGH) – VBE ( 1.1V) ICT 3μA CT 3011 F04 – + PWRGD 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 table lists thermal resistance for several different board sizes and copper areas. All measurements were taken in still air on 3/32" FR-4 board with one ounce copper. Table 1. MSOP Measured Thermal Resistance COPPER AREA BOARD AREA THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) TOPSIDE BACKSIDE 2500 sq mm 2500 sq mm 2500 sq mm 52°C/W 1000 sq mm 2500 sq mm 2500 sq mm 54°C/W 225 sq mm 2500 sq mm 2500 sq mm 58°C/W 100 sq mm 2500 sq mm 2500 sq mm 64°C/W Table 2. DFN Measured Thermal Resistance COPPER AREA BOARD AREA THERMAL RESISTANCE (JUNCTION-TO-AMBIENT) TOPSIDE BACKSIDE 2500 sq mm 2500 sq mm 2500 sq mm 52°C/W 1000 sq mm 2500 sq mm 2500 sq mm 54°C/W 225 sq mm 2500 sq mm 2500 sq mm 58°C/W 100 sq mm 2500 sq mm 2500 sq mm 64°C/W The thermal resistance junction-to-case ( θJC), measured at the Exposed Pad on the back of the die, is 16°C/W. Continuous operation at large input/output voltage dif- ferentials and maximum load current is not practical due to thermal limitations. Transient operation at high input/ output differentials is possible. The approximate thermal time-constant for a 2500sq mm 3/32" FR-4 board, with maximum topside and backside area for one ounce cop- per, is three seconds. This time-constant will increase as more thermal mass is added (i.e., vias, larger board and other components). For an application with transient high power peaks, average power dissipation can be used for junction temperature calculations as long as the pulse period is significantly less than the thermal time constant of the device and board. Figure 4. PWRGD Circuit Block Diagram APPLICATIONS INFORMATION |
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