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LM3557SDX-2 데이터시트(PDF) 4 Page - National Semiconductor (TI) |
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LM3557SDX-2 데이터시트(HTML) 4 Page - National Semiconductor (TI) |
4 / 11 page Note 3: Current flows into the pin. Note 4: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. See Thermal Properties for the thermal resistance. The maximum allowable power dissipation at any ambient temperature is calculated using: PD(MAX) = (TJ(MAX) – TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature. Note 5: The on threshold indicates that the LM3557 is no longer switching or regulating LED current, while the off threshold indicates normal operation. Note 6: All voltages are with respect to the potential at the GND pin. Note 7: Junction-to-ambient thermal resistance ( θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC standard JESD51-7. The test board is a 4 layer FR-4 board measuring 102 mm x 76 mm x 1.6 mm witha2x1 array of thermal vias. The ground plane on the board is 50 mm x 50 mm. Thickness of copper layers are 36 µm/18 µm/18 µm/36 µm (1.5 oz/1 oz/1 oz/1.5 oz). Ambient temperature in simulation is 22˚C, still air. Power dissipation is 1W. In applications where high maximum power dissipation exists, special care must be paid to thermal dissipation issues. For more information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Layout Guidelines section of this datasheet. Note 8: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Note 9: Feedback pin voltage is with respect to the voltage at the Sw2 pin. Note 10: The Power Switch Current Limit is tested in open loop configuration. For closed loop application current limit please see the Current Limit vs Temperature performance graph. Block Diagram Operation The LM3557 is a current-mode controlled constant- frequency step-up converter optimized for the facilitation of white LED driving/current biasing. The LM3557’s operation can be best understood by the following device functionality explanation. For the following device functionality explanation, the block diagram in Figure 2 serves as a functional schematic representation of the underlying circuit blocks that make up the LM3557. When the feedback voltage falls below, or rises above, the internal reference voltage, the error amplifier outputs a signal that is translated into the correct amount of stored energy within the inductor that is required to put the feedback voltage back into regulation when the stored inductor energy is then trans- ferred to the load. The aforementioned translation is a con- version of the error amplifier’s output signal to the proper on-time duration of the N1 power field effect transistor (FET). This conversion allows the inductor’s stored energy to in- crease, or decrease, to a sufficient level that when trans- ferred to the load will bring the feedback voltage back into regulation. 20131603 FIGURE 2. Block Diagram www.national.com 4 |
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