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LTM4676A Arkusz danych(PDF) 65 Page - Linear Technology |
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LTM4676A Arkusz danych(HTML) 65 Page - Linear Technology |
65 / 136 page LTM4676A 65 4676afa For more information www.linear.com/LTM4676A applicaTions inForMaTion It is recommended that all command writes (write byte, write word, etc.) be preceded with a polling loop to avoid the extra complexity of dealing with busy behavior and unwanted ALERT notification. A simple way to achieve this is by creating SAFE_WRITE_BYTE() and SAFE_WRITE_ WORD()subroutines.Theabovepollingmechanismallows one’s software to remain clean and simple while robustly communicating with the part. For a detailed discussion of these topics and other special cases please refer to the application note section located at www.linear.com/ designtools/app_notes. When communicating using bus speeds at or below 100kHz, the polling mechanism shown here provides a simplesolutionthatensuresrobustcommunicationwithout clock stretching. At bus speeds in excess of 100kHz, it is strongly recommended that the part be configured to en- able clock stretching. This requires a PMBus master that supports clock stretching. System software that detects and properly recovers from the standard PMBus NACK/ BUSY faults as described in the PMBus Specification v1.2, Part II, Section 10.8.7 is required to communicate above 100kHz without clock stretching. Clock stretching will not extend the PMBus speed beyond the specified 400kHz. THERMAL CONSIDERATIONS AND OUTPUT CURRENT DERATING The thermal resistances reported in the Pin Configuration section of this data sheet are consistent with those pa- rameters defined by JESD51-12 and are intended for use with finite element analysis (FEA) software modeling tools that leverage the outcome of thermal modeling, simula- tion, and correlation to hardware evaluation performed on a µModule package mounted to a hardware test board. The motivation for providing these thermal coefficients is found in JESD51-12 (“Guidelines for Reporting and Using Electronic Package Thermal Information”). Many designers may opt to use laboratory equipment and a test vehicle such as the demo board to predict the µModule regulator’s thermal performance in their appli- cation at various electrical and environmental operating conditions to compliment any FEA activities. Without FEA software, the thermal resistances reported in the Pin Con- figuration section are, in and of themselves, not relevant to providing guidance of thermal performance; instead, the derating curves provided in this data sheet can be used in a manner that yields insight and guidance pertaining to one’s application-usage, and can be adapted to correlate thermal performance to one’s own application. The Pin Configuration section gives four thermal coeffi- cients explicitly defined in JESD51-12; these coefficients are quoted or paraphrased below: 1 θJA, the thermal resistance from junction to ambi- ent, is the natural convection junction-to-ambient air thermal resistance measured in a one cubic foot sealed enclosure. This environment is sometimes referred to as “still air” although natural convection causes the air to move. This value is determined with the part mounted to a JESD51-9 defined test board, which does not reflect an actual application or viable operating condition. 2 θJCbottom, the thermal resistance from junction to the bottom of the product case, is determined with all of the component power dissipation flowing through the bottom of the package. In the typical µModule regulator, the bulk of the heat flows out the bottom of the package, but there is always heat flow out into the ambient environment. As a result, this thermal resistance value may be useful for comparing pack- ages but the test conditions don’t generally match the user’s application. 3 θJCtop, the thermal resistance from junction to top of the product case, is determined with nearly all of the component power dissipation flowing through the top of the package. As the electrical connections of the typical µModule regulator are on the bottom of the package, it is rare for an application to operate such that most of the heat flows from the junction to the top of the part. As in the case of θJCbottom, this value may be useful for comparing packages but the test conditionsdon’tgenerallymatchtheuser’sapplication. 4 θJB,thethermalresistancefromjunctiontotheprinted circuit board, is the junction-to-board thermal resis- tance where almost all of the heat flows through the bottom of the µModule regulator and into the board, and is really the sum of the θJCbottom and the thermal |
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