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MIC2590B2YTQ Datasheet(PDF) 21 Page  Micrel Semiconductor 

MIC2590B2YTQ Datasheet(HTML) 21 Page  Micrel Semiconductor 
21 / 23 page Micrel, Inc. MIC2590B September 2008 21 M9999091808 Using this graph is not nearly as daunting as it may at first appear. Taking the simplest case first, we’ll assume that once a fault event such as the one in question occurs, it will be along time, 10 minutes or more, before the fault is isolated and the slot is reset. In such a case, we can approximate this as a “single pulse” event, that is to say, there’s no significant duty cycle. Then, reading up from the Xaxis at the point where “Square Wave Pulse Duration” is equal to 0.1sec (=100ms), we see that the effective thermal impedance of this MOSFET to a single pulse event of this duration is only 6% of its continuous Rθ(JA). This particular part is specified as having an Rθ(JA) of 50°C/W for intervals of 10 seconds or less. So, some further math, just to get things ready for the finale: Assume TA = 55°C maximum, 1 square inch of copper at the drain leads, no airflow. Assume the MOSFET has been carrying just about 5A for some time. Then the starting (steadystate)TJ is: TJ ≅ 55°C + (7.3mΩ)(5A) 2(30°C/W) TJ ≅ 60.5°C Iterate the calculation once to see if this value is within a few percent of the expected final value. For this iteration we will start with TJ equal to the already calculated value of 67°C: RON at TJ = 60.5°C =[1+(60.5°C–25°)(0.5%/°C)]×6.35mΩ RON at TJ = 60.5°C ≅ 7.48mΩ TJ ≅ 55°C + (7.3mΩ)(5A) 2(30°C/W) TJ ≅ 60.6°C At this point, the simplest thing to do is to approximate TJ as 61°C, which will be close enough for all practical purposes. Finally, add (10W)(67°C/W)(0.03) = 21°C to the steady state TJ to get TJ(TRANSIENT MAX) = 82°C. The Si4430DY can easily handle this value of TJ(MAX). A second illustration of the use of the transient thermal impedance curves: assume that the system will attempt multiple retries on a slot showing a fault, with a one second interval between retry attempts. This frequency of restarts will significantly increase the dissipation in the Si4430DY MOSFET. Will the MOSFET be able to handle the increased dissipation? We get the following: The same part is operating into a persistent fault, so it is cycling in a squarewave fashion (no steadystate load) with a duty cycle of (50msec/second = 0.05). On the Transient Thermal Impedance Curves, read up from the Xaxis to the line showing Duty Cycle equaling 0.05. The effective Rθ(JA) = (0.7 x 67°C/W) = 4.7°C/W. Calculating the peak junction temperature: TJ(PEAK MAX) = [(10W)(4.7°C/W) + 55°C] = 102°C And finally, checking the RMS power dissipation just to be complete: ( ) ( ) 0.042W 0.05 7.47mΩ 5A P 2 RMS = = which will result in a negligible temperature rise. The Si4430DY is electrically and thermally suitable for this application. MOSFET and Sense Resistor Selection Guide Listed below, by Manufacturer and Type Number, are some of the more popular MOSFET and resistor types used in PCI hot plug applications. Although far from comprehensive, this information will constitute a good starting point for most designs. MOSFET Vendors Key MOSFET Type(s) Web Address Vishay (Siliconix) Si4430DY (“LittleFoot” Series) Si4420DY (“LittleFoot” Series) www.siliconix.com International Rectifier IRF7413A (SO8 package part) Si4420DY (second source to Vishay) www.irf.com Fairchild Semiconductor FDS6644 (SO8 package part) FDS6670A (SO8 package part) FDS6688 (SO8 package part) www.fairchildsemi.com Resistor Vendors Sense Resistors Web Address Vishay (Dale) “WSL” Series www.vishay.com/docs/wsl_30100.pdf IRC “OARS” Series “LR” Series (second source to “WSL”) irctt.com/pdf_files/OARS.pdf irctt.com/pdf_files/LRC.pdf 
