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## ADP3207 Datasheet(PDF) 21 Page - ON Semiconductor

 Part No. ADP3207 Description CPU Synchronous Buck Controller Download 29 Pages Scroll/Zoom 100% Maker ONSEMI [ON Semiconductor] Homepage http://www.onsemi.com Logo

## ADP3207 Datasheet(HTML) 21 Page - ON Semiconductor

 21 / 29 page ADP3207Rev. 1 | Page 21 of 29 | www.onsemi.comThe following procedure and equations yield values for RCS1, RCS2,and RTH (the thermistor value at 25°C) for a given RCS value:1.Select an NTC to be used based on type and value. Becausethere is no value yet, start with a thermistor with a valueclose to RCS. The NTC should also have an initial toleranceof better than 5%.2.Based on the type of NTC, find its relative resistance valueat two temperatures. Temperatures that work well are 50°Cand 90°C. These are called Resistance Value A (A isRTH(50°C)/RTH(25°C)) and Resistance Value B (B isRTH(90°C)/RTH(25°C)). Note that the relative value of NTCis always 1 at 25°C.3.Next, find the relative value of RCS that is required for eachof these temperatures. This is based on the percentage ofchange needed, which is initially 0.39%/°C. These arecalled r1 and r2.()()251125112211−×+=−×+=TTCrTTCr(9)where:TC = 0.0039T1 = 50°CT2 = 90°C.4.Compute the relative values forrCS1, rCS2, and rTH using()()()()()()()122121211221211111111111CSCSTHCSCSCSCSrrrrrArArBArABrBArABrBArrBAr−−=−−−−=−−×−×−×−××−×+×−×−××−=(10)5.CalculateRTH = RTH × RCS, then select the closest value ofthermistor that is available. Also, compute a scaling factork based on the ratio of the actual thermistor value relativeto the computed one)()(CALCULATEDTHACTUALTHRRk =(11)6.Finally, calculate values forRCS1 and RCS2 using()()()22111CSCSCSCSCSCSrkkRRrkRR×+−×=××=(12)This example starts with a thermistor value of 100 kΩ and usesa Vishay NTHS0603N04 NTC thermistor (a 0603 size thermistor)with A = 0.3359 and B = 0.0771. From this data, rCS1 = 0.359,rCS2 = 0.729 and rTH = 1.094. Solving for RTH yields 240 kΩ, so220 kΩ is chosen, making k = 0.914. Finally, RCS1 and RCS2 are72.3 kΩ and 166 kΩ. Choosing the closest 1% resistor valuesyields a choice of 71.5 kΩ and 165 kΩ.COUT SELECTIONThe required output decoupling for processors and platforms istypically recommended by Intel. The following guidelines canalso be used if both bulk and ceramic capacitors in the system:•Select the total amount of ceramic capacitance. This is basedon the number and type of capacitors to be used. The bestlocation for ceramics is inside the socket; 20 pieces ofSize 0805 being the physical limit. Additional capacitorscan be placed along the outer edge of the socket.•Select the number of ceramics and find the total ceramiccapacitance (CZ). Combined ceramic values of 200 μF to300 μF are recommended and are usually made up ofmultiple 10 μF or 22 μF capacitors.•Note that there is an upper limit imposed on the totalamount of bulk capacitance (CX) when considering theVID on-the-fly output voltage stepping (voltage step VV intime tV with error of VERR), and also a lower limit based onmeeting the critical capacitance for load release at a givenmaximum load step ΔIO. For a step-off load current, thecurrent version of the IMVP-6 specification allows amaximum VCORE overshoot (VOSMAX) of 10 mV, plus 1.5% ofthe VID voltage. For example, if the VID is 1.150 V, thenthe largest overshoot allowed is 27 mV.()⎟⎟⎟⎟⎟⎠⎞⎜⎜⎜⎜⎜⎝⎛−×⎟⎟⎠⎞⎜⎜⎝⎛Δ+×Δ×≥zVIDOOSMAXOOMINxCVIVRnILC(13)zOVVIDvVIDV2O2MAXXCLnKRVVtVVRnKLC−⎟⎟⎟⎠⎞⎜⎜⎜⎝⎛−⎟⎟⎠⎞⎜⎜⎝⎛×+××≤112)((14)where:⎟⎟⎠⎞⎜⎜⎝⎛−=VERRVVnK1(15)To meet the conditions of these equations and transientresponse, the ESR of the bulk capacitor bank (RX) should be lessthan two times the droop resistance, RO. If the CX(MIN) is largerthan CX(MAX), the system does not meet the VID on-the-flyand/or deeper sleep exit specification and can require a smallerinductor or more phases (the switching frequency can also haveto be increased to keep the output ripple the same).