an97-1 ? 1998 semtech corp. 652 mitchell road newbury park ca 91320 POWERING intel's pentium ii microprocessor march 11, 1998 pentium ii processor is a trademark of intel corp. 1 introduction modern microprocessors demand more and more stringent requirements from their power sources. de- creasing working voltages, increasing current draw and faster changes in load demand all combine to make the power supply designer?s life more interest- ing. these requirements also mandate the use of local regulation or voltage conversion as close as possible to the microprocessor itself. intel has defined the power requirements for their pentium ii microprocessor and semtech has designed the sc1151cs single chip pwm controller to meet those requirements. semtech has also designed the mp60-f voltage regulator module (vrm) both to demonstrate the sc1151cs and also to serve as a plug-in vrm solution for those not wishing to develop an on-board solution. this application note describes the sc1151cs, it?s design philosophy and practical use in a pentium ii power supply application. design criteria output filter components probably the most striking design criterium for a power system to meet pentium ii requirements is the need to maintain regulated output voltage during very fast load transients. it is impossible for even the fastest pwm controller to respond to the required load transient in the time available. this means that short term transient load requirements will have to be met using the output capacitors of the power system. un- fortunately all practical capacitors have parasitic char- acteristics, the most important of which for our pur- poses are a) equivalent series resistance (esr) b) equivalent series inductance (esl) the output capacitor equivalent circuit is shown in fig.1. the combined effect of all these parasitics dur- ing a fast increase in load current is shown in fig.2 the esr of the capacitor causes a downward volt- age step (together with a short spike due to esl). the output circuit of the converter must be designed so that the additional output voltage sag caused by dis- charging output capacitors is offset by the reducing esr drop due to less current demanded from the out- put capacitors. in simple terms we are looking for low esr, high capacitance and low inductance in the con- verter output circuit. the choice of output capacitors therefore be- comes very important. the first criteria is that esr should be less than 10m w to keep the leading edge transient to less than 120mv for a 12a load tran- sient. the choice of cost and technology leads to three approaches. a) multiple aluminum electrolytic capacitors - typi- cally to meet esr requirements 7x1000f will be re- quired. b) multiple os-con type capacitors - this ap- proach typically results in 4x330f but at approxi- mately 5x the cost of a). c) multiple tantalum chip capacitors - typically 10 to 20 giving a total output capacitance of 1000 to 2000f and an even higher cost solution than b). fig.1 output capacitor with parasitics c esl esr fig.2 output voltage during load transient io vo esr esl recovery of properly designed circuit recovery of improperly designed circuit inductor current POWERING intel's pentium ii microprocessor using the sc1151cs and the mp60-f vrm tel:805-498-2111 fax:805-498-3804 web:http://www.semtech.com
an97-1 ? 1998 semtech corp. 652 mitchell road newbury park ca 91320 POWERING intel's pentium ii microprocessor march 11, 1998 pentium ii processor is a trademark of intel corp. 2 for a cost effective solution the only real option is to use multiple low esr aluminum electrolytics, with a total capacitance of approximately 7000f. the intel specified decoupling at the processor socket will fur- ther reduce the leading edge esr transient but is not included in results presented in this application note. the output inductor value on a ?buck? converter is set from a consideration of the allowable peak to peak current ripple. this in turn will determine output volt- age ripple and the output current below which the op- erating mode will be discontinuous. a high value of output inductor will result in low ripple and the capabil- ity of achieving low output current in continuous mode, but will suffer from poor transient response. an optimum value is 4h for the semtech applica- tion circuit. semtech does not recommend changing this value by more than 25% without careful evalua- tion in the intended application. the specified inductor may be wound with 9 turns of 16awg wire on a mi- crometals t60-52 core. choice of fets there are many suitable choices of fets from many different manufacturers. the requirements are, as a minimum, logic level drive, 22m w rds(on), vdss 3 30v and a package consistent with the power dissi- pation and heatsinking requirements. for example, if a 22m w fet is chosen it should be in to-220 packag- ing with a 10 o c/w (or better) heatsink used, if a 10m w fet is chosen it could be in a to-263 package, sol- dered down to the pcb with sufficient copper area to alleviate the need for additional heatsinking. choice of schottky again the schottky diode choice is governed by the trade off between cost and performance. as a mini- mum, vr 3 30v and vf 0.5v @ if = 14a at tj = 100 o c. better vf characteristics will improve efficiency and reduce power dissipation in the schottky, espe- cially at the lower output voltages. packaging should be to-220 and a heatsink of 10 o c/w or better is re- quired. in applications at output voltages in the high range around 2.8v, it is possible to use a to-220 or to-263 packaged schottky soldered down to about 1 sq. in of copper. in these applications, the lowest vf schottky should be used and performance evaluated in the ap- plication. thermal impedance will be approximately 30 o c/w . sc1151 the sc1151cs contains the control circuitry to im- plement a pentium ii power solution. it is a simple, voltage mode, non synchronous pwm controller with integrated 5 bit d/a converter, and power monitoring circuitry. the operating frequency is fixed at 100khz. a higher operating frequency would normally allow smaller filter components however in this application, the output capacitors are fixed by esr requirements. using a lower value inductor at higher operating fre- quencies would require a change of core material and much increased cost or, using the same core material, would have minimal effect on the required core size. the standard buck topology was chosen to minimize the implementation costs. mp60-f the mp60-f is a self contained, plug-in voltage regulator module (vrm) designed to power the intel pentium ii microprocessor. it uses the semtech sc1151cs. the circuit diagram is shown in fig.3 and board layout in figs. 4 through 7. performance data in this application note relates to this board. pcb layout guidelines good analog layout techniques are important to the successful implementation of a pentium ii power solu- tion using the sc1151cs. a multilayer board will greatly ease the layout task, although it is perfectly feasible to produce a working system with only a dou- ble sided pcb. the mp60-f is a perfect example. a continuous ground plane should be used underneath the chip, and all ground pins should be directly con- nected to this plane. it is preferable, though not strictly necessary, that no large ground currents flow in the plane beneath the chip, they should be routed around it. the chip decoupling capacitor (c13 in fig.3) should be as close to the chip as possible, with the ground side connecting to the ground plane close to pin 1 of the chip. c11 should also be mounted as close to the chip as possible, as should r3 and r4 and to a lesser extent r1. the current sense traces from r1 should be as short as reasonably possible and should run close and parallel to each other. the fet drive con- nection (pin 8 to fet gate) should be kept short. high current paths should be kept wide and short, see the mp60-f layout (fig.4 and fig.5) for a guide.
an97-1 ? 1998 semtech corp. 652 mitchell road newbury park ca 91320 POWERING intel's pentium ii microprocessor march 11, 1998 pentium ii processor is a trademark of intel corp. 3 outen vss vid0 5vin 5vin vid3 vid2 vid1 vccvid pwrgd up# vid4 c12 0.1uf c1 0.1uf c5 1000uf + c6 1000uf + c7 1000uf + c8 1000uf + q1 buk556 c4 1000uf + c3 1000uf + c2 1000uf + l1 4uh j1 ampmod2_40 a1 a2 a3 a4 a5 a6 a7 a8 a9 a10 a11 a12 a13 a14 a15 a16 a17 a18 a19 a20 b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12 b13 b14 b15 b16 b17 b18 b19 b20 r2 10k r3 2.32k r4 1.00k c9 1000uf + c10 1000uf + r1 5mohm d1 psr16c30ct u1 sc1151cs gnd 1 vcc 2 ovp 3 pwrgood 4 csm 5 csp 6 pgnd 7 dh 8 bst 9 shutdown 10 vosense 11 vid4 12 vid3 13 vid2 14 vid1 15 vid0 16 r5 10 c13 0.1uf c11 0.1uf circuit diagram fig.3 application circuit schematic
an97-1 ? 1998 semtech corp. 652 mitchell road newbury park ca 91320 POWERING intel's pentium ii microprocessor march 11, 1998 pentium ii processor is a trademark of intel corp. 4 fig.5 bottom copper layout fig.4 top copper layout fig.7 bottom component layout fig.6 top component layout bill of materials qty reference part/description vendor notes 4 c1,c11,c12,c13 0.1f ceramic various 9 c2,c3,c4,c5,c6, c7,c8,c9,c10 1000f/6.3v sanyo 6.3mv1000gx or equiv. low esr 1 d1 psr16c30ct photron or lower vf device from various vendors 1 l1 4h 9 turns 16awg on micrometals t60-52 core 1 q1 buk556 philips logic level fet, 22m w , 30v 1 r1 5m w irc oar-1 series 1 r2 10k w , 5%, 1/8w various 1 r3 2.32k w , 1%, 1/8w various 1 r4 1.00k w , 1%, 1/8w various 1 r5 10 w , 5%, 1/8w various 1 u1 sc1151cs semtech 2 heatsinks various 10c/w
an97-1 ? 1998 semtech corp. 652 mitchell road newbury park ca 91320 POWERING intel's pentium ii microprocessor march 11, 1998 pentium ii processor is a trademark of intel corp. 5 performance typical performance characteristics are shown below -0.020 -0.010 0.000 0.010 0.020 0.030 0.040 0.050 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 voltage change (v) normalized at io=2a output current (amps) sc1151 regulation in application circuit 16935f application circuit 16935f ripple, 3.1v, 10a out 70.0% 75.0% 80.0% 85.0% 90.0% 95.0% 100.0% 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 3.50v set 3.00v set 2.40v set 2.00v set efficiency output current (amps) sc1151 efficiency in application circuit 16935f application circuit 16935f transient response, 3.1v, 0.3a to 10a vo at io=300ma steady state vo at io=10a steady state steady state voltage levels are superimposed for reference.
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