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Fairchild RC5040 manual

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Table of contents for the manual

  • Page 1

    Application Note 42 Implementing the RC5040 and RC5042 DC-DC Conver ters on Pentium ® Pr o Motherboar ds www .fair childsemi.com Intr oduction This document describes how to implement a switching v olt- age regulator using an RC5040 or an RC5042 high speed controller , a po wer inductor , a Schottky diode, appropriate capacitors, and external po w[...]

  • Page 2

    AN42 APPLICATION NOTE 2 T able 2. Intel P entium Pro and OverDrive Processor P ower Specifications Notes: 1. Maximum power values are measured at typical V CC P to take into account the thermal time constant of the CPU package. 2. Flexible motherboard specifications are recommendations only. Actual specifications are subject to change. CPU Model &[...]

  • Page 3

    APPLICATION NOTE AN42 3 RC5040 and RC5042 Description Simple Step-Down Con ver ter Figure 1. Simple Buck DC-DC Converter Figure 1 illustrates a step-down DC-DC con verter with no feedback control. The basic step-do wn conv erter serves as the basis for deriving the design equations for the RC5040 and RC5042. From Figure 1, the basic operation begin[...]

  • Page 4

    AN42 APPLICATION NOTE 4 High Current Output Drivers The RC5040 contains two identical high current output driv ers that use high speed bipolar transistors in a push-pull confi guration. Each driv er is capable of delivering 1A of cur - rent in less than 100ns. Each driv er’ s power and ground are separated from the chip power and ground for addi[...]

  • Page 5

    APPLICATION NOTE AN42 5 Over-V oltage Protection The RC5040 and RC5042 constantly monitor the output voltage for protection against o ver voltage. If the v oltage at the VFB pin exceeds 20% of the selected program voltage, an ov er-v oltage condition is assumed, and the controller dis- ables the output driv e signal to the external MOSFET(s). Short[...]

  • Page 6

    AN42 APPLICATION NOTE 6 Figure 4. Synchronous DC-DC Converter Application Schematic Using RC5040 Figure 5. Non-Synchronous DC-DC Converter Application Schematic Using RC5042 VO GND VID3 VID2 VID1 VID0 8 7 6 5 4 3 2 1 9 10 11 12 13 14 15 16 17 18 19 20 VCC PWRGD RC5040 VCC VCC OUTEN R5 10K C10 0.1 µ F C11 0.22 µ F R6 10K R4 10K 10K 10K 10K R3 R2 R[...]

  • Page 7

    APPLICATION NOTE AN42 7 MOSFET Selection This application requires the use of N-channel, Logic Lev el Enhancement Mode Field Effect Transistors. The desired characteristics of these components are: • Low Static Drain-Source On-Resistance R DS,ON < 37 m Ω (lo wer is better) • Low g ate driv e voltage, V GS ≤ 4.5V • Power package with lo[...]

  • Page 8

    AN42 APPLICATION NOTE 8 MOSFET Gate Bias The MOSFET(s) can be biased using one of two methods: Charge Pump or 12V Gate Bias. Charge Pump (or Bootstrap) Figure 6 employs a char ge pump to provide the MOSFET gate bias. The char ge pump capacitor, CP , is used as a flying capacitor to boost the voltage of the RC5040 or RC5042 out- put driv er . When [...]

  • Page 9

    APPLICATION NOTE AN42 9 Con ver ter Efficiency Losses due to parasitic resistance in the switches, inductor , and sense resistor dominate at high load-current lev els. The major loss mechanisms under heavy loads, in order of importance, are: • MOSFET I 2 R Losses • Inductor Losses • Sense Resistor Losses • Gate-Charge Losses • Diode-Cond[...]

  • Page 10

    AN42 APPLICATION NOTE 10 Selecting the Inductor Selecting the right inductor component is critical in the DC-DC con verter application. The inductor’ s critical param- eters to consider are inductance (L), maximum DC current (I O ), and coil resistance (R l ). The inductor core material is crucial in determining the amount of current it can withs[...]

  • Page 11

    APPLICATION NOTE AN42 11 where: • V IN = Input V oltage to the Con verter • V SW = V oltage Across the MOSFET = I LOAD x R DS,ON • V D = Forw ard V oltage of the Schottky Diode • T = The Switching Period of the Con verter = 1/f S , Where f S = Switching Frequency . For an input v oltage of 5V , an output voltage of 3.3V , an inductor value [...]

  • Page 12

    AN42 APPLICATION NOTE 12 T able 8. R sense for v arious load currents Discrete Sense Resistor Discrete iron alloy resistors come in a v ariety of tolerances and power ratings, and are ideal for precision implementa- tions. Either an MnCu alloy wire resistor or an CuNi alloy wire resistor is ideal for a low cost implementation. Embedded Sense Resist[...]

  • Page 13

    APPLICATION NOTE AN42 13 currents to change. Therefore, combining an embedded resistor with a discrete resistor may be a desirable option. This section discusses a design that provides fle xibility and addresses wide tolerances. Refer to Figure 12. In this design, the user has the option to choose either an embedded or a discrete MnCu sense resist[...]

  • Page 14

    AN42 APPLICATION NOTE 14 Figure 14A. V CCQP Output Waveform for Normal Operation Condition with V out = 3.3V@10A Figure 14B. V CCQP Output Waveform for Output Shorted to Ground The Schottky diode has a po wer dissipation consideration during the short circuit condition. During normal operation, the diode dissipates power when the po wer MOSFET is o[...]

  • Page 15

    APPLICATION NOTE AN42 15 T able 10. Schottky Diode Selection T able Output Filter Capacitors Output ripple performance and transient response are functions of the filter capacitors. Since the 5V supply of a PC motherboard may be located sev eral inches away from the DC-DC con verter , the input capacitance can play an impor- tant role in the load [...]

  • Page 16

    AN42 APPLICATION NOTE 16 Refer to Appendix A for Directory of component suppliers. Notes : 1. In synchronous mode using the RC5040, a 1A schottky diode (1N5817) may be substituted for the MBR1545CT. 2. MOSFET M3 is only required for the RC5040 synchronous application. C13, C14, C15 Sanyo 6MV1500GX 1500 µ F 6.3V electrolytic capacitor 10mm x 20mm E[...]

  • Page 17

    APPLICATION NOTE AN42 17 PC Motherboard La yout and Gerber File A reference design for motherboard implementation of the RC5040 and RC5042 along with the Layout Gerber File and Silk Screen are presented below . The actual PCAD Gerber File can be obtained from Raytheon Electronics Semicon- ductor Division’ s Marketing Department at (415) 966-7819.[...]

  • Page 18

    AN42 APPLICATION NOTE 18[...]

  • Page 19

    APPLICATION NOTE AN42 19 Guidelines f or Debug ging and P erformance Ev aluations Debug ging Y our First Design Implementation Use the following procedure to help you deb ug your design implementation: 1. Note the VID pins settings. The y tell you what voltage is to be expected. 2. Do not connect any load to the circuit. While monitoring the output[...]

  • Page 20

    AN42 APPLICATION NOTE 20 4. Premature shut down can be caused by an inappropriate value of sense resistor . See the Sense Resistor section. 5. A poor load regulation can hav e many causes. Y ou should first check the voltages and signals at the critical pins. 6. The VREF pin should be at the v oltage set by the VID pins. If the power supply pins a[...]

  • Page 21

    APPLICATION NOTE AN42 21 Note: Load regulation is expected to be typically around 0.8%. The load regulation performance for this device under evaluation is excellent. Output V oltag e Load T ransients Due to Load Current Step This test is performed using Intel P6.0/P6S/P6T V oltage T ransient T ester . Note: Excellent transient voltage response. Tr[...]

  • Page 22

    AN42 APPLICATION NOTE 22 Component Case T emperature Note: Case temperatures are all within guidelines. Our guideline is that case temperatures for all components should be below 105 ° C @25 ° C Ambient. Comments: Excellent input ripple voltage. Input ripple voltage is recommended to be less than 5% of the output voltage. Device Description Case [...]

  • Page 23

    APPLICATION NOTE AN42 23 Attachment C Attachment D Attachment E Attachment F Attachment G Summary This application note cov ers for implementation of a DC-DC con verter on a Pentium Pro motherboard using the RC5040 and RC5042. The detailed discussion includes Pentium Pro processor power requirements, RC5040 and RC5042 description, design considerat[...]

  • Page 24

    AN42 APPLICATION NOTE 2/98 0.0m Stock#AN30000042  1998 Fairchild Semiconductor Corporation LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support de[...]