Gebruiksaanwijzing /service van het product RC5040 van de fabrikant Fairchild
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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 o.
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.
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.
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.
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).
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.
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.
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 .
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.
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 ).
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 .
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.
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.
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.
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.
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.
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.
AN42 APPLICATION NOTE 18.
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.
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.
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.
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.
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.
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.
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