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Application Note 42
Implementing the RC5040 and RC5042
®
DC-DC Converters on Pentium Pro Motherboards
®
Introduction Pentium Pro and OverDrive
Processor Power Requirements
This document describes how to implement a switching volt-
age regulator using an RC5040 or an RC5042 high speed
Use Intel’s AP-523 Application Note, Pentium® Pro
controller, a power inductor, a Schottky diode, appropriate
Processor Power Distribution Guidelines, November 1995
capacitors, and exte
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AN42 APPLICATION NOTE Table 2. Intel Pentium Pro and OverDrive Processor Power Specifications Voltage Maximum Maximum Thermal 1 Specification Current Design Power CPU Model & Features V P (VDC) I P (A) (W) CC CC 150MHz – 256K L2 Cache 3.1 – 5% 9.9 29.2 166MHz – 512K L2 Cache 3.3 – 5% 11.2 35.0 180MHz – 256K L2 Cache 3.3 – 5% 10.1 31.7 200MHz – 256K L2 Cache 3.3 – 5% 11.2 35.0 200MHz – 512K L2 Cache 3.3 – 5% 12.4 37.9 OverDrive Processors 150 MHz 2.5 – 5% 11.2 26.7 180 MHz 12.5 29.7 200 MHz 13.9
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APPLICATION NOTE AN42 The RC5040 and RC5042 Controllers RC5040 and RC5042 Description The RC5040 is a programmable synchronous-mode DC-DC Simple Step-Down Converter converter controller. The RC5042 is a non-synchronous ver- sion of the RC5040. When designed with the appropriate S1 external components, either device can be configured to L1 + deliver more than 14.5A of output current. During heavy loading conditions, these controllers function as current- V mode PWM step-down regulators. Unde
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AN42 APPLICATION NOTE Main Control Loop RC5040 +5V OSCILLATOR VIN – + – + VO DIGITAL CONTROL 1.24V 4-BIT VREF REFERENCE DAC POWER PWRGD GOOD 65-5040-01 VID0 VID1 VID2 VID3 Figure 2. RC5040 Block Diagram High Current Output Drivers Power Good (PWRGD) The RC5040 contains two identical high current output The RC5040 and RC5042 Power Good function has been drivers that use high speed bipolar transistors in a push-pull designed according to Intel’s Pentium Pro DC-DC converter configuration. Each dri
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APPLICATION NOTE AN42 In general, a lower operating frequency increases the peak Over-Voltage Protection ripple current flowing through the output inductor, allowing The RC5040 and RC5042 constantly monitor the output the use of a larger inductor value. Operation at lower fre- voltage for protection against over voltage. If the voltage at quencies increases the amount of energy storage that the the VFB pin exceeds 20% of the selected program voltage, bulk output capacitors must provide du
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AN42 APPLICATION NOTE L2 VCC C4 2.6μH C1 C2 C5 C3 0.1μF 1000μF 0.1μF 1000μF 1000μF DS2 C9 C8 1N5817 0.1μF 0.1μF M1 M2 C12 2SK1388 11 10 12 9 1μF 2SK1388 L1 R 13 8 SENSE R7 VO 14 7 10K 1.3μH C6 8mΩ 15 6 RC5040 4.7μF 16 5 VREF 17 4 M3 C7 DS1 18 3 2SK1388 1N5817 19 2 0.1μF 20 1 GND C EXT 39pF R1 10K VID3 VCC 10K R2 R6 VID2 10K 10K R3 VID1 PWRGD 65-AP42-04 R4 10K C11 VCC VID0 0.22μF R5 OUTEN 10K C10 0.1μF Figure 4. Synchronous DC-DC Converter Application Schematic Using RC5040 L2 VCC 2.6μH C4 C3 C5
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APPLICATION NOTE AN42 MOSFET Selection • Power package with low thermal resistance • Drain current rating of 20A minimum This application requires the use of N-channel, Logic Level • Drain-Source voltage > 15V. Enhancement Mode Field Effect Transistors. The desired characteristics of these components are: The on-resistance (R ) is the main parameter for MOS- DS,ON FET selection. It determines the MOSFET’s power dissipa- • Low Static Drain-Source On-Resistance tion, thus significantly affectin
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AN42 APPLICATION NOTE 12V Gate Bias MOSFET Gate Bias Figure 7 illustrates how an external 12V source can be used The MOSFET(s) can be biased using one of two methods: to bias VCCQP. A 47 W resistor is used to limit the transient Charge Pump or 12V Gate Bias. current into the VCCQP pin, and a 1m F capacitor filter is used to filter the VCCQP supply. This method provides a Charge Pump (or Bootstrap) higher gate bias voltage (V ) to the MOSFET, and there- GS Figure 6 employs a charge pump to pr
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APPLICATION NOTE AN42 Converter Efficiency Losses due to parasitic resistance in the switches, inductor, • Sense Resistor Losses and sense resistor dominate at high load-current levels. The • Gate-Charge Losses major loss mechanisms under heavy loads, in order of • Diode-Conduction Losses importance, are: • Transition Losses • Input Capacitor Losses 2 • MOSFET I R Losses • Losses Due to the Operating Supply Current of the IC. • Inductor Losses Efficiency of the converter under heavy loads can be
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AN42 APPLICATION NOTE PD = 2.19W + 1.0W + 0.65W + 0.045W + 1.35W + 0.010W + 0.37W + 0.2W = 5.815W LOSS 3.3 · 10 --------------------------------------- \ Efficiency = » 85% 3.3 · 10 + 5.815 Table 6. RC5040 and RC5042 Short Circuit Comparator Selecting the Inductor Threshold Voltage Selecting the right inductor component is critical in the Short Circuit Comparator DC-DC converter application. The inductor’s critical param- V (mV) threshold eters to consider are inductance (L), maximum DC curr
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APPLICATION NOTE AN42 (I – I ) where: PK min I ‡ I = I + ----------------------------- = 14.5 + 1 = 15.5A SC inductor Load, max 2 • V = Input Voltage to the Converter IN For continuous operation at 14.5A, the short circuit detection • V = Voltage Across the MOSFET = I x R SW LOAD DS,ON threshold must be at least 15.5A. • V = Forward Voltage of the Schottky Diode D • T = The Switching Period of the Converter = 1/fS, The next step is to determine the value of the sense resistor. Where f =
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AN42 APPLICATION NOTE Table 8. R for various load currents Table 9 is a summary of tolerances for the Embedded PC sense Trace Resistor. R R SENSE SENSE I PC Trace Discrete Load,max Table 9. Summary PC Trace Resistor Tolerance (A) Resistor (mW ) Resistor (mW ) Tolerance due to sheet resistivity variation 16% 10.0 6.5 8.6 Tolerance due to L/W error 1% 11.2 5.8 7.8 Tolerance due to temperature variation 12% 12.4 5.3 7.1 Total Tolerance for PC Trace Resistor 29% 13.9 4.8 6.4 14.0 4.7 6.3 Design ru
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APPLICATION NOTE AN42 Embedded Sense Resistor IFBH MnCu Discrete Resistor R21 R22 IFBL Output Power Plane (Vout) R-Δr R R+Δr Figure 12. Short Circuit Sense Resistor Design Using PC Trace Resistor and Optional Discrete Sense Resistor currents to change. Therefore, combining an embedded The converter exhibits at normal load regulation until the resistor with a discrete resistor may be a desirable option. voltage across the resistor reaches the internal short circuit This section discusses a de
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AN42 APPLICATION NOTE P = I · V · (1 – DutyCycle) = D, Diode F, ave F 14 · 0.45 · 0.8 » 5W Thus for the Schottky diode, the thermal dissipation during a short circuit is greatly magnified and requires that the thermal dissipation of the diode be properly managed by the appropriate choice of a heat sink. In order to protect the Schottky from being destroyed in the event of a short, we should limit the junction temperature to less than 130 C. Using the equation for maximum junction temperat
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APPLICATION NOTE AN42 With this in mind, correct calculation of the output capaci- Table 10. Schottky Diode Selection Table tance is crucial to the performance of the DC-DC converter. Manufacturer Forward Voltage The output capacitor determines the overall loop stability, Model # Conditions V F output voltage ripple, and load transient response. The calcu- Philips I = 20A; T =25 C < 0.84v F j lation is as follows: PBYR1035 I = 20A; T =125 C < 0.72v F j I · D T O Motorola I = 20A; T =25 C
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AN42 APPLICATION NOTE Table 11. Bill of Materials for a 14.5A Pentium Pro Motherboard Application C13, C14, C15 Sanyo 1500m F 6.3V electrolytic ESR < 0.047 W 6MV1500GX capacitor 10mm x 20mm DS1 Motorola Shottky Diode Vf<0.72V @ If = 15A (note 1) MBR1545CT DS2 General Instruments 1N5817 Schottky Diode 1A, 20V L1 Skynet 320-8107 1.3m H inductor L2* Skynet 2.5m H inductor *Optional – will help re- 320-6110 duce ripple on 5v line M1, M2, M3 Fuji N-Channel Logic Level RDS(ON) < 37m ohm (note 2) 2SK1
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APPLICATION NOTE AN42 Good layout Bad layout 10 10 11 RC5040 11 RC5040 12 9 12 9 13 13 8 8 7 7 14 14 15 6 15 6 16 5 16 5 17 17 4 4 18 18 3 3 19 2 19 2 20 20 1 1 = “Quiet” Pins Figure 16. Example of Proper MOSFETs Placements File can be obtained from Raytheon Electronics Semicon- PC Motherboard Layout and Gerber File ductor Division’s Marketing Department at (415) 966-7819. A reference design for motherboard implementation of the RC5040 and RC5042 along with the Layout Gerber File and Silk S
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AN42 APPLICATION NOTE 18
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APPLICATION NOTE AN42 5. Apply load at 1A increments; an active load (HP6060B Guidelines for Debugging and or equivalent) is suggested. Performance Evaluations 6. In case of poor regulation, refer to the procedures in the Debugging Your First Design Implementation Troubleshooting section. Use the following procedure to help you debug your design implementation: Troubleshooting 1. If no voltage is registered at the output and the circuit is 1. Note the VID pins settings. They tell you wha
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AN42 APPLICATION NOTE Load Regulation 4. Premature shut down can be caused by an inappropriate value of sense resistor. See the Sense Resistor section. VID I (A) V (V) load out 5. A poor load regulation can have many causes. You 0100 0.5 3.0904 should first check the voltages and signals at the critical pins. 1.0 3.0825 2.0 3.0786 6. The VREF pin should be at the voltage set by the VID pins. If the power supply pins are correct and the VID 3.0 3.0730 pins are correct, the VREF should be at
