AN908

® AN908 APPLICATION NOTE HIGH PERFORMANCE VRM USING L4990A, FOR Pentium Pro® PROCESSOR by Nicola Tricomi SYSTEM DESCRIPTION The actual generation of Pentium Pro® microprocessors, requires a power supply with a programmable output voltage that decreases with the increasing of the processor internal clock. At the same time higher output currents are required and superior load transient performance has to be supplied due to very high dI/dt requests. The VRM (Voltage Regulator Module) proposed (Fig 1) is a step down converter based on the L4990A, very fast current mode controller. A complete system solution is given and described. A connector with a standard VRM pinout is used, with 4 bit for Voltage Identify system, Power Good, Enable and constant limiting current functions are available. Output Over Voltage Protection is also available. ELECTRICAL SPECIFICATION Input Characteristics Symbol Vi di/dt Vbias Parameter Input Voltage Slew Rate Bias Voltage Load Transient I12 = 45mA 11.4 12 Test Condition Min. 4.75 Typ. 5 Max. 5.25 0.04 14 Unit V A/µs V Output Characteristics Symbol Parameter Output Voltage range Vonom Io Nominal Output Voltage Output Current Peak Current Regulation Pulse by pulse mode Including variation due to: - Io = min-max - Vo imitial set point accuracy - Line regulation - Output ripple and noise - Load transient dIo/dt = 30A/µs - Temperature Vo = min-max; Io = min-max Vo = min-max Short circuit condition 70 85 1.18 ⋅ VO 12 300 A kHz 0.4 13 ±5 Test Condition By 4 bits VID Min. 2.1 2.9 12.5 Typ. Max. 3.5 Unit V V A A % η Efficiency Output Over Voltage Protection Foldback Current 93 % fsw Switching frequency July 1999 1/9 2/9 R2 5V ±5% Vi L3 L2 50T D2 D1 STPS 20L25CT R3 10K VCC 8 10 OUT R6 510 9 14 15 VC DIS DC_LIM Q1 STP80N03 L1 2.6µH Vo=2.1V-3.5V Io=0.3A-12.5A C5-8 330µF-6.3V SANYO-OSCON C18 C1-4 330µF-25V LXF-NCC R8 4.7 VCC 12V C13 47µF C20 220nF SYNC 1 VREF AN908 APPLICATION NOTE 4 L4990A IC1 13 6 7 COMP SS VFB C11 150pF R5 27K R9 1K(0.1%) R27 C12 220pF R7 3.3 RR1 11 5 PGND ISEN R1 6.8K VREF Figure 1. Voltage Regulator Module. RCT 2 12 C19 1µF C10 680pF SGND 10K R14 4k9 9(0.1%) Q2 BC337 R4 4.7K R23 R24 R25 3k09 6k19 12k4 (0.1%) (0.1%) R26 24k9 A R10 20K R16 10K + LM339/A IC2 D3 R22 3K Q3 BC327 R21a 3K PWRG EN + R17 6.2K R18 R21 10K - C21 220nF D0 B D 6.2K C R10a 2K C16 1µF R19 1.5K D4 R13 2K + B M74HC125 IC3 D1 R11 2K C14 5.6nF - C D5 Vref R15 2K R17 6.2K C17 + A - D2 R12 22K D C22 220nF C15 5.6nF D97IN516B D3 AN908 APPLICATION NOTE Supply Voltages The VRM is supplied by the 5V and the 12V standard outputs available from the conventional PC power supplies. The 12V is used here to supply the L4990A and the 5V power bus is used to supply the output power on the regulated DC output. An input inductor, L3, is necessary to limit the dI/dt of the input current during load transients conditions. This value must not exceed 0.1 A/us to prevent the intervention of protections on the main power supply. Control Inputs Enable: The module accept an open collector signal that disable the power supply in the low state. This function is realized by discharging the soft start capacitor. This solution allows the module to restart with the soft start sequence. Voltage Identification: The module is able to accept 4 open collectors input signals coming from the microprocessor. Table 1 shows the VID0-3 code. The voltage identification is implemented with R23-26, R9 and R14 resistors, to allow to programm the converter output voltage at a value lower than the L4990A internal feedback reference, of 2.5V. The four voltage identification bit are buffered with a 3-State buffer, 74HCT125. Table 1. VID3 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 VID2 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 VID1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 VID0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 Vo no def 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 Output Control Power Good Output: A PWRG signal is available. It’s in the Low State when the programmed output voltage is inside the +/-10% of the nominal voltage, and in the High state outside the mentioned voltage range. This function is realized by using two comparators inside the quad LM339. Internal Controls Current limiting: The current limiting intervention is fixed at 13A to guarantee a good functionality at 12A continuous output current. Behind 13A, the controller is going into pulse-bypulse constant limiting current. A foldback type limiting current is also available. In case a hiccup limiting current is accepted, the L4990A controller has this function on board. Over Voltage Protection: a circuit is detecting an Overvoltage condition when output voltage is 18% above the nominal output voltage. In this case, the VRM is disabled. Connector description The connector is standard 40 pin, with a current capability of 3A/pin (type AMPMOD2 part number 532956-7 or equivalent) and the pinout is shown in Tab2. Table 2. Connector pinout. Pin # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ROW A Vin (5V) Vin (5V) Vin (5V) Vin (12V) Reserved Reserved VID0 VID2 Reserved Vo GND Vo GND Vo GND Vo GND Vo GND Vo ROW B Vin (5V) Vin (5V) Vin (5V) Vin (12V) Reserved OUTEN VID1 VID3 PWRGD GND Vo GND Vo GND Vo GND Vo GND Vo GND 0 = Processor pin connected to ground 1 = Processor pin Open 3/9 AN908 APPLICATION NOTE Figure 2. Efficiency vs. Output Current η% 94 92 90 88 86 84 82 80 78 0 2 4 6 8 10 12 14 Output Current (A) Vo=3.3V Figure 3. Input Current CH1: Iinput (5V) (1A/div) CH2: Vo = 2.9V (50mV/div) Time: 10µs/div Vo=2.9V dIi/dt = 0.04A/µs Figure 4. Load Transient Io = 0.5A to 12A CH1: Vo = 2.9V (50mV/div) CH2: Io (5A/div) Time: 5µs/div Figure 5. Load Transient Io = 12A to 0.5A CH1: Vo = 2.9V (50mV/div) CH2: Io (5A/div) Time: 5µs/div Vo = 2.9V Vo = 2.9V dIo/dt = 30A/µs dIo/dt = 30A/µs 4/9 AN908 APPLICATION NOTE COMPONENT LIST R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R10a R11 R12 R13 R14 R15 R16 R17 R18 6.8k 1.8 10k 4.7 27k 510 3.3 4.7 1k 20k 2k 2k 22k 2k 4.99k (0.1%) 2k 10k 6.2k 6.2k R19 R20 R21 R21a R22 R23 R24 R25 R26 R27 RR1 C1-4 C5-8 C9 C10 C11 C12 C13 C14-15 3.6k 2.7k 10k 3k 3k 3.09k (0.1%) 6.19k (0.1%) 12.4k 24.9k xx 10k (5+1) 470µF/25V HFQ Panasonic 330µF/6.3V SANYO-Oscon 33nF 680pF 150pF 220pF 47 µF/50V 5.6nF C16 C17 C18 C19 C20-22 Q1 Q2 Q3 D1 D2-5 IC1 IC2 IC3 L1 L2 L3 1 µF/50V 220nF 1.2nF 1 µF/50V 220nF STP80N03L-06 BC337 BC327 STPS20L25CT 1N4148 L4990A LM339 M74HC125 77120(6T) Magnetics 40705-TC(50T) Magnetics T38-52(4T) Micrometals Figure 6. PCB Layout - Xerigraphy (77.47 x 78.74mm) 5/9 AN908 APPLICATION NOTE Figure 7. PCB Layout - Layer 1 (77.47 x 78.74mm) Figure 8. PCB Layout - Layer 2 (77.47 x 78.74mm) 6/9 AN908 APPLICATION NOTE Figure 9. PCB Layout - Layer 3 (77.47 x 78.74mm) Figure 10. PCB Layout - Layer 4 (77.47 x 78.74mm) 7/9 AN908 APPLICATION NOTE APPLICATION IDEAS Figure 11. Voltage Mode Regulator. OUT VCC 12V C13 47 ∝F VCC R13 4.7 VC SYNC VREF 4 R1 6.8K C8 1∝F C10 680pF RCT SS C9 220nF 2 7 12 SGND 13 ISEN 15 DC LIM 14 5 VFB PGND 11 DIS 9 6 1 8 10 DC 3 Q1 STP60N03L R8 4.7 R2 10K R5 27K VI 5V±5% C1-4 220∝F-10V SANYO-OSCON COMP L1 2∝H R9 820 R10 2.4K VO=3.3V IOmax=12.5A C5-7 220∝F-10V SANYO-OSCON C11 D1 STPS10L2S D99IN1068A Figure 12. Simplified Schematic. VCC C13 47µF C20 220nF SYNC 1 VREF R1 6.8K C19 1µF C10 680pF SGND 4 8 V CC 9 VC DIS 14 DC-LIM 15 OUT R8 4.7 Vi 5V±5% Q1 STP80N03 L3 L2 50T D2 L1 2.6µH D1 STPS202S R3 10K V O=2V-3.5V IO=0.3A-12.5V C5-8 330µF-5.3V SANYO OSCON C1-4 330µF-25V LXF-NCC L4990A RCT IC1 2 12 PGND 11 VFB 5 COMP C11 150pF 6 SS 7 ISEN R6 510 C12 220pF R7 3.3 R14 R5 27K R23 R4 4.7K R24 R25 R9 1K R26 D0 D1 Q2 BC337 R10 10K R10a 10K R11 2K C21 220nF R20 + - R16 10K B C16 1 µF A R17 6.2K PWRG R18 6.2K + - D R20 6.2K R21 10K + C D2 D3 R19 1.5K D4 LM339/A IC2 D3 + R12 22K R22 3K D5 C17 EN R21a 3K Q3 BC327 D99IN1069 8/9 AN908 APPLICATION NOTE I nformation furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specification mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics © 1999 STMicroelectronics – Printed in Italy – All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta - Morocco Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A. http://www.st.com 9/9