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| AN718 Vishay Siliconix The amount of output capacitance at the output of the converter also determines the transient response of a converter. The greater the output capacitance, the less converter bandwidth is required to meet the transient regulation. A 4-A transient response has been simulated using a SPICE program for various output capacitance characteristics (Table 1). A SPICE simulation with an 800-F output capacitor with 0.0125- ESR reveals that converter with a 15-s response time will meet the voltage regulation limit of the Pentium VRE processor (Figure 4). This translates into a converter with approximately 32-kHz unity gain bandwidth, assuming a second-order response system with a damping coefficient of 0.9. In reality, approximately 25% of the requisite regulation limit will be utilized by variations in the reference voltage and in the resistor dividers. To increase power supply manufacturing yields, it is also necessary to allow an additional tolerance in voltage regulation of 10% or more, further increasing the bandwidth requirement. If remote sensing is not available, delayed sensing of the processor voltage will cause a further drop. Together, these constraints reduce the regulation limit of VRE Pentium converter from 75 mV to approximately 45 mV. To meet the 45-mV regulation limit requires a converter unity gain bandwidth of approximately 100 kHz. Obtaining a closedloop bandwidth of 100 kHz requires a converter switching frequency of 375 kHz or greater. It also requires a ultra-fast error amplifier. Typically, the error amplifier must provide a bandwidth between 10 and 20 times the switching frequency to correctly respond to the stimuli. In the case of the Pentium converter, large bulk capacitors at the output of the converter and at the microprocessor require even greater bandwidth. FIGURE 3. Pentium Converter Requirements TABLE 1. Transient Response Output Capacitance (F) 800 700 600 500 400 ESR 0.0125 0.0143 0.0167 0.020 0.025 Response Time for 45-mV Regulation (s) 5 4 3 2 1 BW for 45-mV Regulation (kHz) 95 120 160 240 480 Response Time for 75-mV Regulation (s) 15 14 13 12 11 BW for 75-mV Regulation (kHz) 32 34 37 40 43 THE LINEAR REGULATION SOLUTION In the past, linear regulators were the ideal solution for lowpower, 5-V/3-V conversion. Linear regulators provided an inherently quiet power system, eliminating EMI/EMC problems. Stability and compensation issues were also minimal, making the application and analysis as simple as possible. The advantages of linear regulators remained significant until power demands increased. Under these new conditions, their disadvantages become obvious. With a 7-A output current, a VRE converter will dissipate over 10 W of power into an already blazing hot system, requiring cumbersome heat sinks. These increase manufacturing difficulties and labor costs, which could easily offset the price advantage of the linear regulator solution. Meanwhile, the physical dimension requirements, 2.60 x 1.81 x 0.8 inches, could be easily exceeded with a large heat sink. Additionally, the cost savings of a linear regulator solution would be transferred to the increased costs of a larger, noisier fan required to circulate the additional heat over the circuit. 2 FaxBack 408-970-5600, request 70588 www.siliconix.com AN718 Vishay Siliconix ranging from an obvious, flagged software/hardware error to insidious, subtle, and unpredictable computational errors. Current mode can also decrease noise immunity if the slope of the inductor current is too small to increase efficiency. By having the ramp voltage always close to the error voltage, a small noise injection into the current ramp could cause a large variation in the duty cycle. Current mode also requires a slope compensation for duty cycles greater than 50%, adding parts and cost. For 5-V to 3.525-V conversion, the duty cycle exceeds 70% without counting FET and parasitic losses. The transfer function for the voltage-mode control buck converter is stated below. V IN 1 Voc = -------- --------------------------------------------------------- Vs L 2 1 + S ------ + S L C o RL VIN = input voltage Vs = ramp voltage L = inductance FIGURE 4. Transient Response CURRENT MODE VS. VOLTAGE MODE The Pentium's exacting dynamic load requirements makes it crucial for the designers to choose an optimal control method to provide voltage regulation during the transients. There are two modes of control for a buck converter operating in fixed frequency: voltage mode or current mode. The transfer function of current mode control with the converter's loop gain from output to control voltage operating in continuous inductor current can be derived using the state space average model and stated below. RL 1 Voc = -------- ------------------------------------ R cs 1 + S C o R L RL = load resistance Rcs = current sense resistance Co = output capacitance In the voltage mode control converter, loop gain is a function of input voltage instead of load resistance. Load resistance affects the Q-ing of the converter. Since the input voltage of the Pentium converter is virtually fixed, loop gain is considered constant. Input voltage for the Pentium varies from 4.75 V to 5.25 V, less than 1-dB variation. With the loop gain constant, the converter's bandwidth can be maximized and that same wide bandwidth can be maintained, irrespective of load changes. Therefore, the voltage-mode controlled converter is an ideal control method for the Pentium converter. With virtually fixed input voltages and a wide load variation, bandwidth can be maximized and transient response times minimized. Voltage-mode control does have a minor disadvantage. As revealed in the above equation, its double-pole filter is generally more complicated to compensate than the singlepole filter of current-mode control. This means the addition of a pole-zero pair compensation network. Siliconix' application circuit solves this problem with optimal compensation values. As the above equation shows, current mode control has inherently good input line regulation since the transfer function is unaffected by the input voltage. As the input voltage changes, the slope of the inductor current changes instantaneously in compensation. Unfortunately, loop gain is load dependent. As the output load varies from minimum to maximum, as in the case of the Pentium, RL ranges from 0.5 to 17.6 , and the loop gain varies by approximately 31 dB. This could cause the power supply to oscillate, if the loop is not compensated correctly for all load conditions. Typically the power supply is compensated for the maximum load resistance and the design must somehow accommodate the loop bandwidth reduction during the minimum load resistance. With -1 slope, loop bandwidth can decrease by more than a 1.5 decades in frequency. This decrease in bandwidth could have catastrophic effects on the dynamic transient response of the Pentium. Once its output voltage regulation is exceeded, performance could also be impaired. Violation of output voltage regulations could cause the processor to exhibit failure characteristics TRANSIENT RESPONSE Maintaining the output voltage regulation within the 45-mV during the 4-A transient is not a trivial task for any power supply designer. Present solutions advertised by other PWM manufacturers using current-mode control lack bandwidth or dc gain to satisfy the voltage regulation of the VRE Pentium microprocessor. Without a voltage-mode PWM IC capable of switching at 375 kHz or above, designers have been forced to use current-mode controllers with much lower switching frequencies. Operating at a lower switching frequency generally yields slightly greater efficiency, but the disadvantages far outweigh the efficiency tradeoff. A lower operating frequency forces the designers to use much larger inductance and capacitance to maintain the same ripple FaxBack 408-970-5600, request 70588 www.siliconix.com 3 AN718 Vishay Siliconix voltage compared to the higher frequency operation, increasing space and cost. This increase in inductance also has a detrimental effect during the dynamic load transients. During the transition from maximum to minimum load, energy stored in the inductor makes a forced discharge into the output capacitance. Therefore, the larger the inductance, the more energy is stored in the inductor, causing a larger overshoot on the output voltage during the unloading transition. During the minimum-to-maximum current transition, a larger inductor delays the ramping of current demanded by the load, further sagging the output voltage. The advantages of the Si9145BY, with its higher switching frequency and wide bandwidth error amplifier, are clearly demonstrated by comparing the two converters outlined in Table 2. A complete VRE-specified converter schematic operating at 375 kHz is shown in Figure 5. TABLE 2. Converter Response Comparison Converter #1 Switching Frequency Closed Loop BW Output Capacitance Inductance 375 kHz 100 kHz 800 F 2.4 H Converter #2 125 kHz 33 kHz 2400 F 7.2 H 4 FaxBack 408-970-5600, request 70588 www.siliconix.com AN718 Vishay Siliconix Notes a. R24 and R25 are designed in the circuit to provide the flexibility to remote sense. Remove R24 if remote sensing is utilized. Remove R25 if remote sensing is not utilized. b. Remove R17 to float the PWRGOOD signal. The new version of the Intel specification requires the PWRGOOD signal to be an open-collector output or floating. FIGURE 5. Pentium Converter FaxBack 408-970-5600, request 70588 www.siliconix.com 5 AN718 Vishay Siliconix SYNCHRONOUS RECTIFICATION PIN DESCRIPTION Pin 1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B 7A 7B 8A 8B 9A 9B 10A 10B 11A 11B 12A 12B 13A 13B 14A 15B 15A 15B CONN2XL15_ALPHA Description VSS VSS VSS VSS ND VI/O VI/O VI/O +3.3 V +3.3 V +3.3 V +3.3 V VCORE VCORE VCORE VCORE VSS VCORE VCORE VCORE PWRGOOD UPVRM# SENSE DISABLE VSS VSS +5.0 V +5.0 V +5.0 V +5.0 V A continuous inductor current mode is necessary to sustain the converter's large bandwidth, irrespective of control method (voltage or current). If the mode of operation changes from continuous to discontinuous inductor current mode, the transfer function of the converter will change drastically. This is precisely why synchronous rectification should be utilized. FIGURE 6. 100-kHz BW Transient Response FIGURE 7. 33-kHz BW Transient Response Synchronous rectification ensures continuous inductor current, regardless of output current. By maintaining continuous inductor current, the transfer function remains constant, preserving its large bandwidth. Figure 8 shows the bode plot of a non-synchronous converter. Notice the drastic decrease in the loop bandwidth from 100 kHz to 4.2 kHz as a result of the transformation into discontinuous mode. Synchronous rectification also buys greater efficiency compared with using a Schottky diode as the rectifier, particularly when used in conjunction with low-on-resistance power MOSFETs built on an innovative Trench technology. Figures 6 and 7 show the dynamic response to 4-A transients. Notice that the settling time of the 375-kHz converter is three times faster than that of the 125-kHz converter. The magnitude of regulation was identical for both converters. Note, however, that the 125-kHz converter's output capacitor and inductor size and value were increased by factor of three to maintain the consistency shown in Table 2. The +45-mV transient regulation result reaffirms the need for a converter bandwidth of 100 kHz or more to meet the regulation needs of the Pentium VRE processor. 6 FaxBack 408-970-5600, request 70588 www.siliconix.com AN718 Vishay Siliconix For example, the Si4410DY Trench MOSFET from Siliconix offers an rDS(on) of 0.02 at VGS = 4.5 V. At a 7-A output current, the MOSFET drops only 0.14 V, compared with a typical voltage drop of 0.60 V across the Schottky diode. Efficiency can be increased considerably, since the synchronous rectifier conducts for approximately 30% of the period. Figure 9 shows the efficiency of the Pentium converter using two p-channel Si4435DYs as the high-side switch and one n-channel Si4410DY as the low-side switch. FIGURE 8. Non-Synchronous Bode Plot FIGURE 9. Efficiency Curve COMPONENT SUPPLIER LIST Reference Designator C1-C8, C23-C26 D1 L1 P1 R8 U1 U2, U3 U4 U6 U7 Part Number TPSD107K010R DIFS4 CTX07-12717-1 66527-015 SL-2 Si9145BY Si4435DY Si4410DY LM4040BIM LM393M Description Tantalum Capacitor 100 F, 10 V 1.1 A, 40 V 2.4 H, 7 A 15 2 0.01 , 2 W PWM IC P-Ch MOSFET N-Ch MOSFET Reference Comparator Pattern D IF Vendor AVX Shindegen Coiltronics Berg Electronics KRL SO-16 SO-8 SO-8 SOT-23 SO-8 Vishay Siliconix Vishay Siliconix Vishay Siliconix National National FaxBack 408-970-5600, request 70588 www.siliconix.com 7 AN718 Vishay Siliconix BILL OF MATERIAL FOR PENTIUM CONVERTER Part 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 Used 2 1 10 2 1 1 1 1 1 1 1 1 4 4 1 1 1 1 1 1 1 1 12 1 2 1 1 1 1 1 2 1 Part Type 0 0.01, 2 W 0.1 F 1N4148 1 F 2.4 H 2k 3M 6.8 k 6.8 k 8.2 pF 10 10K 10K, 0.1% 12K 24 24.3K, 0.1% 40.2 k 47 pF 89.8 100 100 k 100 F 220 k 220 pF CONN2X15 ALPHA LM393M MICREL LM4040BIM DIFS4 Si4410DY Si4435DY Si9145BY Designators R24 R25, CRCWD805000RT1 R8, SL2 C9 C10 C11 C14 C15 C19 C20 C22 C27 C28, VJ0805Y104KXAAT D2 D3 C17, VJ1825Y105KXAAT L1, CTX07-12717-1 R18, CRCW0805202JRT1 R23, CRCW1206305JRT1 R15, CRCW0805682JRT1 R22, CRCW1206682JRT1 C12, VJ0805A8R2KXAAT R2, CRCW080510RJRT1 R5 R12 R14 R17, CRCW0805103JRT1 R10 R19 R20 R21, TNPW12061W2BT-9 R3, CRCW1206123JRT1 R1, CRCW080524RJRT1 R11, TNPW12062432BT-9 R6, CRCW08054022FRT1 C13, VJ0805A470KXAAT R16, CRCW120689R8FRT1 R4, CRCW0805101JRT1 R7, CRCW0805104JRT1 C1 C2 C3 C4 C5 C6 C7 C8 C23 C24 C25 C26, TPSD107K010R R13, CRCW0805224JRT1 C16 C18, VJ0805A221KXAAT P1, 66527-015 U7 U6 D1 U4 U2 U3 U1 Footprint Pattern 0805 9433 2W 0805 MLL34 1825 INDUCTOR 0805 1206 0805 1206 0805 0805 0805 1206 1206 0805 1206 0805 0805 1206 0805 0805 7374 0805 0805 CONN VRM SO-8 SOT-23-5 6032 SO-8 SO-8 SO-16 Vendor Vishay Dale KRL Vishay Vitramon Vishay Siliconix Vishay Vitramon Coiltronix Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Vitramon Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Vitramon Vishay Dale Vishay Dale Vishay Dale AVX Vishay Dale Vishay Vitramon Berg Electronics National Semiconductor Micrel Shindegen Vishay Siliconix Vishay Siliconix Vishay Siliconix 8 FaxBack 408-970-5600, request 70588 www.siliconix.com |
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