Design Solutions 8 April 1999 2-Step Power Conversion: Portable Power for the Future As microprocessor operating voltages continue to decrease, power conversion for CPU core power is becoming a daunting challenge. A core power supply must have fast transient response, good efficiency, and low heat generation in the vicinity of the processor. These factors will soon force a move away from 1-step power conversion from battery or wall adapter to processor, to 2-step conversion where the CPU core power is obtained from the 5V supply. While new to the portable arena, distributed power systems using 5V as a bus voltage have been used in large systems for many years. And although it may not be absolutely necessary to adopt this architecture in portables today, the clock is ticking for the old brute-force approach. Let’s start with the biggest argument against 2-step conversion: the perceived drop in efficiency and attendant heat generation in the 5V supply. Quick calculations may give a false impression that efficiency significantly decreases. It does not! Later in this paper we will show accurate calculations of efficiency for 2-step power conversion based on actual demo board measurements that show overall efficiency numbers equal to 1-step high efficiency converters. On the other hand, many benefits result from 2-step conversion: more symmetrical transient response, lower heat generation in the vicinity of the processor, and easy modification for lower processor voltages in the future. Peak currents taken from the battery are also reduced, which leads to better battery efficiency that can often improve upon the efficiency measured using laboratory power supplies. Consequently, battery life in a real notebook computer may actually exceed that of 1-step architectures. , LTC and LT are registered trademarks of Linear Technology Corporation. 1-Step Approach for CPU Power 2-Step Approach for CPU Power LDO 3 TO 4 SERIES Li-Ion CELLS (8.1V TO 16.8V) 12V 0.2A 5V 2.5A (TOT 3A) LTC1628 DUAL OUTPUT DC/DC CONVERTER 12V 0.2A LDO 3 TO 4 SERIES Li-Ion CELLS (8.1V TO 16.8V) 5V 2.5A (TOT 6.2A) LTC1628 DUAL OUTPUT DC/DC CONVERTER 3.3V 3A (TOT 3.5A) LDO LTC1735 OR LTC1736 (VID) DC/DC CONVERTER 3.3V 3A (TOT 5.3A) 2.5V 0.5A LDO 1.XV 9A+ 2.5V 0.5A 1.XV 9A LTC1702 OR LTC1703 (VID) DC/DC CONVERTER LTC1624 DC/DC CONVERTER 1.8V 3A 1.8V 3A Figure 1. As CPU Core Voltages Continue to Drop, the Traditional 1-Step Approach Will Become Obsolete Due to Infinitesimal Duty Cycles and Severely Skewed Transient Behavior. The 2-Step Approach Eliminates These Issues by Splitting the Conversion Into Two Stages, Thus Yielding Faster Transient Response and Lower Heat Generation Near the CPU. 1 Design Solutions 8 The culprit lies in the duty cycle for a step-down switching regulator, given by the ratio of VOUT to VIN. In 1-step power conversion, the switch on-time must be very short because the step-down ratio is large. This gives a very fast inductor current ramp-up and a much slower current ramp-down. The inductance value must be large enough to keep the current under control during the ramp-up. This requires a larger inductance than for operation with a low input voltage. Fast current rise and slow current decay means that the transient response of the regulator is good for load increases but poor for load decreases. The lower, constant input voltage for a 2-step conversion process not only yields a more symmetrical transient response, but it completely eliminates the headaches associated with optimizing loop dynamics over widely varying battery and wall adapter voltages. voltage converter can add up to five points of efficiency, thereby minimizing heat generation near the processor. Because the duty cycles are closer to 50% with 2-step conversion, and there is less switching loss due to the lower voltage swings, the switching frequency may also be increased. This allows smaller, lower cost external components to be used and further aids the transient response. The “Total Power Out” term must include not only the power ultimately supplied to the CPU core, but also the additional power supplied at each conversion from which the CPU core voltage is derived. The “Total Power Lost” term is the sum of the powers lost at every conversion and is calculated from the respective operating efficiencies. To minimize the high current PCB trace lengths, the core supply must be located near the processor. With a 1-step converter, the power lost is significantly higher than for the second step of a 2-step conversion. Switching regulators for converting high input to low output voltages rarely approach 90% efficiency. A properly designed 5V to core Table 1 compares the power lost at each stage for 1-step and 2-step CPU power conversions from a 12V input voltage. Note that when all of the Figure 1 outputs are considered, the overall efficiency is identical for 1-step and 2-step conversion. A common mistake when computing the efficiency of a 2-step power conversion system is to simply multiply the efficiency of the first conversion by the efficiency of the second conversion. While expedient, this method does not reveal the overall system efficiency nor the distribution of losses on the board. The correct approach to evaluate 2-step power conversion efficiency is to return to the definition of efficiency: Efficiency = Total Power Out • 100% (Total Power Out + Total Power Lost) Table 1. 1-Step vs 2-Step Operating Efficiency, VIN = 12V 1-Step for CPU Power OUTPUT /ILOAD† 3.3V/1.5A 5V/1A 2-Step for CPU Power DC/DC OUTPUT POWER EFFICIENCY* POWER LOST OUTPUT/ILOAD† DC/DC OUTPUT POWER EFFICIENCY* POWER LOST 5.61W 93% 0.42W 3.3V/1.5A 8.54W†† 93% 0.64W 14.78W†† 94% 0.94W 5W 95% 0.26W 5V/1A 1.6V/5.5A 8.8W 86% 1.43W 1.6V/5.5A 8.8W 90% 0.98W 1.8V/1.5A 2.7W 80% 0.68W 1.8V/1.5A 2.7W 92% 0.23W 2.5V/0.2A 0.5W 76% 0.16W 2.5V/0.2A 0.5W 76% 0.16W CCFL 8W 90% 0.89W CCFL 8W 90% 0.89W TOTAL POWER TO LOADS 30W 88.6% 3.84W TOTAL POWER TO LOADS 30W 88.6% 3.84W Total Output Power × 100% Total Output Power + Total Power Lost †Average output current; the 12V output is normally off. ††Including the additional power required for the 1.6V and 1.8V outputs. *Efficiency = 2 Design Solutions 8 And what about the increased burden on the 5V regulator? Table 1 reveals that while the power lost in the 5V supply does increase with 2-step conversion, it is still less than that lost in the 1-step CPU core supply. Furthermore, power lost in the core and I/O supplies is in the worst possible thermal environment for a notebook computer— next to the processor. In this example, 2-step conversion reduced the power dissipated in the vicinity of the CPU by 0.9W. An additional concern sometimes voiced by power supply designers is that there might be pitfalls from loading the output of one switching regulator with the input of another. In fact, the input current of a switching regulator is directly proportional to its output voltage and current and inversely proportional to its input voltage. This represents a benign load for an upstream switching regulator, and cascaded switching regulators have been used in a host of different power distribution applications over the years. Today’s desktop computers, for example, use precisely the same architecture as proposed here for portables. As time goes forward, microprocessor fabrication lithography will continue to shrink and force still lower CPU core operating voltages and higher operating currents. 1.1V supplies and 15A peak operating currents are already on the horizon for portable systems. These demands will render the traditional 1-step conversion approaches unworkable as a result of infinitesimal duty cycles and severely skewed transient behavior. Linear Technology has developed a third generation of high efficiency DC/DC converters with unique features that are ideal for implementing 2-step conversion strategies. The LTC®1628 2-phase dual system power supply controller is an ideal solution for providing 3.3V and 5V system power and the first conversion step in a 2-step solution. For the highest first step conversion efficiency, an LTC1625 No RSENSETM current mode controller can be used to provide 5V at up to 10A. For the second step, the LTC1702 and LTC1703 (VID option) 2-phase dual controllers convert 5V or 3.3V to CPU core and I/O supplies at efficiencies of up to 95%. The LTC1702/LTC1703 require no sense resistors and operate at 550kHz for fast transient response and low external component cost. No RSENSE is a trademark of Linear Technology Corporation. LTC1703 Load Transient Response VID VOUT 50mV/DIV 1.5V ±7.5% 1.5V VOUT 1.3V 100µs/DIV 2STEP F2 Figure 2. Operating at 550kHz from a 5V Supply Allows the Use of a Much Smaller Inductor, Resulting in Excellent Transient Response 100µs/DIV 2STEP F3 Figure 3. The LTC1703 Exhibits a Settling Time Less Than 100µs When VOUT is Changed Using the VID Control Inputs Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 3 4 1 2 3 4 5 6 7 8 9 10 Linear Technology Corporation (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com E8 GND E7 VOUT4 1.5V/12A C53 1µF RESET SW1 5V ENABLE FREQSET STBYMD FLTCPL FCB STDBY3 3V STDBY 5V 3.3V ENABLE + C33 180µF 4V R5 33k C4 330pF + C5 56pF + R6 33k C6 330pF R18 1M C34 180µF 4V R2 1M COREVENABLE FAULT FCB 1.8V ENABLE D5 C35 180µF MBRD835L 4V L3 0.8µH ETQP6FOR8L C7 56pF R4 1M C2 0.1µF JP1 LATCH-OFF DISABLE 8 7 6 5 Q8 IRF7811 3 2 1 TG1 RUN/SS2 TG2 SW2 BOOST2 BG2 PGND INTVCC EXTVCC BG1 VIN BOOST1 SW1 C37 0.22µF 15 16 17 18 19 20 21 22 23 24 25 26 27 28 C40 220pF + C13 4.7µF C15 0.1µF R11 10Ω R19 18.7k 1% C36 0.1µF D6 MBR0520LT1 C10 0.1µF C11 D2 0.1µF CMDSH-3 C12 1µF D1 CMDSH-3 C14 0.1µF 50V VID0 VID1 VID2 VID3 VID4 R21 1k R20 C38 C39 100k 220pF 15pF 8 7 6 5 Q9 IRF7811 3 2 1 8 7 6 5 Q7 IRF7811 3 2 1 SENSE2+ SENSE2– VOSNS2 ITH2 3.3VOUT SGND ITH1 FCB STBYMD FREQSET VOSNS1 SENSE1– SENSE1 + FLTCPL U1 LTC1628CG RUNSS1 C8 1000pF 14 13 12 11 10 9 8 7 6 5 4 3 2 1 C9 1000pF 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Q1 IRF7805 IMAX2 B2 B3 B4 VCC FB2 COMP2 RUN/SS2 FAULT PGND SW2 TG2 BG2 BOOST2 LOCATE LTC1703 NEAR PROCESSOR B1 B0 SENSE FB1 SGND COMP1 RUN/SS1 FCB IMAX1 SW1 TG1 BG1 BOOST1 PVCC U2 LTC1703CG C41 1µF 15 16 17 18 19 20 21 22 23 24 25 26 27 R22 24.9k 1% 28 Q5 IRF7807 5 6 7 8 1 2 3 C42 0.1µF C43 0.22µF D7 MBR0520LT1 L2 4.6µH ETQP6F4R6H C44 15pF R23 100k 7 8 Q10A NDS8926 1 C47 0.22µF C48 1µF + + 2STEP F04 C46 180µF 4V R24 8.06k 1% C45 220pF + + C29 0.1µF 50V C21 180µF 4V C19 150µF 6V C16 0.1µF 50V 5 6 Q10B NDS8926 3 L4 2.2µH DO3316P-222 R13 0.010Ω + R12 0.005Ω R9 0.007Ω C17 150µF 6V D4 MBRS130T3 D3 MBRD835L L1 2.9µH ETQP6F2R9L Q2 IRF7805 Q4 IRF7807 5 6 7 8 1 2 3 5 6 7 8 1 2 3 5 6 7 8 Q3 IRF7805 1 2 3 1 2 3 5 6 7 8 C26 47pF R16 20k 1% R25 10.2k 1% C49 150µF 6V R26 1k C50 2200pF C51 150µF 6V C28 100pF C25 47pF R15 20k 1% R17 63.4k 1% C23 100pF R14 105k 1% + C22 22µF 50V Figure 4. The LTC1628 and LTC1703 Are 2-Phase, Dual Output DC/DC Controllers That, When Combined, Form a High Efficiency 4-Output Notebook PC Power Supply that Requires a Minimum of PC Board Area + C32 180µF 4V C3 0.1µF C1 0.1µF R3 1M R1 1M R7 1M R8 1M + J1 HEADER 10 R27 10Ω C52 1µF SGND GND L5 0.33µH DO3316P-331HC C27 10µF 6.3V C24 10µF 6.3V E10 GND E9 VOUT3 1.8V/3A E6 GND E5 VOUT2 3.3V/5A E4 GND E3 VOUT1 5V/4A E2 GND E1 VIN 7V TO 20V Design Solutions 8 dsol8 LT/TP 0499 2K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 LINEAR TECHNOLOGY CORPORATION 1999