DESIGN IDEAS VRM8.5 Design with the LTC3720 Achieves Small Size and Fast Transient Response by David Chen Several different brands of CPUs fall into Intel® VRM8.5 category. Depending upon clock frequency and computation power, these CPUs consume different levels of supply current ranging from several amperes to 30A. The newly released LTC3720 single- 3.3V POWER GOOD BAT54C 3 CSS 0.1µF 1 0.01µF INT VCC 2 4 CC 220pF R VP2 105k RVP1 21.0k 5 6 20k 220pF 7 David Chen Lower the Output Voltage Ripple of Positive-to-Negative DC/DC Converters with Optimum Capacitor Hook-Up ................................................... 22 Keith Szolusha Use a Single Input to Acquire Two Similar Signals Simultaneously and Other AC Techniques for the LTC1864 ................................................... 24 Derek Redmayne and Mark Thoren 2.5A, 4MHz Monolithic Synchronous Regulator Offers a High Efficiency, Compact Solution by Reducing External Component Count and Size ................................................... 27 Joey M. Esteves White LED Driver in Tiny SC70 Package Achieves 84% Efficiency ................................................... 29 Pit-Leong Wong Boost DC/DC Converter Synchronizes to any Frequency ......................... 30 Gary Shockey Monolithic Synchronous Step-Down Regulators Pack 600mA Current Rating in a ThinSOT™ Package ................................................... 31 Jaime Tseng Inductorless, Low Noise Step-Down DC/DC Converter Saves Space and Provides Efficient 1.5V Output .... 33 Bill Walter Low Voltage, High Current DC/DC Power Supply with Load Sharing and Redundancy ................................ 34 Henry J. Zhang and Wei Chen 10 1k High Performance Op Amps Deliver Precision Waveform Synthesis .... 21 Jon Munson VIN 4.5V TO 21V 2k 8 CION 1000pF 9 510k VIN 100pF 11 220pF 12 VID25mV VID0 13 14 PGOOD BOOST RUN/SS TG CIN 10µF 25V ×4 27 1µF 25V M1 26 VON SW VRNG SENSE+ FCB SENSE– DB CMDSH-3 25 L1 24 23 ITH PGND LTC3720 SGND 22 M2 BG 21 SGND D1 INTVCC 20 ION 4.7µF 6.3V VIN 19 VFB 0.1µF EXTVCC 18 VFB VOSENSE VID4 VID0 VID3 VID1 VID2 5V 3.3V VCC 17 VID3 PGND SGND 0.1µF 16 VID2 15 VID1 + M1: IRF7811A ×2 M2: IRF7822 ×2 D1: UPS840 L1: 0.8µH CEP125U-0R8 COUT 680µF 2.5V ×2 VOUT 1.075V TO 1.800V 20A 1µF 6.3V Figure 1. A 20A VRM8.5 design using the LTC3720 phase PWM controller is designed for CPUs that consume up to 20A. It features a valley current control architecture that speeds up the VRM response to step load changes, two on-chip high current gate drivers for N-channel power MOSFETs, a current sensing mechanism that does not require an additional sense resistor and a 5-bit VID table that is compatible with Intel VRM 8.5. The resulting VRM 8.5 design has a small size and a fast transient response. The LTC3720 also achieves a minimum on-time below 100ns and a wide input range from 4V to 36V. These are important characteristics for notebook CPU applications where the input-to-output ratio is usually Intel is a registered trademark of Intel Corporation 20 10Ω CB 0.22µF 28 high. Other LTC3720 features include a programmable current limit, an output overvoltage soft latch, a capacitor-programmable soft start, an continued on page 32 90 88 86 EFFICIENCY (%) DESIGN IDEAS VRM8.5 Design with the LTC3720 Achieves Small Size and Fast Transient Response ..................... 20 VRON 84 82 80 78 76 74 VIN = 12V VOUT = 1.475V 72 70 0 2 4 6 8 10 12 14 16 18 20 LOAD CURRENT (A) Figure 2. Better than 80% efficiency is achieved over a 1A–20A load range. Linear Technology Magazine • August 2002 DESIGN IDEAS 100 95 VIN = 2.7V 90 VOUT 100mV/DIV AC COUPLED 90 80 IL 500mA/DIV ILOAD 500mA/DIV EFFICIENCY (%) EFFICIENCY (%) 85 VIN = 3.6V 80 VIN = 4.2V 75 70 60 VIN = 3.6V 50 VIN = 2.7V 40 70 30 VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 50mA TO 600mA Figure 3. LTC3406B-1.8 Transient Response to a 50mA to 600mA Load Step occurs to maintain regulation. Efficiency in pulse skipping mode is lower than Burst Mode operation at light loads, but comparable to Burst Mode operation when the output load exceeds 50mA. 1.8V/600mA Step-Down Regulator Using All Ceramic Capacitors Figure 1 shows an application of the LTC3406/LTC3406B-1.8 using all ceramic capacitors. This particular design supplies a 600mA load at 1.8V with an input supply between 2.5V and 5.5V. Ceramic capacitors have the advantages of small size and low equivalent series resistance (ESR), making possible for very low ripple 65 VIN = 4.2V 20 60 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 10 0.1 Figure 4. Efficiency vs Load Current for LTC3406-1.8 voltages at both the input and output. For a given package size or capacitance value, ceramic capacitors have lower ESR than other bulk, low ESR capacitor types (including tantalum capacitors, aluminum and organic electrolytics). Because the LTC3406/LTC3406B’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used to achieve very low output ripple and small circuit size. Figures 2 and 3 show the transient response to a 50mA to 600mA load step for the LTC34061.8 and LTC3406B-1.8, respectively. Authors can be contacted at (408) 432-1900 1 100 10 OUTPUT CURRENT (mA) 1000 Figure 5. Efficiency vs Load Current for LTC3406B-1.8 Efficiency Considerations Figure 4 shows the efficiency curves for the LTC3406-1.8 (Burst Mode operation enabled) at various supply voltages. Burst Mode operation significantly lowers the quiescent current, resulting in high efficiencies even with extremely light loads. Figure 5 shows the efficiency curves for the LTC3406B-1.8 (pulse skipping mode enabled) at various supply voltages. Pulse skipping mode maintains constant-frequency operation at lower load currents. This necessarily increases the gate charge losses and switching losses, which impact efficiency at light loads. Efficiency is still comparable to Burst Mode operation at higher loads. LTC3720, continued from page 20 optional short-circuit latch-off, a Power Good indicator of output regulation and a current limit foldback for overload protection. A selectable discontinuous conduction mode of operation maintains high efficiency at light loads, when the CPU is running at deep sleep mode, for example, thereby improving battery life in portable applications. Figure 1 shows the schematic diagram of a 20A VRM8.5 design for an Intel processor operating at 1.2GHz. Efficiency is greater than 80% over a wide load range, as shown in Figure␣ 2. With two 680µF Sanyo POSCAPs, the output voltage deviation remains within the VRM8.5 specification when 32 load current switches between CPU leakage and full load, as shown in Figure 3. The entire VRM design fits into a 1.25"×1.5", double-sided PCB area with an overall height below 0.35". In summary, the LTC3720 is an ideal device for low current CPU power supplies. Its unique control architecture and its powerful gate drivers facilitate the design of space-saving VRMs that have a fast transient response. For CPUs that consume more than 20A, the LTC1709-85 dualphase controller addresses the current distribution and thermal management issues associated with higher current applications. 1.535V VOUT 1.370V 20A ILOAD 1A 25µs/DIV Figure 3. With two POSCAPs at output, the design in Figure 1 meets VRM 8.5 transient requirements with significant margin. Linear Technology Magazine • August 2002