LINEAR TECHNOLOGY SEPTEMBER 2003 IN THIS ISSUE… COVER ARTICLE Multiphase Power Conversion for Portable and Point-of-Load Boost Applications..........................1 David Salerno Issue Highlights .............................2 LTC® in the News ............................2 DESIGN FEATURES CompactPCI® Hot Swap™ Controller with I2C™ Interface, Bus Precharge and On-Chip LOCAL_PCI_RESET# Logic...............6 Victor Fleury Low Noise, Micropower Precision Op Amp Swings Outputs from Rail to Rail .............10 Kris Lokere and Glen Brisebois Addressable Bus Buffer Provides Capacitance Buffering, Live Insertion and Nested Addressing in 2-Wire Bus Systems ..................13 John Ziegler Versatile Hot Swap Controller with Open Circuit Detect, Foldback Current Limiting and Much More ..17 Mark Belch Micropower SOT-23 Boost with Integrated Schottky Diode Provides Output Disconnect and Short Circuit Protection ........20 Leonard Shtargot Amplifier with Integrated Filter Offers the Best High Speed, Low Noise Interface for Differential DACs and ADCs..........22 Michael Kultgen VOLUME XIII NUMBER 3 Multiphase Power Conversion for Portable and Point-of-Load Boost Applications Introduction Multiphase power converters offer the advantages of higher efficiency, smaller size and lower capacitor ripple currents over their single phase counterparts. The higher effective switching frequency and phased ripple currents significantly reduce the size and cost of the filter capacitors and lower output ripple, while allowing the use of several small inductors. This has made them popular in many high current buck (step-down) applications, especially where space is a concern. With the LTC3425, the industry’s first multiphase monolithic boost converter, you can achieve the same performance and size benefits in boost (step-up) applications. This 4-phase synchronous boost converter can deliver over 12W of by David Salerno power in a smaller size, with higher efficiency and lower output ripple than is achievable with a comparable singlephase boost converter. The LTC3425 can startup with as little as 1V, and operate with inputs up to 4.5V, making it suitable for a variety of input voltage applications. The output voltage range is 2.4V to 5.25V, and peak current capability is over 5A. Multiphase Converters: They’re Not Just for Buck Applications Anymore The high frequency (up to 8MHz) 4-phase architecture allows the use of small, low cost inductors rather than a single large, bulky inductor, and requires much less output filter capacitance than the equivalent continued on page 3 Low Voltage Amplifiers Give Choice of Accuracy or Speed.........25 Frank Johnston, Glen Brisebois and Danh Tran Feature-Rich Battery Charger that Manages Both Battery Charging and Bus Voltage Regulation..........28 John Shannon DESIGN IDEAS ............................................... 30–38 (complete list on page 30) New Device Cameos.......................38 Design Tools .................................39 Sales Offices.................................40 Figure 1. How does a multiphase boost converter improve on its single phase counterpart? First of all, a multiphase topology saves space and simplifies layout by removing bulky, hard-to-place components and replacing them with easier-to-fit, low profile components. Inductor and output capacitor size comparison of single-phase and 4-phase circuits. , LTC, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load, DirectSense, FilterCAD, Hot Swap, LinearView, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational Filter, PowerPath, PowerSOT, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products. DESIGN FEATURES LTC3425, continued from page 1 Table 1. Comparison of typical inductors used for 1-phase and 4-phase designs 1-Phase 4-Phase Inductor Coilcraft DO3316 TDK RLF5018T Inductance (µH) Required Qty Total Area (mm2) Height (mm) Max Total DC Resistance (Ω) Total Peak Current Rating (A) 2.2 1 122 5.21 0.012 7.0 2.7 4 29.12 × 4 = 116.5 2.05 0.033/4 = 0.0083 1.8 × 4 = 7.2 single-phase circuit. This is ideal for space-constrained boards, Point-ofLoad regulators, and portable devices that demand the use of low-profile components. For example, in a 2-cell NiCd or NiMH to 3.3V/2A boost application, the peak inductor current required for a single-phase design is nearly 5A. Figure 1 shows the size difference between a typical single inductor that would be required to handle this current, and the inductors that could be used in a 4-phase design. Figure 1 also compares the output capacitors required to achieve the same output ripple voltage in single-phase and 4-phase applications. Table 1 shows specifications for the inductors pictured in Figure 1—not only are the four small inductors much thinner, but they also have a lower combined DC resistance for improved efficiency. discontinuous mode operation. In discontinuous mode, an internal resistor is placed across the inductor when the synchronous rectifier turns off, damping any high frequency ringing. A single error amplifier is used for all four phases, and controls the peak current required to maintain regulation. Referring to Figure 2, the loop compensation components are connected between COMP and GND. Soft-start time is set by the CSS capacitor, which ramps the current limit up to its final value during startup Each VOUT pin should have its own ceramic filter capacitor located as close as possible to the VOUT and GND pins in that phase. These are typically 0805 size parts. The pinout of the LTC3425 lends itself to a tight symmetrical layout of the power components. With the 4-phase architecture, low output voltage ripple is achieved using only the four small ceramic capacitors, Easy to Use even at load currents of 2A or more. Designing a converter using the An optional bulk capacitor on VOUT LTC3425 is no different than design- can be added to improve transient ing a traditional single phase boost response with dynamic loads. This converter. All the power switches are can be a ceramic, tantalum, or an internal, so the 4-phase operation is OSCON-style capacitor. transparent. Current limit and switchThe output disconnect feature efing frequency for all four phases are fectively eliminates the PMOS body each programmed by a single resistor, diode between the switch node and as in single phase designs. Setting the VOUT during shutdown, allowing VOUT output voltage and compensating the to discharge to zero volts, all while loop are also no different than in other achieving less than 1µA shutdown familiar designs. current. The disconnect feature also blocks unwanted current flow between Circuit Description: Four VIN and VOUT, eliminating the large inIndependent Power Stages rush currents during startup that are Each of the four phases has an NMOS inherent to most boost converters. and a PMOS power switch, and conThe internal oscillator, programmed trols its own inductor current using by a resistor from R T to GND, genera peak current mode control loop, ates four internal clock pulses, each consisting of a current comparator phase shifted by 90o. The switching with adaptive slope compensation frequency can be set from 100kHz to as and a reverse current comparator for high as 2MHz per phase, for an effecLinear Technology Magazine • September 2003 tive frequency of 8MHz as seen at the output filter cap. Maximum duty cycle for each phase is set to 90%. A sync input and oscillator output are provided for synchronizing the converter to a system clock, or synchronizing two converters together. Note that the sync input and clock output are at four times the switching frequency of each phase. In Burst Mode® operation, only phase A is active, reducing switching and quiescent losses for maximum efficiency. In this mode, phase A operates with a fixed peak inductor current of 0.6A typical. Drawing just 12µA of quiescent current in Burst Mode operation allows the LTC3425 to operate with high efficiency during very light load conditions. 2-Cell to 3.3V/2.2A Boost Application, with Automatic Burst Mode Operation Figure 2 shows a typical application circuit using the LTC3425 to boost from two NiCd or NiMH cells to 3.3V. This design can supply over 2A of load current with efficiencies up to 94% while switching at 1MHz per phase (4MHz output ripple frequency). Maximum component height is a slim 2.05mm. High efficiency is maintained over a very wide load range, as shown in Figure 3. A key feature of the LTC3425 is the programmable automatic Burst Mode operation, which allows the user to set the load current where the converter enters Burst Mode operation, extending the efficiency at light load. This is ideal for systems where the mode cannot be controlled manually by the host. Since the Burst Mode circuit monitors average output current (rather than peak inductor current), the mode threshold is not affected by input 3 DESIGN FEATURES 100 90 CIN 2.2µF L1 2.7µH VIN SWA L1 2.7µH L1 2.7µH SWC SWB SHDN REFOUT CCM REFEN BURST R4 17.4k RT RT 15k RLIM 75k 80 SWD VOUTS VOUTA VOUTB VOUTC VOUTD COUT 4.7µF ×4 LTC3425 SYNCIN C3 0.1µF L1 2.7µH ILIM GNDA SGND GNDB CIN: TAIYO YUDEN JMK107BJ225MA COUT: TAIYO YUDEN JMK212BJ475MG (×4) FB COMP SS SYNCOUT PGOOD GNDC GNDD RFF 10k CFF 22pF R2 1M C2 47pF R1 590k + CSS 0.047µF EFFICIENCY (%) VIN 2V TO 3V VOUT 3.3V 2.2A CIN 2.2µF CBULK 220µF 4V 40 30 10000 Figure 3. Efficiency vs load of 3.3V boost, using automatic Burst Mode operation pin, allowing the use of large value feedback resistors for maximum light load efficiency. 3.3V/Li-Ion to 5V/2.4A Boost Application with Active Clamp Because of the bulk capacitor on VOUT in this example, only a single compensation capacitor is required. The feed-forward network, consisting of RFF and CFF, reduces output ripple in Burst Mode operation and further improves transient response during load steps. It also lowers the high frequency impedance at the FB L1 2.7µH 50 VIN = 2.4V VOUT = 3.3V 10 f = 1MHz/PHASE L = 2.7µH 0 1 10 0.1 100 1000 LOAD CURRENT (mA) Figure 2. 2-cell to 3.3V boost application VIN 3.3V 60 FIXED FREQUENCY MODE 20 L1-L4: TDK RLF5018T-2R7M1R8 CBULK: SANYO 4TPC220M voltage variation. In this example, the Burst Mode threshold is set by R4 to 100mA. When the average load current drops below 100mA, the part enters Burst Mode operation, when the load current increases again, it leaves Burst Mode operation and returns to fixed frequency operation. Capacitor C3 filters the switching ripple at the Burst pin. Burst Mode OPERATION 70 L2 2.7µH L3 2.7µH L4 2.7µH Figure 4 shows the LTC3425 in a 5V boost application. This circuit can deliver 5V at 2.4A from a single Li-Ion cell, or from a 3.3V supply. That’s 12W of output power in a 475mm2 (0.74in2) footprint with a component height of only 2.5mm. As Figure 5 shows, the efficiency peaks at 95%. Output ripple D1 CS 0.47µF ×2 D2 D3 D4 VIN VOUT OFF ON REFOUT SWA SWB SHDN REFOUT C1 0.1µF CCM OFF ON REFOUT OFF ON CCM REFEN SYNC SYNCIN BURST ON OFF BURST ILIM RLIM 75k SGND SWD VOUTS VOUTA VOUTB VOUTC VOUTD Q1 COUT 4.7µF ×4 LTC3425 RT RT 15k SWC GNDA GNDB CIN: TAIYO YUDEN JMK107BJ225MA CS: TAIYO YUDEN LMK107BJ474KA COUT: TAIYO YUDEN JMK212BJ475MG (×4) CBULK: SANYO 6TPC150M FB COMP SS SYNCOUT PGOOD GNDC GNDD C2 68pF CSS 0.047µF R3 330k R2 1M R1 324k RFF 10k CFF 15pF R4 100k + CBULK 150µF 6.3V VOUT 5V 2.5A PGOOD D1 TO D4: MOTOROLA MBR0520L L1 TO L4: TDK RLF5018T-2R7M1R8 Q1: ZETEX ZXM61P02F Figure 4. Li-Ion/3.3V to 5V boost application with active clamp 4 Linear Technology Magazine • September 2003 DESIGN FEATURES 100 90 VIN = 3.6V BURST MODE EFFICIENCY (%) 80 VOUT 50mV/DIV VIN = 3.3V BURST MODE 70 60 VIN = 3.6V 50 1MHZ/PHASE 40 VOUT 20mV/DIV VIN = 3.3V 1MHZ/PHASE 30 2.2A IOUT 1A/DIV 20 200mA 10 0 0.1 1 VIN = 3.6V 500ns/DIV VOUT = 5V CO = 4 • 47µF + 150µF POSCAP 10k 10 100 1k LOAD CURRENT (mA) Figure 6. Li-Ion to 5V output voltage ripple at 2.5A load Figure 5. Efficiency vs load of 5V boost VIN = 3.6V 100µs/DIV VOUT = 5V CO = 4 • 47µF + 150µF POSCAP COMP = 330k +68pF FF = 10k + 15pF TDK INDUCTORS Figure 7. Step response for a 2A load step at full load, shown in Figure 6, is less than 20mVP–P. In this application, Schottky diodes are used as part of an active clamp to limit the peak voltage seen at the switch nodes during the anti-crossconduction time between the turn-on and turn-off of the internal NMOS and PMOS switches. The use of the external SOT-23 P-channel MOSFET (Q1) and 0.47µF capacitors (CS) preserves the output disconnect feature of the LTC3425, allowing VOUT to go to 0V in shutdown and limits the inrush current. If output disconnect is not required, Q1 and CS can be eliminated, and the Schottky diodes can be tied directly from SW to VOUT. This circuit also illustrates the features and flexibility of the LTC3425. There is a 1.22V, short circuit pro- + VIN = 2V TO 3V CIN 2.2µF L1 2.2µH VIN SWA L2 2.2µH SWB SHDN REFOUT CCM REFEN BURST C3 0.022µF RT RT 15k RLIM 100k ILIM SGND CBULK: AVX TPSD157M004R0050 CIN: TAIYO YUDEN JMK107BJ225MA L3 2.2µH SWC L4 2.2µH SWD VOUTS VOUTA VOUTB VOUTC VOUTD GNDA GNDB FB COMP SS SYNCOUT PGOOD GNDC GNDD COUT 4.7µF ×4 Low Cost, Very Low Profile 5W Boost Application Using All Ceramic Caps Many portable applications have strict limitations on component height. This can be a challenge for a power converter, since the inductor and filter capacitors are usually among the tallest components. The LTC3425’s 4-phase architecture is ideal for these applications. An example of a two cell to 3.3V/1.6A boost converter with a component height of only 1.55mm is shown in Figure 8. In this design, the only output filter capacitors needed are the four 0805 CSS 0.047µF Figure 8. Low cost, 1.55mm profile 3.3V boost example R2 499k VOUT 3.3V 1.6A R1 294k C1 220pF COUT: TAIYO YUDEN JMK212BJ475MG (×4) L1 TO L4: MURATA LQH32CN2R2M51 Linear Technology Magazine • September 2003 tion. This can improve the transient response by bringing the converter out of Burst Mode operation prior to a large load step. A scope photo of the output step response, while operating in fixed frequency mode, is shown in Figure 7. continued on page 9 LTC3425 SYNCIN R4 20k tected reference output that can be turned on or off (for higher efficiency at very light loads), a sync input for synchronizing the internal oscillator to an external clock, and an open-drain Power Good output that monitors the output voltage. The CCM input allows the user to force continuous conduction mode, which eliminates pulse skipping at light loads for noise sensitive applications. When CCM is pulled high, the synchronous rectifier stays on until a reverse inductor current of about 0.6A is sensed. Note that this lowers the efficiency at light load, and should only be used during fixed frequency mode operation. In this example, the BURST pin is used to manually command either fixed frequency or Burst Mode opera- R3 100k Figure 9. 3.3V boost demo (circuit shown in Figure 8) 5 DESIGN FEATURES ADDRESS BYTE SCL DATA BYTE 1 2 3 4 5 6 7 0 1 ADDR 4 ADDR 3 ADDR 2 ADDR 1 8 9 1 2 3 4 5 6 7 8 9 ACK S7 S6 S5 S4 S3 S2 S1 S0 ACK STOP START SDA ADDR 0 R/WR=1 Figure 7. Receive byte timing Table 3. Supply causing fault Table 2. Receive byte definition S7 Logic State of the PRSNT2# Pin FAULTCODE0 FAULTCODE1 FAULT Supply Causing Fault S6 Logic State of the PRSNT1# Pin LOW LOW LOW 3VIN S5 Logic State of the PWRGD Pin LOW HIGH LOW 5VIN S4 Logic State of the RESETOUT Pin HIGH LOW LOW 12VIN S3 Logic State of the RESETIN Pin HIGH HIGH LOW VEEIN S2 FAULTCODE1 (See Table 3) X X HIGH None S1 FAULTCODE0 (See Table 3) S0 Logic State of the FAULT pin The LTC4240 incorporates an I2C compatible 2-wire (SCL, SDA) interface that allows the user to easily query and control the status of the LTC4240. A single analog pin selects 1 of 32 allowed addresses. The LTC4240 supports send byte and receive byte LTC3425, continued from page 5 size, 4.7µF ceramics, with a height of 1.35mm. Output voltage ripple is under 50mVP–P at full load. The four low-cost inductors are only 1.55mm high, with a 3.2mm by 2.5mm footprint. The entire 5W power converter can fit into a 20mm by 16mm space, as seen in Figure 9. 2- or 3-Phase Operation For cost-sensitive applications or for reduced board area with lower maximum current capability, the LTC3425 can be used as a 2- or 3-phase converter by simply de-populating one or two of the inductors. Figure 10 illustrates the typical efficiency difference between 2-, 3- and 4-phase operation. In Burst Mode, there is no efficiency penalty, since only phase A is used. Linear Technology Magazine • September 2003 Conclusion The LTC4240 provides a comprehensive solution to CompactPCI Hot Swap applications. An integrated I2C-compatible interface allows software control and monitoring of device function and power supply status. The LTC4240 control functions allow the plug-in board to be safely inserted or removed from a live CompactPCI slot without disturbing the system power supplies or I/O lines. Conclusion: Good Things Do Come in Small Packages The examples here illustrate the performance, flexibility, small size and ease-of-use of the LTC3425. The synchronous 4-phase architecture achieves high efficiency over a wide range of loads while enabling the use of low-profile components. The four-toone reduction in output ripple current makes it possible to achieve very low output voltage ripple using small, lower cost ceramic capacitors. Users can choose between automatic or manual Burst Mode operation, pulse skipping mode or forced continuous conduction mode for noise sensitive applications. All these features, along with output disconnect, soft-start, 1µA shutdown current, anti-ringing control, thermal 98 TJ = 25°C 96 VIN = 2.4V VOUT = 3.3V 94 1MHz/PHASE EFFICIENCY (%) Control and Monitor Card Power with I2C Interface commands. Figure 5 and Table 1 depict the timing and bit definition of the send byte command. Figure 6 schematically outlines some of the command bit functions. Figure 7 shows the timing of the receive byte command. Tables 2 and 3 define the data byte. If a fault occurs, the FAULTCODE bits can be used to determine which supply generated the fault. 92 90 4 PHASE 88 86 84 2 PHASE 3 PHASE 82 80 100 1000 LOAD (mA) 10000 Figure 10. Typical efficiency with 2, 3 and 4 phases (fixed frequency mode) shutdown, a buffered reference output and a Power Good output are packed in a small 5mm by 5mm, thermally enhanced QFN package. 9