July 2010 I N T H I S I S S U E analog multiplier monitors instantaneous power and simplifies design of power control loops 10 regulator with accurate input current limit safely extracts maximum power from USB 22 Virtual Remote Sensing Improves Load Regulation by Compensating for Wiring Drops Without Remote Sense Lines Tom Hack and Robert Dobkin clean, efficient, high current point-of-load power for FPGA and server backplanes 28 POWER SUPPLY Volume 20 Number 2 CONNECTOR DROPS WIRING DROPS Accurately regulating a voltage at a load can be difficult when there are significant voltage drops between the power supply and the load. Even if a regulator produces a perfectly regulated voltage at its own output, variations in load current affect the IR drop along the wiring, resulting in significant voltage fluctuations at the load (see Figure 1). CONNECTOR DROPS The conventional solution to improving regulation at the load involves adding extra wires for remote sensing (Figure 2a), but adding extra wires is not always desirable, or even possible. A new control method, Virtual Remote Sensing™ (VRS™), easily replaces and avoids the pitfalls of conventional solutions and in some instances solves the previously insoluble. LOAD LOAD-END REGULATION BEFORE VRS CONNECTOR DROPS WIRING DROPS CONNECTOR DROPS Figure 1. The simplest model for load regulation over resistive interconnections. Virtual Remote Sensing solves the problem of maintaining load regulation at the end of long wiring runs. VRS is easier to implement and generally performs better than conventional remote sensing techniques such as direct remote voltage sensing, voltage-drop compensation, and load-end regulation. The first conventional technique, direct remote sensing (Figure 2a), produces excellent load-end regulation, but it requires two pairs of wires: one pair to provide the load current and a second pair to measure the voltage at the load for proper regulation. Traditionally, remote sensing requires foresight—it must be (continued on page 2) w w w. l inear.com …continued from the cover In this issue... VRS avoids the limitations of conventional voltage drop compensation techniques while producing impressive load regulation over a wide range of conditions. COVER ARTICLE Virtual Remote Sensing Improves Load Regulation by Compensating for Wiring Drops Without Remote Sense Lines Tom Hack and Robert Dobkin 1 designed into the system. Unless an extra pair of sense wires is ready and waiting, remote sensing is impossible to implement after the fact. DESIGN FEATURES Unique Analog Multiplier Continuously Monitors Instantaneous Power and Simplifies Design of Power Control Loops Mitchell Lee and Thomas DiGiacomo (LT4180, continued from the cover) 10 2-Phase Synchronous Step-Down DC/DC Controller with Programmable Stage Shedding Mode and Adaptive Voltage Positioning for High Efficiency and Fast Transient Response The second conventional technique, voltage drop compensation, doesn’t require extra wires, but it does require careful estimation of the voltage drop of the load lines. The supply voltage is adjusted to make up for the estimated interconnection voltage drop. However, since the drop is only an estimated value and not measured, the accuracy of this method is questionable at best. DESIGN IDEAS The third conventional technique involves placing a voltage regulator directly at the load (Figure 2b). This provides both accuracy and simplified wiring, but the regulator consumes valuable space at the load end, reduces overall power system efficiency and power dissipation near the load increases. In industrial and automotive systems, it may be impossible to place a regulator in the harsh environment at the load end. Fast Time Division Duplex (TDD) Transmission Using an Upconverting Mixer with a High Side Switch VRS avoids all of these limitations while producing impressive load regulation over a wide range of conditions. Jian Li and Charlie Zhao 19 Choose a Regulator with an Accurate Input Current Limit to Safely Extract Maximum Power from USB Albert Lee Vladimir Dvorkin 22 25 Driving Lessons for a Low Noise, Low Distortion, 16-Bit, 1Msps SAR ADC Guy Hoover 26 Ultrafast, Low Noise, Low Dropout Linear Regulators Running in Parallel Produce Clean, Efficient, High Current Point-of-Load Power for FPGA and Server Backplanes Kelly Consoer Figure 1. A simple model shows the problem of load regulation over resistive interconnections. POWER SUPPLY CONNECTOR DROPS WIRING DROPS CONNECTOR DROPS WIRING DROPS CONNECTOR DROPS LOAD CONNECTOR DROPS 28 Tiny Digital Predistortion Receiver Integrates RF, Filter and ADC Todd Nelson (continued on page 4) 30 Dual Output High Efficiency Converter Produces 3.3V and 8.5V Outputs from a 9V to 60V Rail Victor Khasiev 33 product briefs 34 back page circuits 36 Figure 2. Two conventional methods for solving the problem of wiring voltage drops: (a) Remote sensing solves the problem of load regulation, but adds wires across the divide, (b) local regulation stabilizes load voltage but is inefficient. VOUT+ WIRING & CONNECTOR DROPS SENSE+ (a) POWER SUPPLY LOAD SENSE– VOUT– WIRING & CONNECTOR DROPS WIRING & CONNECTOR DROPS (b) LOAD VOLTAGE REGULATOR POWER SUPPLY WIRING & CONNECTOR DROPS 2 | July 2010 : LT Journal of Analog Innovation LOAD The LT4180 works with nearly any power supply or regulator: linear or switching, isolated or non-isolated. (LT4180, continued from page 2) Figure 3 shows a simplified schematic of a Virtual Remote Sense system consisting of a power supply or regulator driving a load over a resistive interconnection (consisting of wiring plus connectors). Without VRS, supply voltage (VSUPPLY) and DC current (ILOAD) are known, but there is no way to determine how much voltage is delivered to the load and how much voltage is lost in the wiring, so proper load voltage regulation can’t be achieved. The LT4180 VRS solves this problem by interrogating the line impedance and dynamically correcting for the voltage drops. It works by alternating the output current between 95% of the required output current and 105% of the required output current. In other words, the LT4180 forces the supply to provide a DC current plus a current square wave with peakto-peak amplitude equal to 10% of the DC current. Decoupling capacitor C (which normally insures low impedance for load transients in non-VRS systems) takes on an additional role by filtering out voltage transients from the VRS square wave. Because C is sized to produce an “AC short” at the square wave frequency, the interrogating voltage square wave produced at the power supply is equal to VSUPPLY(AC) = 0.1 × IDC × R, measured in VP-P. The voltage square wave measured at the power supply has a peak-to-peak amplitude equal to one tenth the DC wiring drop. This is not an estimate—it is a direct measurement of the voltage drop across the wiring over all load currents. 4 | July 2010 : LT Journal of Analog Innovation Figure 3. Virtual Remote Sensing is easy to implement. ILOAD POWER SUPPLY LOAD RWIRE + VSUPPLY + CLOAD – VLOAD – POWER WIRING VFB, VC OR ITH VLOAD REMAINS CONSTANT Virtual Remote Sense CONTROLLER ISENSE VIN GND VSUPPLY VARIES TO KEEP VLOAD CONSTANT EVEN AS ILOAD AND RWIRE CHANGE Minor signal processing creates a DC voltage from this AC signal, which is introduced into the supply’s feedback loop to provide accurate load regulation. spectrum operation to provide partial immunity from single-tone interference. Its large input voltage range simplifies design. SO HOW WELL DOES VRS WORK? Besides offering an alternative to conventional techniques, VRS opens up opportunities previously unavailable in battery charging, industrial and Ethernet, lighting, well logging and other applications. Static load regulation for the LT4180 is shown in Figure 4. In this case, load current was increased from zero until it produced a 2.5V drop in the wiring. The voltage at the load dropped only 73mV at maximum current from what it would be at no current. Even with an in-thewire voltage drop equivalent to 50% of the nominal load voltage, the voltage at the load stayed within 1.5% of the no load current value. Less dramatic wiring drops produced even better results. VRS IS EXTREMELY FLEXIBLE The LT4180 works with nearly any power supply or regulator: linear or switching, isolated or non-isolated. Power supplies can be synchronized to the LT4180 or not. To accommodate a variety of system and power supply requirements, VRS operating frequency can be adjusted over more than three decades. It also offers spread SOLVING THE IMPOSSIBLE WITH VRS 5.00 4.99 4.98 4.97 VLOAD (V) WHAT IS VRS? 4.96 4.95 4.94 4.93 4.92 4.91 0 0.5 1 1.5 2 2.5 3 VWIRING (V) Figure 4. Static load regulation for the LT4180 is impressive over an extreme range of regulator-toload wiring voltage drops. design features The LT4180 VRS solves the problem of line voltage drops by interrogating the line impedance and dynamically correcting for the voltage drops. Improve Battery Chargers Figure 5 illustrates a poorly conceived power system for notebook computers, PDAs, cell phones or portable entertainment devices. An external power supply/ battery charger is used to minimize the size of the portable electronic device. The Figure 5. A (flawed) battery charging architecture aims to reduce the device size with an external battery charger. BATTERY CHARGER charger only works properly when the device is off and not drawing current. As the battery approaches full capacity, battery charging current (IBAT) is nearly zero. With I = 0, the battery charger voltage VSUPPLY equals the battery float voltage and charge termination works properly. PORTABLE ELECTRONIC DEVICE + RWIRE I VOLTAGE REGULATOR VSUPPLY – BATTERY CHARGER The conventional solution uses a complex architecture like that shown in Figure 6, which incorporates the charger and a power path controller into the device. While this reduces wiring-related charging errors, it increases the size of the device and the power dissipation within the device because the charger and power path controller must be packed inside. POWER WIRING IBAT VBAT Figure 6. Typical battery charging architecture without VRS LOAD Li Ion PORTABLE ELECTRONIC DEVICE + RWIRE I POWER PATH CONTROL & VOLTAGE REGULATOR VSUPPLY – LOAD POWER WIRING VI BATTERY CHARGER But what happens if the system voltage regulator is drawing current? The battery voltage VBAT can be less than the needed battery charger voltage, VSUPPLY, thus slowing charging or even stopping it altogether. Interconnection resistance can’t be lowered enough to solve this. The 1% Li-ion float voltage accuracy requirement translates into a 42mV float voltage error (for a one cell Li-ion battery). Because there are other float-voltage error sources, the wiring drop must be kept well below this. Li Ion Figure 7 shows the no-compromise solution using VRS. Charger voltage is properly controlled at the device, independent of load current (I), so an external battery charger supply can be used and a power path controller eliminated. Easily Compensate Line Drops in Power over Ethernet Applications Figure 7. Simplified battery charging with VRS reduces the overall device size, achieving what the solution in Figure 5 could not. BATTERY CHARGER PORTABLE ELECTRONIC DEVICE + RWIRE I VOLTAGE REGULATOR VSUPPLY – POWER WIRING IBAT VBAT Virtual Remote Sense CONTROLLER Li Ion LOAD Power over Ethernet and industrial applications also benefit from VRS. VRS allows low voltage devices (with high operating current) to operate over CAT5 and CAT6 cable—without the drops caused by long runs. Even 10V-20V line drops can be compensated, allowing either no regulator or a simple linear regulator at the far end. July 2010 : LT Journal of Analog Innovation | 5 A VRS system can be used to improve lighting. For medium and large lighting systems, the improvement in energy efficiency easily pays for the upgrade from a standard transformer to a DC/DC converter. Additional benefits of using a VRS system include better color-temperature control and longer, more consistent bulb lifetimes. Retrofit Industrial Applications lifetime and color temperature as shown in Figure 8, and as follows: VRS can also be used to simplify system retrofits for industrial applications. For example, a pair of power wires is available for new equipment, but regulation at the load-end is not up to the equipment spec. VRS can be easily dropped in to control the existing power supply or regulator. This is far easier and cheaper than adding another pair of wires for remote sensing or adding load-end regulation. savings. So to produce light at 11V that is equivalent to that produced at 12V would require 25% more bulbs running relatively less efficiently. Simply put, running •Light output is approximately proportional to V3.4 •Power consumption is approximately proportional to V1.6 •Lifetime is approximately inversely proportional to V16 •Color temperature is approximately proportional to V0.42 NORMALIZED PARAMETER 100 Normally these devices operate at 12V, but their operating current is relatively high, so line drops between the regulator and the light can be high. In this case, the load-end discrepancy can easily reach 1V or more. A 12V halogen operated at 11V produces 25% less light than when operated at 12V, with only a 13% power Increase the Efficiency and Light Output of High Intensity Lighting Applications While incandescent lighting is on the decline, high intensity halogen lights continue to be popular. The operating voltage of halogens directly affects their light output, efficiency, 10 1 LIFETIME POWER CONSUMPTION LIGHT OUTPUT 0.1 0.01 0.7 0.8 0.9 1.0 1.1 1.2 1.3 NORMALIZED VOLTAGE Figure 8. Lamp parameters vs normalized lamp voltage show that better voltage regulation at the lamp improves output, saves energy and prolongs lamp life. Figure 9. An automotive halogen headlamp power supply D1 PD51045 L1 6.8µH VIN 9V TO 15V + CIN1 6.8µF 50V CIN2 10µF 63V VCC 4.7µF 10V 200k C1 10µF 50V ×2 VIN SHDN/UVLO 43.2k + COUT1 22µF CER ×3 L2 6.8µH INTVCC RSENSE2 0.015Ω 42.2k 1% FBX LT3757 GATE OSC RT GND 42.2k Q1 SI7850DP VCC SYNC 10k 4.12k 1% SS VC COUT2 10µF OSCON ×2 CLOAD TOTAL RWIRE ≤ 1Ω 1000µF 25V 1µF 1µF FB RUN VIN SENSE DIV1 DIV2 DIV0 INTVCC VPP OSC GUARD2 GUARD3 GUARD4 SPREAD GND 3.4k 1% OV 6.65k 1% LT4180EGN DRAIN COMP CHOLD1 CHOLD2 CHOLD3 CHOLD4 6.8k 47pF 10nF 14.7k 1% CIN1: TDK C4532X7R1H685M CIN2: SANYO 63CE10FS C1: TAIYO YUDEN UMK325BJ106MM-T 3.3nF COSC ROSC 150pF 470pF 470pF 6 | July 2010 : LT Journal of Analog Innovation HALOGEN LAMP 4.99k 1% 0.1µF 100pF + 84.5k 1% SENSE 0.005Ω 1W VOUT 12V, 30W 0.1nF COUT1: TAIYO YUDEN TMK325BJ226MM-T COUT2: OSCON 20SVP10 L1, L2: VISHAY IHLP4040D2ER6R8M11 42.2k 1% design features VIN 20V Q1 IRLZ440 RSENSE 0.1Ω 1% C4 10µF 25V C1 4.7µF 25V R3 27k TOTAL RWIRE ≤ 8Ω R7 10k A VRS system can be used to accurately maintain correct bulb intensity. A capacitor is placed in the vicinity of the bulbs, and the voltage is controlled at that point. For medium and large lighting systems, the improvement in energy efficiency easily pays for the upgrade from a standard transformer to a DC/DC converter. Additional benefits of using a VRS system include better color-temperature control and longer, more consistent bulb lifetimes. A SEPIC-based automotive halogen headlight power supply (Figure 9) improves bulb reliability while also ensuring optimum illumination. The design maintains 12V at headlight voltage over a 9V to 15V input voltage range. It works well up to 1Ω interconnection resistance. Using VRS allows the SEPIC converter to be placed far from the load—say in the passenger compartment, away from extreme under-the-hood environments, thus improving reliability. Residential and commercial track-style lighting also benefit. The cost of properly regulating lamp voltage is quickly recouped in the form of lower power consumption and higher efficiency. Two to three kilowatt-hours can be saved per day on a 250W string while maintaining the same amount of light. Color temperature (while not as dependent on voltage as other lamp parameters) also benefits. VRS allows remote voltage regulation of a single lamp, or provides C2 1µF R2 64.9k 1% R4 3.74k 1% halogens at the correct voltage offers more precise lighting control, more predictable color temperature and better efficiency. R6 2.21k 1% R8 20k CLOAD 100µF 25V FB RUN VIN LOAD DIV2 DIV1 DIV0 INTVCC VPP SENSE OSC GUARD2 GUARD3 GUARD4 SPREAD GND OV LT4180EGN DRAIN C5 22pF + C3 1µF R5 5.36k 1% INTVCC Q2 VN2222 VOUT 12V, 500mA COMP CHOLD1 CHOLD2 CHOLD3 CHOLD4 C6 1nF C8 470pF C7 4.7nF C9 470pF C10 1.5nF COSC C11 470pF ROSC R9 41.2k 1% Figure 10. A full featured VRS linear supply first-order regulation of several lamps distributed over a single power rail. VRS Might be the Only Solution When the Line Lengths Are in Miles VRS can be used in oil and gas well logging applications where instrumentation is often connected by cables from thousands, to tens of thousands of feet long. A COLLECTION OF APPLICATIONS The LT4180 includes all components needed for a linear power supply (except for the pass transistor). Undervoltage lockout, overvoltage lockout and soft-correct are also available, so a full featured linear VRS power supply can be built with few components (Figure 10). The linear supply in Figure 10 provides 12V at 500mA with an 18V input. Pass transistor Q1 is driven via R3, R7 and Q2 via the DRAIN pin. Q2 serves to keep DRAIN pin voltage below the absolute maximum rating. C5, R8, and C6 provide compensation. R2, R4, R5, and R6 set output voltage and lockout thresholds. R1 is the current sense resistor. C7–C10 are hold capacitors used by the VRS, while C11 and R9 set the square wave frequency. Typical load-step response is shown in Figures 11 and 12 with 4Ω wiring resistance and 100µF and 1100µF load-end VSENSE 2V/DIV VLOAD 2V/DIV (AC COUPLED) VSENSE 2V/DIV VLOAD 2V/DIV (AC COUPLED) ILOAD 0.2A/DIV ILOAD 0.2A/DIV 2ms/DIV Figure 11. Load step response of the linear supply shown in Figure 10 with 100µF decoupling capacitance. 2ms/DIV Figure 12. Load step response of the linear supply shown in Figure 10 with 1100µF decoupling capacitance. July 2010 : LT Journal of Analog Innovation | 7 Figure 13. It’s easy to add VRS control to a power supply module. RSENSE 0.033Ω 1% VOUT 3.3V OR 5V, 2.5A TOTAL RWIRE ≤ 0.5Ω 3.3VOUT 5VOUT 11.52k 17.4k 523Ω 4.64k 2.74k 1.69k 5.36k 5.36k 1µF CLOAD 2200µF 10V + LOAD 1µF VIN+ VIN+ VOUT+ VIN– VICOR VSEN+ MODULE TRIM VI-230-EX VSEN– 48V VIN– FB 2.4k RUN VIN OV VOUT– DIV0 DIV1 DIV2 INTVCC VPP SENSE OSC GUARD2 GUARD3 GUARD4 SPREAD GND LT4180EGN DRAIN 1% RESISTORS COMP CHOLD1 CHOLD2 CHOLD3 CHOLD4 10k 47pF 10nF 3.3nF 3.3nF ROSC COSC 1nF 0.1µF 42.2k 1% 178k 10nF capacitances. VRS transient response is well controlled with widely varying CLOAD. changing the values of the feedback and overvoltage resistors. Nominal input voltage is 48V. VRS is produced via the module’s trim pin. This design works with 0.5Ω wiring resistance and 2200µF decoupling capacitance. Figure 13 shows how the LT4180 interfaces to a Vicor power module, providing Virtual Remote Sensing for a 3.3V/2.5A or 5V, 2.5A load through 0.5Ω wiring resistance. Output voltage is adjusted by A fully isolated flyback converter capable of supplying 3.3V at 3A from an 18V to 72V input is shown in Figure 14. It is designed to correct for up to 0.4Ω of wiring resistance. Recommended loaddecoupling capacitance is 940µF. Isolation is achieved through T1 and Figure 14. A fully isolated VRS flyback converter RSENSE 0.018Ω 1% T1 VIN 18V TO 72V 10k 4700pF CIN 1µF 100V ×2 4.7µF 50V 51.1Ω 1% D2 D1 COUT 100µF 6.3V ×2 VOUT 3.3V, 3A CLOAD 470µF 10V ×2 TOTAL RWIRE ≤ 0.4Ω 1µF 2k VIN 0.1µF 105k 1% SS VC SHDN/ UVLO VIN Q1 INTVCC GATE U2 LT3758 SENSE SYNC FB RT 8.66k 1% LOAD 0.01µF D3 17.4k 100pF + GND 36.5k 1% 13.7k 1% 1µF 1Ω 523Ω 1% 100Ω FB RCS1 0.040Ω U3 PS2801-1 8 | July 2010 : LT Journal of Analog Innovation VIN SENSE DIV2 OV 5.36k 1% DIV1 DIV0 INTVCC VPP OSC GUARD2 GUARD3 GUARD4 SPREAD LT4180EGN DRAIN COMP GND CHOLD1 CHOLD2 CHOLD3 CHOLD4 47pF COUT: MURATA GRM32ER60J107M CLOAD: AVX TPSE477M010R0050 D1: BAV21W D2: UPS840 D3: BAS516 Q1: Si4848DY T1: PULSE ENGINEERING PA1277NL RUN 2.74k 1% 4.7nF 13k 1% 0.015µF ROSC 470pF 470pF 470pF 2200pF 250V COSC 0.1µF 41.2k 1% design features The LT4180 gives power supply designers a valuable new tool, enabling use of Virtual Remote Sensing for accurate load voltage regulation over highly resistive interconnections. Virtual Remote Sensing provides alternatives previously unavailable for simplifying or improving designs. opto-isolator U3. While not shown in this design, it is also possible to provide an opto-isolated OSC signal from the LT4180 to a power supply for synchronization. In contrast, a Virtual Remote Sense system produces excellent regulation at the load, with none of the drawbacks of wired remote sense. Unlike other compensation schemes such as negative resistance, Virtual Remote Sensing continuously corrects the output—even if the linedrop resistance changes—by determining real-time wire drops and connector drops. The additional noise on the power supply lines from the Virtual Remote Sense circuitry is easily removed by the capacitor at the load, which is always included in remote sense systems anyway. Figure 15 shows a buck regulator capable of supplying 12V at 1.5A to a load with up to 2.5Ω of wiring resistance. 470µF load decoupling capacitance is recommended. Input voltage range is 22V to 36V. CONCLUSION While conventional 2-wire remote sensing gives proper voltage at the load, there are many drawbacks. The sense wires are an additional cost in the system as well as consuming connector space for the system. Reliability issues can occur if the sense wires are disconnected or broken. the cost of adding a VRS IC to a power supply system is much less than laying wires for traditional remote sensing. The LT4180 gives power supply designers a valuable new tool to accurately regulate load voltage over highly resistive interconnections. Virtual Remote Sensing provides alternatives previously unavailable for simplifying or improving designs. The LT4180 VRS works with virtually any power supply or regulator: switching or linear, isolated or non-isolated, synchronized or unsynchronized. It contains a VRS regulation circuit and a variety of features such as undervoltage and overvoltage lockout, and opto-isolator drivers. n The LT4180 can interface with IC regulators as well as preconfigured purchased offline supplies. In most cases, Figure 15. A VRS buck converter VIN 22V TO 36V RSENSE 0.033Ω 1% + CIN1 22µF 50V CIN2 1µF 50V INTVCC 30.1k CRUN 0.1µF 50V LT3685 PG D1 68.1k 1% SYNC LOAD 1µF 3.65k 1% COUT 22µF 25V FB RUN VIN SENSE DIV2 DIV1 DIV0 INTVCC VPP OSC GUARD2 GUARD3 GUARD4 SPREAD GND 2k 1% OV VC INTVCC 5.36k 1% D2 LT4180EGN DRAIN COMP CHOLD1 CHOLD2 CHOLD3 CHOLD4 47pF 1k CIN1: 22µF 50V CIN2: 1µF 50V COUT: TAIYO YUDEN TMK325 BJ226MM D1: DFLS240 D2: CMDSH-3 L1: VISHAY 1HLP2020CZ-11 + 1µF 61.9k 1% L1 10µH VIN BD BOOST SW RUN/SD FB RT 10k CLOAD 470µF 25V TOTAL RWIRE ≤ 2.5Ω 0.47µF 100k VOUT 12V, 1.5A 4.7nF COSC 470pF 470pF ROSC 330pF 10nF 22.1k 1% 31.6k 1% 3.3nF July 2010 : LT Journal of Analog Innovation | 9