LTC3308AIV#TRMPBF

Vol 53 No 4, October 2019 High Power Density in a Small Form Factor Steve Knoth Background Sophisticated high power density digital integrated circuits (ICs), such as graphics processor units (GPUs) and field programmable gate arrays (FPGAs), can be found in a broad range of feature-rich electronic environments, including: XX Automotive XX Medical XX Telecom XX Datacom XX Industrial XX Communications XX Gaming XX Consumer audio/video Switching Regulators vs. Charge Pumps vs. LDO Regulators With this level of market penetration, it is no surprise that the global demand for high current, low voltage digital ICs is exploding. The current global market is assessed at more than US $1.8B, and this is expected to rise annually by 10.87% to reach US $3.7B over the 2018 to 2025 period. As one of the biggest slices of this market, FPGAs account for a projected US $1.53B by the end of 2025. The rest of the digital IC market is represented by GPUs, microcontrollers and microprocessors, programmable logic devices (PLDs), digital signal processors (DSPs), and application-specific integrated circuits (ASICs). High power density digital ICs have penetrated virtually every embedded system. FPGAs enable cutting-edge applications in the market segments listed above. For example, in automotive applications, advanced driver assistance systems (ADASs) and collision avoidance systems prevent catastrophe due to human error. Likewise, government-mandated safety features such as antilock brake systems, stability control, and electronically controlled independent suspension systems require FPGAs to function. In consumer electronics, the demand for Internet of Things (IoT) functionality, sophisticated graphic engine functionality, and machine-to-machine (M2M) functionality call for advanced digital ICs. Massive data storage and cloud computing centers and expansive networks of optical networking modules drive the need for FPGAs and digital ICs.   //  These digital ICs are powerful, yet temperamental, especially regarding power requirements. Traditionally, efficient switching regulator controllers that drive high power MOSFETs have been used to power FPGAs and ASICs, but these controller-based power schemes have potential noise interference issues, relatively slow transient response, and layout limitations. In recent years, small and quiet low dropout (LDO) regulators that minimize heat have been used as an alternative, but not without their own set of limitations. Recent power conversion innovations have introduced high power monolithic switching regulators that are able to efficiently power digital ICs with low noise and high efficiency while minimizing space requirements.   //    //    //    //  Visit analog.com Low voltage, high current step-down conversion and regulation can be achieved via a variety of methods, each with its own performance and design trade-offs. Switching regulator controllers feature high efficiency at high load currents over a wide range of voltages, but they require several external components such as inductors, capacitors, and FETs to operate; and they can be a source of high and low frequency noise. Inductorless charge pumps (or switched capacitor voltage converters) can also be used to produce low voltages, but are limited in output current capability, suffer from poor transient performance, and require several external components. For these reasons, charge pumps are not commonly found in digital IC power applications. Linear regulators—especially LDO regulators—are simple in that they only require two external capacitors to operate. However, they may be power limited depending on the size of the input-to-output voltage differential across the IC and how much current is demanded by the load, plus the thermal resistance characteristics of the package. This certainly limits their ability to power digital ICs. Monolithic Buck Converter Design Challenges Moore’s Law has proven to be visionary and valid since its debut in 1965. Wafer fabrication technology line widths are continually being reduced, pushing digital IC voltages lower. Smaller geometry processes allow higher integration of more power-hungry features in the end product. For example, modern computer servers and optical communication routing systems demand higher bandwidth to process more computing data and internet traffic; these systems also generate a lot of heat, and therefore highly efficient ICs are required. Cars have more on-board electronics for entertainment, navigation, self-driving features, and even engine control. As a result, there is an increase in both the system’s current consumption and the associated total power required. Therefore, state-of-the-art packaging and innovative, internal power-stage design are required to drive the heat out of the power IC while delivering unprecedented power. XX Multiphase operation enables parallel operation for high output currents and reduced ripple XX Low EMI to meet low system noise requirements XX Synchronous operation for high efficiency and minimal power loss XX Easy design simplifies design cycle, qualification, and testing High power supply rejection ratio (PSRR) and low output voltage noise, or ripple, are important considerations. A device with high supply rejection can filter and reject noise at the input, resulting in a clean and stable output. Furthermore, power solutions with low output voltage noise across a wide bandwidth or low output ripple are desirable—modern digital systems have several rails where noise sensitivity is a major design consideration. As speed requirements for high end FPGAs increase, supply noise tolerance decreases in order to minimize bit errors. Noise-induced digital faults drastically reduce the effective data throughput speeds for these high speed PLDs. Input supply noise at high current is one of the more demanding specifications placed on power supplies. XX Very low output ripple XX Fast transient response time XX Operation over a wide input/output voltage range XX High output current capability XX Excellent thermal performance XX Compact footprint Higher transceiver speeds—in FPGAs, for example—dictate high current levels due to high power consumption from fine geometry circuit switching. These ICs are fast. They may cycle load current from near-zero to several amps within tens to hundreds of nanoseconds, requiring a regulator with ultrafast transient response. With board area reserved for the power regulators ever-decreasing, many system designers turn to monolithic switching regulators operating at fast switching frequencies to reduce the size of external components and total solution size—accepting the trade-off of some efficiency loss due to switching losses at higher frequencies. This trade-off is eliminated by a new generation of monolithic switching regulators. These new regulators feature synchronous operation with integrated high-side and low-side switches, allowing for tight control of switch gate voltages, greatly reducing dead time, and resulting in higher efficiencies even at high frequencies. One of the biggest challenges with high current monolithic switchers is their ability to dissipate heat that results from power loss in the IC. This challenge can be met by using multiple power and ground pins, plus thermally enhanced laminate-based packages with copper (Cu) pillars where the heat can be easily transferred from the IC into the board. The large copper planes on the board connected to these power pins allow for the heat to spread more evenly. These features can be found in Analog Devices’ Power by Linear™ LTC33xx family of monolithic high, medium, and low current buck regulators. The highest current member is the LTC3310S, a 5 V, 10 A, high power density, low EMI, Silent Switcher® 2, monolithic, synchronous buck converter in a 9 mm2 package (power density = 1.11 A/mm2). The device’s fixed frequency peak current-mode architecture is ideal for applications that demand fast transient response. The LTC3310S uses the Silent Switcher 2 architecture with integrated hot loop bypass capacitors to deliver a highly efficient, small footprint solution at frequencies up to 5 MHz with excellent EMI performance. Multiphase operation enables direct paralleling of up to four devices for higher current up to 40 A. The LTC3310S’ 2.25 V to 5.5 V input range supports a wide variety of applications, including most intermediate bus voltages. Integrated low on-resistance MOSFETs deliver continuous load currents as high as 10 A with minimal thermal derating. Output voltages ranging from 0.5 V to VIN are ideal for point-of-load applications such as high current, low voltage DSP/FPGA/GPU/ASIC designs. Other key applications include optical networking, telecom/datacom, automotive systems, distributed power architectures, or any medium-to-high power density systems. Figure 1 shows the simplicity of a typical design, while Figure 2 shows how easy it is to produce a 4-phase, 40 A configuration. VIN 2.25 V to 5 V 22 µF New Family of Silent Switcher Buck Converters It’s clear that buck converter solutions for high performance digital ICs must have the following attributes: XX Fast switching frequency to minimize the size of external components XX Zero dead-time design to maximize efficiency at high frequency XX Monolithic chip on-board power device for smaller solution size 2 // Analog Dialogue 53-10, October 2019 22 µF EN 0.1 µF 10 kΩ VIN SW MODE/SYNC LTC3310S FB PGOOD SSTT AGND ITH RT PGND 100 nH 274 kΩ 220 pF L = XEL4030-101ME, COILCRAFT Figure 1. LTC3310S typical application. VOUT 1.2 V 10 A 15 pF 140 kΩ 22 µF ×3 100 kΩ VIN 2.25 V to 5.5 V 1 µF 47 µF 0.22 µF 0.22 µF VIN PGOOD MODE/SYNC SW EN 100 nH 6.8 pF LTC3310S ITH 10 kΩ FB RT SSTT 390 pF 47 µF 140 kΩ 100 kΩ AGND PGND 22 µF ×2 10 µF ×2 VOUT 1.2 V 40 A 274 kΩ 0.1 µF VIN 1 µF 47 µF 0.22 µF 0.22 µF VIN PGOOD MODE/SYNC SW EN LTC3310S 100 nH 22 µF ×2 10 µF ×2 VIN FB ITH 301 kΩ AGND PGND 47 µF 90° RT 100 kΩ VIN 1 µF 47 µF 0.22 µF VIN EN 0.22 µF PGOOD MODE/SYNC 100 nH SW LTC3310S 22 µF ×2 10 µF ×2 VIN FB ITH AGND PGND 47 µF 180° RT VIN 1 µF 47 µF 0.22 µF 0.22 µF VIN EN PGOOD MODE/SYNC 100 nH SW LTC3310S VIN FB ITH 100 kΩ AGND PGND 47 µF 22 µF ×2 10 µF ×2 270° RT 301 kΩ L = COILCRAFT, XEL4030-101ME Figure 2. Four LTC3310S monolithic regulators in parallel, forming a 4-phase, 40 A step-down regulator. The LTC3310S’ low 35 ns minimum on-time enables a large step-down ratio at high frequency, and 100% duty-cycle operation delivers low dropout performance when the input and output voltages are close in value. The operating frequency can be synchronized to an external clock. The LTC3310S total reference voltage accuracy is better than ±1% over the –40°C to +125°C operating junction temperature range. Additional features include a power good signal when the output is in regulation, precision enable threshold, output overvoltage protection, thermal shutdown, die temperature monitor, programmable soft start, tracking, clock synchronization, mode selection, and output short-circuit protection. The LTC3310S is available in a thermally enhanced 18-lead, 3 mm × 3 mm × 0.94 mm LQFN package. The E- and I-grades are specified from the –40°C to +125°C operating junction temperature range, while the J- and H-grades are specified from the –40°C to +150°C operating junction temperature range. High Efficiency, Low EMI, and Fast Transient Response Silent Switcher buck regulator designs offer high efficiency at high switching frequencies (>2 MHz) with ultralow electromagnetic interference (EMI) emissions, offering very compact and quiet step-down solutions. The Silent Switcher family uses special design and packaging techniques to enable >92% efficiency at 2 MHz while easily passing the CISPR 25 Class 5 peak EMI limits. The next-generation Silent Switcher 2 technology internal construction uses copper pillars in lieu of bond wires, adds internal bypass capacitors, and an integrated substrate ground plane to further improve EMI, which is not sensitive to PCB layout, simplifying designs and reducing performance risks. Visit analogdialogue.com // 3 The “S” in the LTC3310S part number indicates its second-generation Silent Switcher technology. The IC has integrated VIN ceramic capacitors to keep all fast ac current loops small, improving the EMI performance. This technology allows fast switching edges for high efficiency at high switching frequencies, while simultaneously achieving good EMI performance (see Figure 3, Figure 4, and Figure 5). Furthermore, it allows for faster, cleaner, low overshoot switching edges, greatly improving efficiency at high switching frequencies. The graph in Figure 6 shows the LTC3310S’s high efficiency performance. 50 Vertical Polarization 45 Peak Detector 40 Amplitude (dBµV/m) 35 30 25 20 15 60 10 50 40 0 0 100 200 300 30 400 500 600 700 10 Figure 5. Radiated emissions for vertical polarization. 0 –10 Peak Limit Peak 0 0 10 20 30 40 50 60 70 80 90 100 110 DC2629A Demo Board (with EMI Filter Installed) 3.3 V Input to 1.2 V Output at 7.5 A, fSW = 2 MHz Figure 3. CISPR 25 conducted EMI emissions with Class 5 peak limits (voltage method). Efficiency (%) Frequency (MHz) 100 3.0 90 2.7 80 2.4 70 2.1 60 1.8 50 1.5 1.2 40 30 50 20 Horizontal Polarization 45 Peak Detector 10 40 Amplitude (dBµV/m) 900 1 GHz DC2629A Demo Board (with EMI Filter Installed) 3.3 V Input to 1.2 V Output at 7.5 A, fSW = 2 MHz 20 0 35 0 1 2 3 4 5 6 7 VIN = 3.3 V VOUT = 1.2 V fOSC = 2 MHz 0.9 Efficiency Power Loss 0.3 8 9 0.6 0 10 Load Current (A) 30 Figure 6. LTC3310S efficiency performance. 25 The LTC3310S’s fixed frequency peak-current mode architecture eases compensation and allows the IC to rapidly respond to transient steps. External compensation components allow the control loop to be optimized for the highest bandwidth and fastest transient response. 20 15 10 Peak Limit Peak 5 0 800 Frequency (MHz) Power Loss (W) Amplitude (dBµV/m) Peak Limit Peak 5 0 100 200 300 400 500 600 700 800 Frequency (MHz) DC2629A Demo Board (with EMI Filter Installed) 3.3 V Input to 1.2 V Output at 7.5 A, fSW = 2 MHz Figure 4. Radiated emissions for horizontal polarization. 900 1 GHz 6 A, 4 A, and 3 A Silent Switcher Bucks in a 2 mm × 2 mm Package For increased power density, first-generation Silent Switcher architecture is a good solution. Silent Switcher topology is like Silent Switcher 2 topology except the VIN bypass capacitors are external instead of within the plastic encapsulation flip-chip laminate style package. For full Silent Switcher, low EMI performance, external VIN bypass capacitors are placed symmetrically, external to the package. This split cap, symmetrical arrangement minimizes the effective hot loop area, thereby reducing EMI and allowing for a smaller package footprint size. LTC3309A, LTC3308A, and LTC3307A are 5 V input regulators that can support 6 A, 4 A, and 3 A (respectively), for high power density, low EMI monolithic synchronous buck conversion. They all operate at up to 3 MHz in a 4 mm2 footprint package (LTC3309A power density = 1.5 A/mm2). 4 // Analog Dialogue 53-10, October 2019 Figure 7 shows a typical LTC3309A application. The fixed frequency peak current-mode architecture is ideal for fast transient response, including fast transient response during Burst Mode® operation (see Figure 8). The LTC3309A features Silent Switcher architecture, utilizing external hot loop bypass capacitors. This design enables highly efficient, small footprint solutions at high operating frequencies with excellent EMI performance. The family’s 2.25 V to 5.5 V input voltage range supports a wide variety of applications, including most intermediate bus voltages, and is compatible with lithium and nickel-based battery types. Integrated low on-resistance MOSFETs deliver continuous load currents as high as 6 A. Output voltages, ranging from 0.5 V to VIN, are ideal for point-of-load applications such as high current/low voltage DSP/FPGA/GPU/ASIC reference designs. Other key applications include telecom/datacom and automotive systems, distributed power architectures, and general-purpose power systems. The LTC3309A, LTC3308A, and LTC3307A operate in forced continuous or pulse skip modes for low noise, or low ripple, low IQ Burst Mode operation for high efficiency at light loads, ideal for battery-powered systems. Low 22 ns minimum on-time enables high step-down ratios even as the power supply operates at high frequency, and 100% duty cycle operation delivers low dropout performance when input and output voltages are the same. The operating frequency can be synchronized to an external clock. The total reference voltage accuracy is better than ±1% over the –55°C to +150°C operating junction temperature range. The device safely tolerates inductor saturation in overload. Additional features include a power good signal when the output is in regulation, internal soft start, precision enable threshold, output overvoltage and short-circuit protection, thermal shutdown, and clock synchronization. The LTC3309A, LTC3308A, and LTC3307A are all pin-compatible devices available in a thermally enhanced, compact, and low profile 12-lead, 2 mm × 2 mm × 0.74 mm LQFN package. The E- and I-grades are specified from a –40°C to +125°C operating junction temperature range. The J- and H-grades are specified from a –40°C to +150°C operating junction temperature range, and the MP-grade is specified from a –55°C to +150°C operating junction temperature range. VIN = 2.25 V to 5.5 V 10 µF 0.1 µF 0201 220 nH 0.1 µF 0201 EN VIN SW SW LTC3309A FB VIN 6.8 pF 10 µF VOUT 1.2 V 6A 140 kΩ 100 kΩ 33 µF ×2 MODE/SYNC AGND RT PGND PGOOD fOSC = 2 MHz Figure 7. LTC3309A typical application circuit. ILOAD 2.5 A/div IL 2 A/div VOUT 50 mV/div 10 µs/div 3.3 VIN to 1.2 VOUT, 2 MHz Typical Application COUT = 66 µF, L = 220 nH Load Step: 0.1 A to 4.5 A in 1 µs Figure 8. LTC3309A transient response in Burst Mode operation. Table 1 compares the features of the members of the LTC33xx Silent Switcher and Silent Switcher 2 family. Table 1. Fault Mode and Supported Range Vendor Part # Topology VIN Range VOUT Range Output Current Efficiency Switching Frequency Control Mode VREF Accuracy Room/Temp Current Limit Accuracy Minimum On-Time Directly Parallelable x Phase? IQ Supply Burst Mode/Non-Burst Package Theta JA Solution Size Package ADI ADI ADI ADI LTC3307A LTC3308A LTC3309A LTC3310S Single synchronous Single synchronous Single synchronous Single synchronous monolithic, Silent Switcher monolithic, Silent Switcher monolithic, Silent Switcher monolithic, Silent Switcher 2 2.25 V to 5.5 V 2.25 V to 5.5 V 2.25 V to 5.5 V 2.25 V to 5.5 V 0.5 V to VIN 0.5 V to VIN 0.5 V to VIN 0.5 V to VIN 3A 4A 6A 10 A 92% 92% 92% 92% (3.3 VIN/1.2 VOUT/2 A) (3.3 VIN/1.2 VOUT/2 A) (3.3 VIN/1.2 VOUT/3 A) (3.3 VIN/1.2 VOUT/2 A) 1 MHz to 3 MHz 1 MHz to 3 MHz 1 MHz to 3 MHz 500 kHz to 5 MHz Constant-frequency, Constant-frequency, Constant-frequency, Constant-frequency, peak current mode peak current mode peak current mode peak current mode ±0.2%/±1% ±0.2%/±1% ±0.2%/±1% ±1% ±15% ±15% ±15% ±9% 22 ns 22 ns 22 ns 35 ns No No No Yes, 4 phase 40 µA BM/1.3 mA 40 µA BM/1.3 mA 40 µA BM/1.3 mA 1.3 mA 51°C/W 51°C/W 51°C/W 40°C/W ~20 mm2 ~20 mm2 ~20 mm2 47 mm2 2 mm × 2 mm × 0.74 mm, 2 mm × 2 mm × 0.74 mm, 2 mm × 2 mm × 0.74 mm, 3 mm × 3 mm × 0.94 mm, 12-lead LQFN 12-lead LQFN 12-lead LQFN 18-lead LQFN Visit analogdialogue.com // 5 Conclusion The trend in high performance digital ICs—such as GPUs, FPGAs, and microprocessors—is rapidly raising current demands coupled with dropping operating voltages, a result of shrinking line width wafer fabrication technologies. Current and voltage demands are only part of the power supply picture. Digital IC advancements come with a host of other requirements, including fast transient response, low EMI, low noise/ripple, and efficient operation to minimize heat. Traditionally, digital ICs has been powered by LDO regulators or inductorbased switching regulator controllers with off-board power devices. With increased power supply performance and space requirements, in many cases these traditional approaches are not up to the task. ADI’s new generation of monolithic power supplies are up to the task, including the LTC3310S, LTC3309A, LTC3308A, and LTC3307A, which support 10 A, 6 A, 4 A, and 3 A, respectively. These high power density Silent Switcher and Silent Switcher 2 buck regulators are housed in thermally efficient, compact flip-chip laminate packages, and have a variety of feature sets to satisfy the requirements of a wide range of digital IC power problems. About the Author Steve Knoth is a senior product marketing manager in Analog Devices’ Power Group. He is responsible for all power management integrated circuit (PMIC) products, low dropout (LDO) regulators, battery chargers, charge pumps, charge pump-based LED drivers, supercapacitor chargers, and low voltage monolithic switching regulators. Prior to rejoining Analog Devices in 2004, Steve held various marketing and product engineering positions since 1990 at Micro Power Systems, Analog Devices, and Micrel Semiconductor. He earned his bachelor’s degree in electrical engineering in 1988 and a master’s degree in physics in 1995, both from San Jose State University. Steve also received an M.B.A. in technology management from the University of Phoenix in 2000. In addition to enjoying time with his kids, Steve is an avid music lover and can be found tinkering with pinball and arcade games or muscle cars, and buying, selling, and collecting vintage toys, movie, sports, and automotive memorabilia. He can be reached at steve.knoth@analog.com. 6 // Analog Dialogue 53-10, October 2019