[Guide]Data processing ICs such as Field Programmable Gate Array (FPGA), System-on-Chip (SoC), and microprocessor continue to expand their applications in the fields of telecommunications, networking, industry, automobiles, avionics, and defense systems. A common feature of these systems is the continuous increase in processing power, which leads to a corresponding increase in raw power requirements. The designer is well aware of the thermal management of high-power processors, but may not consider the thermal management of the power supply. Similar to the transistor package processor itself, when low core voltage requires high current, thermal problems are unavoidable in the worst case-this is the overall power trend of all data processing systems.
Overview of DC-DC converter requirements: EMI, conversion ratio, size, and heat dissipation considerations
Generally, FPGA/SoC/microprocessors require multiple power rails, including 5 V, 3.3 V, and 1.8 V for peripheral and auxiliary power supplies, 1.2 V and 1.1 V for DDR4 and LPDDR4, and processing cores 0.8 V. The DC-DC converters that generate these power rails usually get 12 V or 5 V input voltage from a battery or intermediate DC bus. In order to reduce the DC voltage of these power supplies to the lower voltage required by the processor, switch-mode step-down converters are naturally chosen because of their high efficiency at large step-down ratios. There are hundreds of types of switch-mode converters, but many can be classified as controllers (external MOSFETs) or monolithic regulators (internal MOSFETs). Let’s take a look at the former first.
Traditional controller solutions may not meet the requirements
Traditional switch-mode controller ICs drive external MOSFETs and have external feedback control loop compensation components. The resulting converter is highly efficient and versatile, while providing high power, but the number of discrete components required makes the design relatively complex and difficult to optimize. External switches also limit the switching speed, which is a problem when space is at a premium, such as in an automobile or avionics environment, because the lower switching frequency causes the overall component to be larger.
On the other hand, a monolithic regulator can greatly simplify the design. This article discusses the overall solution in depth, and first introduces the “reducing size while improving EMI” part.
Don’t ignore the minimum on and off time
Another important consideration is the minimum on and off time of the converter, or its ability to operate at a duty cycle sufficient to drop from the input voltage to the output voltage. The larger the step-down ratio, the lower the minimum on-time required (also depends on the frequency). Likewise, the minimum off time corresponds to the differential pressure: how low the input voltage can drop before the output voltage is no longer supported. Although the benefit of increasing the switching frequency is that the overall solution is smaller, the minimum on and off time will set the upper limit of the operating frequency. In short, the lower these values, the more leeway there is when designing small sizes and high power densities.
Pay attention to real EMI performance
For other noise-sensitive devices to operate safely, they also need to have excellent EMI performance. In industrial, telecommunications or automotive applications, an important point in power supply design is to minimize EMI. In order to enable complex Electronic systems to work together and not cause problems due to EMI overlap, strict EMI standards such as CISPR 25 and CISPR 32 radiated EMI specifications are adopted. To meet these requirements, traditional power methods reduce EMI by slowing down the switching edge and lowering the switching frequency-the former reduces efficiency and improves heat dissipation, while the latter reduces power density.
The reduced switching frequency may also violate the EMI requirements for the 530 kHz to 1.8 MHz AM band in the CISPR 25 standard. Mechanical mitigation techniques can be used to reduce noise levels, including complex and large-size EMI filters or metal shields, but these techniques not only increase a lot of costs, but also increase circuit board space, component count and assembly complexity, and further increase heat Complicated management and testing. None of these strategies can meet the requirements of small size, high efficiency and low EMI.
Reduce size while improving EMI, thermal performance and efficiency
Obviously, the power system design has become very complicated, which has brought a heavy burden to the system designer. In order to reduce this burden, a good strategy is to find a power IC solution that can solve many problems at the same time: reduce the complexity of the circuit board, work efficiently, minimize heat dissipation, and generate low EMI. Power ICs that can support multiple output channels can further simplify design and production.
A monolithic power IC with a switch integrated in the package can achieve many of these goals. For example, Figure 1 shows a complete dual output solution board, illustrating the compactness and simplicity of a monolithic regulator. The integrated MOSFET and built-in compensation circuit in the IC used here require only a few external components. The total core size of this solution is only 22 mm × 18 mm, partly through a relatively high 2 MHz switching frequency.
Figure 1. A compact, high-switching frequency, high-efficiency solution with excellent EMI performance.
The schematic diagram of this circuit board is shown in Figure 2. In this solution, the converter uses two channels of the LT8652S, runs at a frequency of 2 MHz, and generates 3.3 V at 8.5 A and 1.2 V at 8.5 A. This circuit can be easily modified to produce output combinations including 3.3 V and 1.8 V, 3.3 V and 1 V, etc. Or, in order to take advantage of the wide input range of the LT8652S, the LT8652S can be used as a secondary converter, and then use a 12 V, 5 V or 3.3 V pre-regulator to improve overall efficiency and power density performance. Due to high efficiency and excellent thermal management, the LT8652S can simultaneously provide 8.5 A for each channel, 17 A for parallel output, and up to 12 A for single-channel operation. With an input range of 3 V to 18 V, the device can cover most input voltage combinations for FPGA/SoC/microprocessor applications.
Figure 2. Dual output, 2 MHz, 3.3 V/8.5 A and 1.2 V/8.5 A applications using two channels of the LT8652S.
Performance of dual output, monolithic regulator
Figure 3 shows the measured efficiency of the solution shown in Figure 1. For single-channel operation, using this solution, when the input voltage is 12 V, the peak efficiency of the 3.3 V power rail reaches 94%, and the peak efficiency of the 1.2 V power rail reaches 87%. For dual-channel operation, the LT8652S achieves 90% peak efficiency per channel at 12 V input voltage and 86% full load efficiency per channel at 8.5 A load current.
Due to the off-time skip function, the extended duty cycle of the LT8652S is close to 100%, and the output voltage is adjusted using the lowest input voltage range. The typical minimum on-time of 20 ns even makes it possible to operate the regulator at high switching frequencies, directly generating an output voltage of less than 1 V from a 12 V battery or DC bus-ultimately reducing the overall solution size and cost, while avoiding AM frequency band. Silent Switcher® 2 technology with integrated bypass capacitors prevents possible layout or production issues, thereby avoiding the impact of excellent benchtop EMI and efficiency performance.
Figure 3. Single and dual output efficiency with 2 MHz switching frequency.
Differential voltage detection for high current loads
For high current applications, every inch of PCB wiring will cause a significant voltage drop. For typical low-voltage, high-current loads that require a very narrow voltage range in modern core circuits, voltage drops can cause serious problems. The LT8652S provides a differential output voltage detection function, allowing customers to create Kelvin connections to achieve output voltage detection and feedback directly from the output capacitor. It can correct the output ground line potential up to ±300 mV. Figure 4 shows that the LT8652S uses differential detection to adjust the load of two channels.
Figure 4. LT8652S uses differential detection function for load adjustment.
Monitor output current
In some high-current applications, output current information must be collected for telemetry and diagnosis. In addition, limiting the maximum output current or reducing the output current according to the operating temperature can prevent damage to the load. Therefore, constant voltage and constant current operation are required to accurately adjust the output current. The LT8652S uses the IMON pin to monitor and reduce the effective regulation current of the load.
When IMON sets the regulation current to the load, you can configure IMON according to the resistance between IMON and GND to reduce this regulation current. The load/circuit board temperature derating can be set using a positive temperature coefficient thermistor. When the circuit board/load temperature rises, the IMON voltage increases. In order to reduce the regulation current, the IMON voltage is compared with the internal 1 V reference voltage to adjust the duty cycle. The IMON voltage can be lower than 1 V, but this will not affect it. Figure 5 shows the output voltage and load current curves before and after activating the IMON current loop.
Figure 5. LT8652S output voltage and current curve.
Low electromagnetic radiation (EMI)
In order to make complex electronic systems work, strict EMI standards are applied to individual component solutions. In order to maintain consistency in multiple industries, various standards have been widely adopted, such as the CISPR 32 industrial standard and the CISPR 25 automotive standard. In order to obtain excellent EMI performance, the LT8652S uses the leading Silent Switcher 2 technology in the EMI elimination design, and uses an integrated loop capacitor to minimize the size of the noisy antenna. Coupled with integrated MOSFET and small size, the LT8652S solution can provide excellent EMI performance. Figure 6 shows the EMI test results of the LT8652S standard demo board shown in Figure 1. Figure 6a shows the CISPR 25 radiated EMI results of the peak detector, and Figure 6b shows the CISPR 32 radiated EMI results.
Figure 6. Radiated EMI test results of the application circuit in Figure 1. VIN = 14 V, VOUT1 = 3.3 V/8.5 A, VOUT2 = 1.2 V/8.5 A.
Parallel operation for higher current and better thermal performance
With the soaring of data processing speed and the doubling of data volume, in order to meet these needs, the capabilities of FPGA and SoC have also expanded. The power supply requires power, and the power supply should maintain power density and performance. However, the advantages of simplicity and robustness cannot be lost in order to increase power density. For processor systems that require more than 17 A current capability, multiple LT8652S can be connected in parallel and run in out-of-phase.
Figure 7 shows that two converters connected in parallel can provide 34 A output current at 1 V. By connecting CLKOUT of U1 to SYNC of U2, the master unit clock is synchronized with the slave unit. The resulting 90° phase difference per channel reduces the input current ripple and spreads the thermal load to the circuit board.
Figure 7. A 4-phase, 1 V/34 A, 2 MHz solution suitable for SoC applications.
To ensure better current sharing during steady state and during startup, connect VC, FB, SNSGND and SS together. It is recommended to use a Kelvin connection for accurate feedback and noise immunity. Place as many thermal vias as possible on the bottom layer near the ground pin to improve thermal performance. The ceramic capacitor of the input thermal circuit should be placed close to the VIN pin.
Because driving conditions may change drastically, frequently, and quickly, SoC must adapt to the rapidly changing load in time. Therefore, the load transient requirements imposed by the automotive SoC may be difficult to meet. The load current slew rate of the peripheral power supply reaches 100 A/μs, and the slew rate of the core power supply is even higher, which is very common. However, under fast load current slew rates, the voltage transient of the power supply output must be minimized. The fast switching frequency of >2 MHz can quickly recover from transients, and the output voltage deviation is minimal. Figure 7 shows the correct loop compensation component values for fast switching frequency and stable dynamic loop response. In the circuit board layout, it is also very important to minimize the circuit inductance from the output capacitance of the circuit to the load.
Figure 8. Load transient response of the circuit of Figure 7.
The processing power of FPGAs, SoCs, and microprocessors continues to increase, and raw power requirements increase accordingly. As the number of required power rails and their carrying capacity increase, it is necessary to consider the design of small power supply systems and speed up system performance. The LT8652S is a current mode, 8.5 A, 18 V synchronous Silent Switcher 2 step-down regulator with an input voltage range of 3 V to 18 V. It is suitable for input source applications from single-cell lithium-ion batteries to automotive inputs.
The operating frequency range of the LT8652S is 300 kHz to 3 MHz, allowing designers to minimize the size of external components and avoid critical frequency bands such as FM radio. Silent Switcher 2 technology can guarantee excellent EMI performance without sacrificing the switching frequency and power density, nor the switching speed and efficiency. Silent Switcher 2 technology also integrates all necessary bypass capacitors in the package, which can greatly reduce unexpected EMI that may be caused by layout or production, thereby simplifying design and production.
Burst Mode® operation reduces the quiescent current to only 16 μA while keeping the output voltage ripple at a low value. The 4 mm × 7 mm LQFN package and very few external components ensure a compact form factor while minimizing the cost of the solution. The 24 mΩ/8 mΩ switch of the LT8652S provides over 90% efficiency, and the programmable undervoltage lockout (UVLO) optimizes system performance. The remote differential detection of the output voltage maintains high accuracy in the entire load range, and is not affected by the line impedance, thereby greatly reducing the possibility of load damage caused by external changes. Other features include internal/external compensation, soft start, frequency foldback and thermal shutdown protection.