Low noise, high current, compact size... Do you love this DC-DC converter?

Typically, an FPGA/SoC/microprocessor requires multiple power rails, including 5 V, 3.3 V, and 1.8 V for peripheral and auxiliary supplies, 1.2 V and 1.1 V for DDR4 and LPDDR4, and 0.8 V for the processing core. The DC-DC converters that generate these rails typically take a 12 V or 5 V input voltage from a battery or intermediate DC bus. To step down these supply DC voltages to the lower voltages required by the processor, switch-mode buck converters are a natural choice because of their high efficiency at large step-down ratios. There are hundreds of types of switch-mode converters, but many can be categorized as either controllers (external MOSFETs) or monolithic regulators (internal MOSFETs). Let's look at the former first.

Traditional switch-mode controller ICs drive external MOSFETs with external feedback control loop compensation components. The resulting converters are highly efficient and versatile while delivering high power, but the number of discrete components required makes the design relatively complex and difficult to optimize. The external switches also limit switching speed, which is a problem when space is at a premium, such as in automotive or avionics environments, as lower switching frequencies result in a larger overall component size.

On the other hand, a monolithic regulator can greatly simplify the design. This article discusses the overall solution in depth, starting with the "Reduce Size While Improving EMI" section.

Another important consideration is the converter’s minimum on and off times, or its ability to operate at a duty cycle sufficient to step down from the input voltage to the output voltage. The larger the step-down ratio, the lower the required minimum on time (also dependent on frequency). Likewise, the minimum off time corresponds to dropout voltage: how low the input voltage can drop before the output voltage is no longer supported. While the benefit of increasing the switching frequency is a smaller overall solution, the minimum on and off times set an upper limit on the operating frequency. In general, the lower these values, the more leeway there is in designing for small size and high power density.

Excellent EMI performance is also required for other noise-sensitive devices to operate safely. In industrial, telecommunication or automotive applications, a key focus of power supply design is to minimize EMI. In order to enable complex electronic systems to work together without problems caused by EMI overlap, strict EMI standards such as CISPR 25 and CISPR 32 radiated EMI specifications are adopted. To meet these requirements, traditional power supply methods reduce EMI by slowing down switching edges and reducing switching frequency - the former reduces efficiency and improves heat dissipation, while the latter reduces power density.

The reduced switching frequency may also violate the 530 kHz to 1.8 MHz AM band EMI requirements in the CISPR 25 standard. Mechanical mitigation techniques can be used to reduce noise levels, including complex, large-sized EMI filters or metal shielding, but these techniques not only add significant cost, but also increase board space, component count, and assembly complexity, and further complicate thermal management and testing. None of these strategies can meet the requirements of small size, high efficiency, and low EMI.

It is clear that power system design has become very complex, which places a heavy burden on system designers. To reduce this burden, a good strategy is to look for power IC solutions with functions that solve many problems at the same time: reduce board complexity, operate at high efficiency, 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 the switch integrated into the package can achieve many of these goals. For example, Figure 1 shows a complete dual-output solution board that illustrates the compact simplicity of a monolithic regulator. The integrated MOSFETs and built-in compensation circuitry in the IC used here require only a few external components. The total core size of this solution is only 22 mm × 18 mm, which is made possible in part by the relatively high 2 MHz switching frequency.

The schematic of this board is shown in Figure 2. In this solution, the converter uses two channels of the LT8652S, running at 2 MHz and generating 3.3 V at 8.5 A and 1.2 V at 8.5 A. This circuit can be easily modified to generate output combinations including 3.3 V and 1.8 V, 3.3 V and 1 V, etc. Alternatively, to take advantage of the wide input range of the LT8652S, the LT8652S can be used as a secondary converter with a 12 V, 5 V, or 3.3 V preregulator to improve overall efficiency and power density performance. Due to high efficiency and excellent thermal management, the LT8652S can provide 8.5 A for each channel simultaneously, 17 A for parallel outputs, and up to 12 A for single-channel operation. With a 3 V to 18 V input range, the device can cover most input voltage combinations for FPGA/SoC/microprocessor applications.

Figure 3 shows the measured efficiency of the solution shown in Figure 1. For single-channel operation, this solution achieves a peak efficiency of 94% for the 3.3 V rail and 87% for the 1.2 V rail at an input voltage of 12 V. For dual-channel operation, the LT8652S achieves a peak efficiency of 90% per channel at a 12 V input voltage and a full load efficiency of 86% per channel at 8.5 A load current.

Thanks to the off-time skipping feature, the LT8652S’s extended duty cycle approaches 100%, regulating the output voltage using the lowest input voltage range. The 20 ns typical minimum on-time even makes it possible to operate the regulator at high switching frequencies, generating output voltages less than 1 V directly from a 12 V battery or DC bus—ultimately reducing overall solution size and cost while avoiding the AM band. Silent Switcher ^® 2 technology with integrated bypass capacitors prevents possible layout or production issues that could compromise the excellent benchtop EMI and efficiency performance.

For high current applications, every inch of PCB trace causes a significant voltage drop. For typical low voltage, high current loads in modern core circuits that require a very narrow voltage range, the voltage drop can cause serious problems. The LT8652S provides differential output voltage sensing, allowing customers to create a Kelvin connection for output voltage sensing and feedback directly from the output capacitor. It can correct the output ground line potential up to ±300 mV. Figure 4 shows the LT8652S using differential sensing to load regulate both channels.

In some high current applications, output current information must be collected for telemetry and diagnostics. In addition, limiting the maximum output current or reducing the output current based on the operating temperature can prevent damage to the load. Therefore, constant voltage, constant current operation is required to accurately regulate 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, IMON can be configured to reduce this regulation current based on the resistor between IMON and GND. Load/board temperature derating can be set using a positive temperature coefficient thermistor. As the board/load temperature rises, the IMON voltage increases. To reduce the regulation current, the IMON voltage is compared to an internal 1 V reference voltage to adjust the duty cycle. The IMON voltage can be lower than 1 V, but this will have no effect. Figure 5 shows the output voltage and load current curves before and after activating the IMON current loop.

In order to enable complex electronic systems to work, strict EMI standards are applied to single component solutions. To maintain consistency across multiple industries, various standards such as CISPR 32 industrial standard and CISPR 25 automotive standard are widely adopted. To achieve excellent EMI performance, LT8652S adopts leading Silent Switcher 2 technology in EMI elimination design and uses integrated loop capacitors to minimize the size of noisy antennas. Coupled with integrated MOSFETs and small size, the LT8652S solution provides excellent EMI performance. Figure 6 shows the EMI test results of the LT8652S standard demonstration 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.

As data processing speeds soar and data volumes multiply, the capabilities of FPGAs and SoCs are expanding to meet these demands. Power supplies require power, and they should maintain power density and performance. However, simplicity and robustness cannot be sacrificed to increase power density. For processor systems that require more than 17 A current capability, multiple LT8652S can be paralleled and run out of phase.

Figure 7 shows two converters in parallel that can deliver 34 A output current at 1 V. The master clock is synchronized with the slave by connecting U1’s CLKOUT to U2’s SYNC. The resulting 90° phase difference per channel reduces input current ripple and spreads the heat load across the board.

To ensure better current sharing in steady state and during startup, connect VC, FB, SNSGND, and SS together. Kelvin connections are recommended for accurate feedback and noise immunity. Place as many thermal vias as possible to the bottom layer near the ground pins to improve thermal performance. The ceramic capacitor of the input hot loop should be placed close to the VIN pin.

Load transient requirements imposed by automotive SoCs can be difficult to meet because driving conditions can change dramatically, frequently, and quickly, and the SoC must adapt to rapidly changing loads in a timely manner. It is common to see load current slew rates of 100 A/μs for peripheral supplies and even higher for core supplies. However, voltage transients at the output of the power supply must be minimized under fast load current slew rates. Fast switching frequencies of >2 MHz provide fast recovery from transients with minimal output voltage excursions. Figure 7 shows the correct loop compensation component values ​​that take advantage of fast switching frequencies and a stable dynamic loop response. It is also critical to minimize the trace inductance from the circuit output capacitor to the load in the board layout.

As the processing power of FPGAs, SoCs, and microprocessors continues to increase, the raw power requirements are also increasing accordingly. As the number of required power rails and their carrying capacity increase, consideration must be given to designing small power systems and accelerating 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 for input source applications from a single Li-Ion battery to an automotive input.

The LT8652S operates from 300 kHz to 3 MHz, allowing designers to minimize external component size and avoid critical frequency bands such as FM broadcast. Silent Switcher 2 technology guarantees excellent EMI performance without sacrificing switching frequency and power density, or switching speed and efficiency. Silent Switcher 2 technology also integrates all necessary bypass capacitors in the package, minimizing unexpected EMI that may be caused by layout or production, thereby simplifying design and production.

Burst Mode ^® operation reduces quiescent current to only 16 μA while keeping output voltage ripple low. A 4 mm × 7 mm LQFN package and very few external components ensure a compact form factor while minimizing solution cost. The LT8652S’s 24 mΩ/8 mΩ switches deliver over 90% efficiency, while programmable undervoltage lockout (UVLO) optimizes system performance. Remote differential sensing of the output voltage maintains high accuracy over the entire load range while being unaffected by line impedance, minimizing the possibility of load damage from external variations. Other features include internal/external compensation, soft-start, frequency foldback, and thermal shutdown protection.