FEATURE COVER STORY
Ultralow noise, high current DC-to-DC converter for signal and data processing
Dong Wang, applications engineer at Analog Devices, Inc. discusses monolithic solutions and how to significantly reduce device size whilst improving EMI performance
D
ata processing ICs - such as field programmable gate arrays (FPGAs),
systems on a chip (SoCs), and microprocessors - continually increase their reach into telecommunications, networking, industrial, automotive, avionics, and defence systems. One common thread throughout these systems is ever increasing processing power, resulting in corresponding increases in raw power requirements. Designers are well aware of the thermal management problems of high power processors but may not consider the thermal management issues of power supplies. Not unlike the transistor- packed processors themselves, worst- case thermal problems are inevitable when low core voltages require high current - the overall trend for the power supplies for all data processing systems.
EMI, CONVERSION RATIO, SIZE, AND THERMAL CONSIDERATIONS Typically, an FPGA/SoC/microprocessor requires a number of power supply rails, including 5V, 3.3V, and 1.8V for peripheral and auxiliary power, 1.2V and 1.1V for DDR4 and LPDDR4, and 0.8V for processing cores. The dc-to-dc converters that produce these rails typically take inputs of 12V or 5V from a battery or intermediate dc bus. To step down these source dc voltages to the much lower voltages required by a processor, switch-mode buck converters are naturally chosen because of their high efficiency at large step-down ratios. Switch-mode converters come in
hundreds of flavours, but many can be categorised as either controllers (external MOSFETs) or monolithic regulators (internal MOSFETs). Let’s look at the former first. A traditional switch-mode controller IC
drives external MOSFETs and features external feedback control loop compensation components. The resulting converter can be very efficient and
Figure 1:
Compact, high switching frequency, high efficiency solution with excellent EMI performance
DON’T OVERLOOK THE MINIMUM ON- AND OFF-TIMES Another important consideration is a converter’s minimum on- and off-times, or its ability to operate at duty cycles sufficient to step down from the input to the output. The larger the step-down ratio, the lower the required minimum on-time (also depending on frequency). In a similar vein, the minimum off-time corresponds to the dropout voltage: how low the input can go before the output is no longer supported. Although increasing switching frequency has the benefit of an overall smaller solution, minimum on- and off-times set the upper limit for operating frequency. In sum, the lower these values, the more leeway you have in designing for small size and high power density.
versatile, while providing high power, but the number of required discrete components make design relatively complicated and difficult to optimise. External switches can also limit the 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 larger sized components throughout. Monolithic regulators, on the other
hand, can greatly simplify design. This article covers monolithic solutions in depth, starting with the “Reducing Size While Improving EMI” section.
Figure 2:
Dual-output, 2MHz, 3.3V/8.5A and 1.2V/8.5A application using two channels of the LT8652S
PAY ATTENTION TO REAL EMI PERFORMANCE Superior EMI performance is also mandatory for safe operation with other noise sensitive devices. In industrial, telecommunications, or automotive applications, minimising EMI can be a significant priority for power supply designs. To enable complex systems of electronics to work in concert - without problems arising from overlapping EMI - stringent EMI standards have been adopted, such as the CISPR 25 and CISPR 32 radiated EMI specifications. To meet these requirements, traditional power supply approaches reduce EMI by slowing down switching edges and lowering switching frequency - the former resulting in lower efficiency and higher thermal dissipation, and the latter in lower power density. Reduced switching frequency also risks
violating the 530kHz to 1.8MHz AM band EMI requirement in the CISPR 25 standard. Mechanical mitigation techniques can be adopted to reduce noise levels, including complex, bulky EMI filters or metal shielding, but these
14 OCTOBER 2020 | ELECTRONICS
/ ELECTRONICS
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