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COVER STORY FEATURE


add significant costs and bulk in board space, component count, and assembly complexity, while further complicating thermal management and testing. None of these strategies meet the requirements of compact size, high efficiency, and low EMI.


REDUCING SIZE WHILE IMPROVING EMI AND THERMAL PERFORMANCE, AND EFFICIENCY It is clear that power system design has reached a point of complexity that places significant burdens on system designers. To relieve some of this burden, a good strategy is to look for power supply IC solutions with features that solve many problems at once: reducing the complexity on the board, operating at high efficiency, minimising heat dissipation, and producing low EMI. Power ICs that can support multiple output channels further simplify design and production. Monolithic power supply ICs, where the


switches are incorporated into the package, can achieve many of these goals. For instance, Figure 1 shows a complete dual-output solution board, illustrating 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 for this solution is only 22mm × 18mm, achieved in part by the relatively high, 2MHz switching frequency. The schematic for this board is shown


in Figure 2. In this solution, the converter runs at 2MHz and produces 3.3V at 8.5A and 1.2V at 8.5A, using two channels of an LT8652S. This circuit can be easily modified to produce output combinations including 3.3V and 1.8V, 3.3V and 1V, etc. Or, to take advantage of the LT8652S’s wide input range, the LT8652S can be used as the second stage converter, following a 12V, 5V, or 3.3V pre- regulator, to enhance the total efficiency and power density performance. The LT8652S can deliver 8.5A for each channel at the same time, 17A for parallel output, and up to 12A for single- channel operation, due to its high efficiency and great thermal management. With a 3V to 18V input range, it can cover most input voltage combinations required for FPGA/SoC/microprocessor applications.


PERFORMANCE OF A DUAL-OUTPUT, MONOLITHIC REGULATOR Figure 3 shows the measured efficiencies for the solution in Figure 1. For single-


channel operation, the solution achieves 94% peak efficiency for a 3.3V rail and 87% peak efficiency for a 1.2V rail with 12V input voltage. For dual-channel operation, the LT8652S has 90% peak efficiency with 12V input and 86% full load efficiency at 8.5A load current for each channel. Due to the off-time skipping function, the LT8652S has an extended duty cycle close to 100%, regulating output voltage with the lowest input voltage range. The 20ns typical minimum on-time even makes it possible to operate the regulator at a high switching frequency, generating a less than 1V output directly from a 12V battery or dc bus - resulting in reduced overall solution size and cost, while avoiding the AM band. Silent Switcher 2 technology with integrated bypass capacitors prevents possible layout or production issues affecting superior benchtop EMI and efficiency performance.


Figure 4:


LT8652S load regulation with differential sensing function.


Figure 3: Efficiency of single and dual output with 2MHz switching frequency


DIFFERENTIAL VOLTAGE SENSING FOR HIGH CURRENT LOADS For high current applications, every linear inch of PCB trace induces significant voltage drop. For the low voltage, high current loads typical in modern core circuitry, which require a very tight voltage range, voltage drops can cause serious issues. The LT8652S features a differential output voltage sensing function, which allows the customer to make a Kelvin connection for output voltage sensing and feedback directly from the output capacitor. It can correct up to ±300mV of the output ground line potential. Figure 4 shows the LT8652S load regulation for both


channels utilising the differential sensing function.


MONITORING OUTPUT CURRENT In some high current applications, output current information must be collected for telemetry and diagnostics purposes. In addition, limiting the maximum output current or derating output current based on 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 pins to monitor and reduce the effective regulated current to the load. While IMON programmes the regulated


current to the load, IMON can be configured to reduce this regulated current based on the resistance between the IMON and GND. The load/board temperature derating is programmed using a positive temperature coefficient thermistor. When the board/load temperature rises, the IMON voltage rises. To reduce the regulated current, the IMON voltage is compared with an internal 1V reference to adjust duty cycle. The IMON voltage can be lower than 1V, but then it will have no effect. Figure 5 shows the output voltage vs. load current profile before and after IMON current loop is activated.


LOW EMI For complex electronics systems to work, stringent EMI standards are applied to the individual component solutions. Standards have been widely adopted for consistency across a number of industries, such as the CISPR 32 for industrial and CISPR 25 for automotive. For superior EMI performance, the LT8652S incorporates leading-edge Silent Switcher 2 technology with EMI cancelling design and integrated hot loop caps that minimise noisy antenna size.


/ ELECTRONICS


ELECTRONICS | OCTOBER 2020 15


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