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FEATURE COVER STORY


Coupled with the integrated MOSFETs and small solution size, an LT8652S solution can provide exceptional EMI performance. Figure 6 shows EMI test results for the LT8652S standard demo board shown in Figure 1. Figure 6a shows CISPR 25 radiated EMI with peak detector, and Figure 6b shows the CISPR 32 radiated EMI result.


PARALLEL OPERATION FOR HIGHER CURRENTS AND BETTER THERMAL PERFORMANCE As data processing speeds soar and the volume of data multiplies, the power of FPGAs and SoCs expands to meet these needs. Power needs power, with supplies expected to keep up in power density and performance. Nevertheless, the merits of simplicity and robustness should not be lost in the quest for increased power density. For processor systems that demand more than 17A current capability, multiple LT8652Ss can be paralleled, mutually running out-of-phase. Figure 7 shows two converters


connected in parallel to provide 34A output current at 1V. The clock from the master unit is synced to the slave unit by tying the CLKOUT of U1 to the SYNC of U2. The resulting per channel 90° phase difference reduces the input current ripple and spreads the thermal load over the board. To ensure better current sharing in


steady state and during startup, VC, FB, SNSGND, and SS are tied together. A Kelvin connection is recommended for accurate feedback and noise immunity. Place as many thermal vias as possible at the vicinity of the ground pins to the bottom layer to improve the thermal performance. Ceramic caps of the input hot loops should be placed close to the VIN pins. Load transient requirements imposed


by automotive SoCs can be difficult to meet as driving conditions can change drastically, frequently, and quickly, where the SoC must adapt to fast-changing loads without delay. It is not uncommon to see a load current slew rate at 100A/μs for peripheral power supplies and even higher for core supplies. Yet the voltage transients at the output of the power supplies must be minimised under fast load current slew rates. A fast switching frequency, >2MHz, enables fast transient recovery, with minimal output voltage excursions. Figure 7 shows the proper loop compensation component values that take advantage of a fast switching frequency and stable dynamic loop response. It is also critical in board


layout to minimise trace inductance from the output capacitors of the circuit to the load. The processing power of FPGAs, SoCs,


and microprocessors continually increases, resulting in corresponding


Figure 8:


Load transient response of the Figure 7 circuit


3.3V/8.5 A, VOUT2


Figure 6 a & b: Radiated EMI test results of the Figure 1 application circuit. VIN


= 1.2V/8.5A = 14V, VOUT1 = The LT8652S has an operating Figure 7:


A 4-phase, 1 V/34A, 2MHz solution for an SoC application


frequency range of 300kHz to 3MHz, enabling designers to minimise external component sizes and avoid critical frequency bands, such as AM radio. Silent Switcher 2 technology guarantees 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 into the package, minimising the chance of layout or production induced EMI surprises - simplifying design and manufacturing. Burst Mode operation reduces quiescent


current to only 16μA while keeping output voltage ripple low. The combination of a 4mm × 7mm LQFN package and very few external components ensures a very compact footprint while minimising solution cost. The LT8652S’s 24mΩ/8mΩ switches deliver efficiencies of over 90%, while programmable undervoltage lockout (UVLO) optimises system performance. Remote differential sensing of output


voltage maintains high accuracy over the whole load range while being immune to trace impedance, minimising the possibility of load damage caused by external variety. Other features include internal/external compensation, soft start, frequency foldback, and thermal shutdown protection.


Figure 5:


LT8652S output voltage vs. current profile


increases in raw power requirements. As the number of required power rails and their carrying capacity increases, it is imperative to think small and fast with regard to design and performance of the power system. The LT8652S is a current- mode, 8.5A, 18V synchronous Silent Switcher 2 buck regulator, which operates from an input voltage range of 3V to 18V, suitable for applications with input sources ranging from a single-cell Li-Ion battery to automotive inputs.


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OCTOBER 2020 | ELECTRONICS


/ ELECTRONICS


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