原标题：Linear LT8645S 65V 8A降压转换器解决方案

　　Linear公司的LT8645S是 65V 8A降压转换器,静态电流只有2.5μA,在任何的PCB上都有超低的EMI/EMC,而在高开关频率提供高的效率.1MHz 12VIN/5VOUT时的效率高达95%,2MHz 12VIN/5VOUT时的效率高达94%,输入电压3.4V-65V,主要用在汽车和工业电源,通用降压转换器和GSM电源.本文介绍了LT8645S主要特性,功能框图和多种应用电路图,以及演示板DC2468A主要特性,电路图和材料清单.

　　The LT®8645S synchronous step-down regulator featuressecond generation Silent Switcher architecture designedto minimize EMI/EMC emissions while delivering high efficiencyat high switching frequencies. This includes theintegration of bypass capacitors to optimize all the fastcurrent loops inside and make it easy to achieve advertisedEMI performance by eliminating layout sensitivity. Thisperformance makes the LT8645S ideal for noise sensitiveapplications and environments.

　　The fast, clean, low-overshoot switching edges enable highefficiency operation even at high switching frequencies,leading to a small overall solution size. Peak current mode control with a 40ns minimum on-time allows high stepdown ratios even at high switching frequencies.

　　Burst Mode operation enables ultralow standby currentconsumption, pulse-skipping mode allows full switchingfrequency at lower output loads, or spread spectrum operation can further reduce EMI/EMC emissions. Soft-startand tracking functionality is accessed via the TR/SS pin,and an accurate input voltage UVLO threshold can be setusing the EN/UV pin. A CLKOUT pin enables synchronizingother regulators to the LT8645S.

　　LT8645S主要特性:

　　Silent Switcher® 2 Architecture

　　Ultralow EMI/EMC Emissions on Any PCB

　　Eliminates PCB Layout Sensitivity

　　Internal Bypass Capacitors Reduce Radiated EMI

　　Optional Spread Spectrum Modulation

　　High Efficiency at High Frequency

　　Up to 95% Efficiency at 1MHz, 12VIN to 5VOUT

　　Up to 94% Efficiency at 2MHz, 12VIN to 5VOUT

　　Wide Input Voltage Range: 3.4V to 65V

　　Ultralow Quiescent Current Burst Mode® Operation

　　2.5μA IQ Regulating 12VIN to 3.3VOUT

　　Output Ripple < 10mVP-P

　　Fast Minimum Switch On-Time: 40ns

　　Low Dropout Under All Conditions: 60mV at 1A

　　Safely Tolerates Inductor Saturation in Overload

　　Adjustable and Synchronizable: 200kHz to 2.2MHz

　　Peak Current Mode Operation

　　Output Soft-Start and Tracking

　　Small 32-Lead 6mm . 4mm LQFN Package

　　LT8645S应用:

　　Automotive and Industrial Supplies

　　General Purpose Step-Down

　　GSM Power Supplies

　　图1.LT8645S功能框图

　　图2.LT8645S应用电路:带软起动和电源良好的6.5V/8A降压转换器

　　图3.LT8645S应用电路:带软起动和电源良好的3.3V/8A降压转换器

　　图4.LT8645S应用电路:广谱超低EMI 5V/8A降压转换器

　　图5.LT8645S应用电路:2MHz 5V/8A降压转换器

　　图6.LT8645S应用电路:2MHz 3.3V/8A降压转换器

　　图7.LT8645S应用电路:12V/8A降压转换器

　　演示板DC2468A

　　Demonstration circuit 2468A is a 65V, 8A micropower synchronousstep-down second generation Silent SwitcherRwith spread spectrum frequency modulation featuring the LT8645S. The demo board is designed for 5V output froma 5.6V to 65V input. The wide input range allows a varietyof input sources, such as automotive batteries and industrial supplies. The LT8645S is a compact, ultralow emission,high efficiency, and high speed synchronous monolithicstep-down switching regulator. The integrated bypasscapacitors optimize all the fast current loops and make iteasier to minimize EMI/EMC emissions by reducing layoutsensitivity. Selectable spread spectrum mode can furtherimprove EMI/EMC performance. Ultralow quiescent currentin Burst ModeR operation achieves high efficiency atvery light loads. Fast minimum on-time of 40ns enableshigh VIN to low VOUT conversion at high frequency.

　　The LT8645S switching frequency can be programmedeither via oscillator resistor or external clock over a200kHz to 2.2MHz range. The default frequency of democircuit 2468A is 2MHz. The SYNC pin on the demo boardis grounded (JP1 at BURST position) by default for lowripple Burst Mode operation. To synchronize to an externalclock, move JP1 to SYNC and apply the external clockto the SYNC terminal. Spread spectrum mode and pulseskipping mode can be selected respectively by movingJP1 shunt. Figure 1 shows the efficiency of the circuit at12V input and 24V input in Burst Mode operation (inputfrom VIN terminal to bypass the EMI filter). Figure 2 shows

　　the LT8645S temperature rising on DC2468A demo boardunder different load conditions. The rated maximum loadcurrent is 8A, while derating is necessary for certain inputvoltage and thermal conditions. Low switching frequencycan extend the output load capability by reducing thepower dissipations. Figure 3 shows the temperature risingat 500kHz switching frequency.

　　The demo board has an EMI filter installed. The EMIperformance of the board (with EMI filter) is shown onFigure 4. The red line in Radiated EMI Performance isCISPR25 Class 5 peak limit. The figure shows that thecircuit passes the test with a wide margin. To achieve EMI/EMC performance as shown in Figure 4, the input EMIfilter is required and the input voltage should be appliedat VEMI terminal. An inductor can be added in the EMIfilter to further reduce the conducted emission. The EMIfilter can be bypassed by applying the input voltage atVIN terminal.

　　The LT8645S data sheet gives a complete description ofthe part, operation and application information. The datasheet must be read in conjunction with this demo manualfor demo circuit 2468A. The LT8645S is assembled in a6mm × 4mm LQFN package with exposed pads for lowthermal resistance. The layout recommendations for lowEMI operation and maximum thermal performance areavailable in the data sheet section Low EMI PCB Layoutand Thermal Considerations.

　　演示板DC2468A材料清单:

　　详情请见:

DC2468A-3-SCH.pdf

DC2468AF.pdf

8645Sf.pdf