JD642电源设计[图]

　　各电源均由5V电源转换得到（所需电流值为(1430*3.3/90%+70*5+850*1.4/85%)/5＝1440mA）。1.4V和3.3V均使用MAX1951ESA提供，1MHz的开关频率能极大的降低滤波需求，虽然芯片成本高一些，但提高了系统的电源利用率，降低了对5V供电的需求，很大程序上提高了模块的可移植性。另外，电源滤波全部使用瓷片电容，不但提高了性能，还能对电源芯片提高的成本有很大补偿。1.8V由LM1117-18得到。 　　FB脚电压达到0.8V时回路稳定，输出电压VO=0.8V*(1+R3/R2)。 　　R1和C2是用于补偿L1和输出滤波电容谐振效果，保证在整个频带内能提供稳定的电源，需根据输出电流大小和L1、C4值进行计算，计算方法见MAX1951数据手册。 　　下面是DM642数据手册里对DSP电源电容分布的要求： In order to properly decouple the supply planes from system noise, place as many capacitors (caps) as possible close to the DSP. Assuming 0603 caps, the user should be able to fit a total of 60 caps, 30 for the core supply and 30 for the I/O supply. These caps need to be close to the DSP power pins, no more than 1.25 cm maximum distance to be effective. Physically smaller caps, such as 0402, are better because of their lower parasitic inductance. Proper capacitance values are also important. Small bypass caps (near 560 pF) should be closest to the power pins. Medium bypass caps (220 nF or as large as can be obtained in a small package) should be next closest. TI recommends no less than 8 small and 8 medium caps per supply (32 total) be placed immediately next to the BGA vias, using the "interior" BGA space and at least the corners of the "exterior". Eight larger caps (4 for each supply) can be placed further away for bulk decoupling. Large bulk caps (on the order of 100 μF) should be furthest away (but still as close as possible). No less than 4 large caps per supply (8 total) should be placed outside of the BGA. 　　综上：DSP芯片附近电源电容需求如下： 　　0402封装1nF电容16*2个 　　0603封装0.1uF电容8*2个 　　10uF瓷片电容2*2个 　　220uF钽电容2*2个 　　上面组合应可满足DSP芯片电源对C、ESR、ESL需求。因采用高频开关电源，钽电容应当可以不焊，视调试情况决定是否保留。(后经样板调试验证可以去掉全部钽电容)