LM2678S-5 DC-DC电源芯片没输出,为什么?????

LM2678S-5 DC-DC电源芯片,标准接法:
Vin=15.6V输入,Cin=1000uf/50V,Cout=220uf/16V,L=22uH,
Cb=0.01uf,Ci=0.47uf,D为5.1V稳压管.
1.没带负载时,LED指示灯能亮,只有4.6V,为什么?????
2.带负载时,LED指示灯灭,只有0.04V,为什么?????
请高手指教 ???
3.On/Off悬空,是否该接上?500) {this.resized=true; this.width=500; this.alt='这是一张缩略图，点击可放大。\n按住CTRL，滚动鼠标滚轮可自由缩放';this.style.cursor='hand'}" onclick="if(!this.resized) {return true;} else {window.open('http://u.dianyuan.com/bbs/u/55/1894101184856725.jpg?x-oss-process=image/watermark,g_center,image_YXJ0aWNsZS9wdWJsaWMvd2F0ZXJtYXJrLnBuZz94LW9zcy1wcm9jZXNzPWltYWdlL3Jlc2l6ZSxQXzQwCg,t_20');}" onmousewheel="return imgzoom(this);">