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LM25085 Vout 接近Vin时工作异常



原理图如附件,RFB1使用RDAC,用于调节Vout电压,纹波注入电路使用datasheet中的“Reduced Ripple Configuration”。

空载时Vin=18V,调节RDAC 使Vout接近Vin时(或分压比算出的Vout大于Vin),Vout电压会不规律的乱跳,如下图,该款DC-DC 是可以工作在duty cycle 100%的,为什么会出现异常呢?

另外空载时如果Vin较低如5V,调节Vout接近或者大于Vin时,上管可以一直打开,Vout正常....