THS3095的PD，REF端的使用问题

如果不需要掉电，PD引脚浮空即可

结合datasheet中Table 3看这段内容：

Note that if the REF pin is left unterminated, it will float to the positive rail and will fall outside of the recommended operating range given above (VS– ≤ VREF ≤ VS+ – 4 V). As a result, it will no longer serve as a reliable reference for the PD pin and the enable/disable thresholds given above will no longer apply. If the PD pin
is also left unterminated, it will also float to the positive rail and the device will be enabled. If balanced, split supplies are used (±Vs) and the REF and PD pins are grounded, the device will be disabled.

另外，

PD ≤ REF + 0.8 V for disable 
PD ≥ REF + 2.0 V for enable

可以看出，PD的disable和enable是和REF电压有关的。如果REF 悬空，那么REF就不在这VS– ≤ VREF ≤ VS+ – 4 V)电压范围之内，因此PD依据REF disable和enable也不再起作用。但如果PD悬空，默认是高电平，THS3095处于enable状态。双电源供电的话，REF和PD都接地，器件disable。

如果REF接地，不使用power down，那么PD需要>=2V 使得器件enable。