Hello
I have a board with an SoC which has a core voltage provided by LTM4616 as an upstream switcher and two LT3071s as downstream LDOs in parallel. The setup resembles the example from Firgure 9 in LT3071 datasheet, but with 2 LT3071s, configured to give 0.8V output.
All three regulators are activated by a common RUN signal that comes from a power manager IC. In principle, everything is fine, the voltage output is stable as desired but I have noticed one peculiar phenomenom: the LTM4616 output is active, as high as 4V, even when RUN signal is low. How can this be?
I payed the attention to the datasheet info, and learned that TRACK mode keeps the part up and running even when RUN is low, so I made sure that TRACK pin is tied to SVIN. What else could be keeping the IC output high? It's worth mentioning that as soon as the RUN signal is high, this voltage drops down to the expected 1.2V, that is 300mV above the LDO output, indicating that the VIOC loop does its job.
Here is the excerpts from my circuit diagramm:
Non related to this issue, the example from the Figure 9 in the LT3071 datasheet specifies that FB should be tied to the input voltage rail. Why is that so? Doesn't it beat the purpose of the switcher regulation when the downstream LDO is inactive?
best regards
