A colleague of mine is using the MPM3683-7 converter to generate 0.85V from 3.3V. The data sheet for the 3683-7 says Vin can go down to 4V without external bias and 2.7V with external bias. However, there is no discussion of this topic in the data sheet. There is no Bias pin, and the Vcc pin is an internal LDO output. On the evaluation board, my colleague said that the part only works if he connects VCC to VIN. This puts 3.3V onto an output pin that is also 3.3V.
Can I conclude that by putting 3.3V into VCC, you are overdriving the internal LDO that is also trying to create 3.3V, and this also serves as the bias supply.
I’m disappointed that there was no section in the data sheet that discusses this issue. This is an important issue that designers need to understand in order to use the part.
Please clarify exactly how you connect the part to have a 3.3V input and a 0.85V output.