MPM3683-7 Bias Supply requirement for low-voltage Vin operation

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.

Thank You.

Hi adam.siegel,

I’m looking into this. Will get back to you asap.


Hi adam.siegel,

Great question.
Yes, you are correct. MPM3683-7 requires an external bias voltage on VCC to operate at a lower input voltage. The external bias voltage overrides the internal LDO reducing power loss and improving the efficiency of the converter.
To operate the part at 3.3Vin and 0.85Vout, you’ll need to apply a 3.3V source on VCC, then use Equation 7 on page 20 of the datasheet to determine R2 and R1:
Table 3 gives a good reference point.

Also, you might find the the “Typical Characteristics” charts on efficiency and line regulation useful. Found on page 8 of the datasheet.


Thanks for asking, Adam. I had this exact question and spent way too long digging through the datasheet trying to figure out how to override the internal LDO with an external bias. This really should be described in the datasheet; I don’t understand why MPS would leave out such a detail.