If the pins corresponding to the unused buck are left floating, then the buck converter will attempt to regulate and LX_ will try to switch. The MAX77812 will then draw 30mA of current. The reason for this is if it is turned ON and SNS_ pins float, then the error amplifier will fire and turn the switched ON for the minimum tON time since it is a COT type control architecture. Thus, the switching frequency will be 7MHz effective and the part will draw 30mA of current.
Therefore, the recommendation is to do the following for the unused buck:
- LX_: Leave unconnected
- SNS_P: Connect to VIN (VIO is an alternative option, but VIN is stronger supply than VIO)
- SNS_N: Connect to AGND