Q: Instead of connecting VN to EP of the MAX14778, can I connect VN and EP directly to GND?
A: No, EP must be either connected to VN, or left unconnected. EP must never be connected to ground.
VN is the output of the negative charge pump and the only allowed connection is a 0.1uF capacitor to ground. EP is connected to the die substrate with conductive epoxy. The substrate must be biased at the most negative voltage potential which is why we recommend connection to VN, but the substrate will self-bias to VN if the EP is left unconnected. The EP also provides a low thermal resistance path from the die to the board. The package thermal characteristics follow JEDEC specification JESD51-7 which assumes that the EP is soldered to a copper pad, therefore, for best thermal performance EP should be connected to an isolated copper pad and to VN. The actual dissipated power in the MAX14778 with a full 300mA rated current through each of the two multiplexer channels will be 2 x ((0.3A)^2 x 1.5 Ohms) = 540mW which is much lower than the package specified continuous power dissipation capability of 2666.7mW.