About the efficiency calculation of LTC3246

Hello,

The answer was in the paragraph preceding the equation. "When the input supply is greater than about two times the output voltage, the LTC3246 will operate in 2:1 step-down mode. Charge transfer happens in two phases. On
the first phase, the flying capacitor (CFLY) is connected between VIN and VOUT. On this phase, CFLY is charged up and current is delivered to VOUT. On the second phase, the flying capacitor (CFLY) is connected between VOUT and GND. The charge stored on CFLY during the first phase is transferred to VOUT on the second phase. When in 2:1 step-down mode, the input current will be approximately half of the total output current."

Basically the input current to the output is only on the first phase. On the second phase the current to the output is from the flying cap and not from the input.