This letter presents a two-phase three-level (2P3L) buck converter with cross-connected flying capacitors (X-C FLY). The proposed converter suppresses the unbalanced inductor currents in two-phase operation, by alternately connecting the flying capacitors to across the branches. Meanwhile, three-level operation reduces the required inductance with lower inductor voltage, together with two-phase operation, helping to achieve fast transient response and small output ripple. Here, we design the power stage and feedback loop aiming for a bandwidth of 10 MHz. Then, the proposed 2P3L buck converter with X-C FLY fabricated and validated in 65-nm standard CMOS achieves a low unbalanced current of about 5 mA, and a small output ripple of 50 mV at a load current of 110 mA, using only 1-nF output capacitor. The measured peak efficiency is 86%. Besides, it can track fast 10-MHz 64QAM LTE envelope signal.