7.3.12 Mode Select (Auto or FPWM)

The LMZM33604 has configurable Auto mode or FPWM mode options. To select Auto mode, connect the SYNC/MODE pin (pin 19) to AGND, or a logic signal lower than 0.3 V. To select FPWM mode, connect the SYNC/MODE pin to a bias voltage or logic signal greater than 0.6 V. When synchronizing to an external clock, the device inherently operates in FPWM mode.

In Auto mode, the device operates in discontinuous conduction mode (DCM) at light loads. In DCM, the inductor current stops flowing when it reaches 0 A. Additionally, at very light loads, the switching frequency reduces (PFM operation) to regulate the required load current, thus improving efficiency by reducing switching losses. At heavier loads, when the inductor current valley is above 0 A, the device operates in continuous conduction mode (CCM), where the switching frequency is fixed and set by the RT pin.

In forced PWM (FPWM) mode, the device operates in CCM (at a fixed frequency) regardless of load. In this mode, inductor current can go negative. At light loads, the efficiency in FPWM mode is lower than in Auto mode, due to higher conduction losses and higher switching losses. The fact that the switching frequency is fixed over the entire load range is beneficial in noise sensitive applications.