When constant frequency operation is more important than light load efficiency, the SYNC/MODE input of the LM53600-Q1 and LM53601-Q1 devices should be pulled high, or a valid synchronization input be provided. Once activated, this feature ensures that the switching frequency will stay above the AM frequency band, while operating between the minimum and maximum duty cycle limits. Essentially, the diode emulation feature is turned off in this mode. This means that the device will remain in CCM under light loads. Under conditions where the device must reduce the on-time or off-time below the ensured minimum to maintain regulation, the frequency will reduce to maintain the effective duty cycle required for regulation. This occurs for very high and very low input/output voltage ratios.

This feature may be activated and deactivated while the part is regulating without removing the load. This feature activates and deactivates gradually, over approximately 40 µs, preventing perturbation of output voltage. When in FPWM mode, a limited reverse current is allowed through the inductor allowing power to pass from the regulators output to its input.

Note that while FPWM is activated, larger currents pass through the inductor, if lightly loaded, than in auto mode. This may result in more EMI, though at a predictable frequency. Once loads are heavy enough to necessitate CCM operation, FPWM mode has no measurable effect on regulator operation.