The LM73605-Q1/6-Q1 pin-configurable auto mode or FPWM options.

In auto mode, the device operates in diode emulation mode (DEM) at light loads. In DEM, inductor current stops flowing when it reaches 0 A. This is also referred to as discontinuous conduction mode (DCM). This is the same behavior as the non-synchronous regulator, with higher efficiency. At heavier load, 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 RT pin.

In auto mode, the peak inductor current has a minimum limit, I_{PEAK_MIN}, in the LM73605-Q1/6-Q1. When peak current reaches I_{PEAK_MIN}, the switching frequency reduces to regulate the required load current. Switching frequency lowers when load reduces. This is when the device operates in pulse frequency modulation (PFM). PFM further improves efficiency by reducing switching losses. Light load efficiency is especially important for battery-operated systems.

In forced PWM (FPWM) mode, the device operates in CCM regardless of load with the frequency set by RT pin or synchronization input. Inductor current can go negative at light loads. At light loads, the efficiency is lower than auto mode, due to higher conduction losses and switching losses. In FPWM, the device has fixed switching frequency over the entire load range, which is beneficial to noise sensitive applications.

Figure 8-2 shows the inductor current waveforms in each mode with heavy load, light load, and very light load. The difference between the two modes is at lighter loads where inductor current valley reaches zero.

Figure 8-2 Inductor Current Waveforms at Auto Mode and FPWM Mode with Different Loads

In CCM, the inductor current peak-to-peak ripple can be estimated by Equation 1:

Equation 1.

The average or DC value of the inductor current equals the load current, or output current I_OUT, in steady state. Peak inductor current can be calculated by Equation 2:

Valley inductor current can be calculated by Equation 3:

In auto mode, the CCM-to-DCM boundary condition is when I_VALLEY = 0 A. When I_Lripple ≥ I_{PEAK_MIN}, the load current at the DCM boundary condition can be found by Equation 4. When the peak-to-peak ripple current is smaller than I_Lripple ≥ I_PEAK-MIN, the PFM boundary is reached first.

when

• I_Lripple ≥ I_{PEAK_MIN}

In auto mode, the PFM operation boundary condition is when I_PEAK = I_{PEAK_MIN}. Frequency foldback occurs when peak current drops to I_{PEAK_MIN}, regardless of whether it is in CCM or DCM operation. When current ripple is small, I_Lripple < I_{PEAK_MIN}, the peak current reaches I_{PEAK_MIN} when it is still in CCM. The output current at CCM PFM boundary can be found by Equation 5:

when

• I_Lripple < I_{PEAK_MIN}

The current ripple increases with reduced frequency if load reduces. When valley current reaches zero, the frequency continues to fold back with constant peak current and discontinuous current.

In FPWM mode, there is no I_PEAK-MIN limit. The peak current is defined by Equation 2 at light loads and heavy loads.

Mode setting only affects operation at light loads. There is no difference if load current is above the DCM and PFM boundary conditions discussed above.

See the Section 8.3.9 section for mode setting options in the LM73605-Q1/6-Q1.