7.4.1 Forced Pulse Width Modulation (FPWM) Mode

The Synchronous Rectifier section gives a brief introduction to the LM5161 diode emulation feature. The FPWM pin allows the power supply designer to select either CCM or DCM mode of operation at light loads. When the FPWM pin is connected to ground or left floating (FPWM = 0), a pulse-skipping mode and the zero-cross current detector circuit is enabled. The zero-cross detector turns off the low-side FET when the inductor current falls close to zero (I_ZX, seeElectrical Characteristics ). This feature allows the LM5161 regulator to operate in DCM mode at light loads. In the DCM state, the switching frequency decreases with lighter loads.

When the FPWM pin is left open or shorted to ground, the user can take the advantage of the internal ripple injection circuit, enabled in this mode, for a typical Buck application circuit. This feature is applicable over the entire load and input voltage ranges. It eliminates the need for an external feedback ripple injection circuit.

For wide VIN applications where VIN > 72 V, an external VCC supply is commonly used to minimize the power dissipation in the IC. In such applications at T_J>125°C, it is recommended to add a BST resistor (> 3Ω) in series with the BST capacitor, in order to protect the internal VCC-BST diode during a full load transient operation. The addition of the external resistor will reduce the fast (dv/dt) of the switch node that can impact the normal IC operation.

If the FPWM pin is pulled high, the LM5161 will operate in CCM mode regardless of the load conditions. The CCM operation reduces efficiency at light load but improves the output transient response to step load changes and provides nearly constant switching frequency. Moreover, the Fly-Buck topology always requires the continuous conduction mode during its operation.

The internal ripple injection circuit is disabled in the CCM mode. An external ripple injection circuit or an additional ESR resistor in series with the output capacitor is required to generate the optimal ripple at the FB node. Also, there is no need to add any BST resistor in series with the BST capacitor in either forced CCM Buck or Fly-Buck application.