Characteristics of PWM control and forced PWM control

    Unlike PWM control (PWM Control) that does not allow the coil current to flow in reverse, PWM control that allows the coil current to flow in reverse is called forced PWM control (Forced PWM Control).

    Detection conditions: Vin=5V, Vout=3.3V/0mA

    Unlike PWM control, forced PWM control does not cause pulse skipping even under light load conditions and can prevent noise.

    The characteristics of PWM control and forced PWM control are as follows:

PWM control

    It is a discontinuous mode under light load conditions, and the larger the output current, the larger the duty.

    Under light load conditions, the ON duty required to maintain the output voltage may be very small, and sometimes it is lower than the minimum duty that the IC can control.

    In this case, pulse skipping (intermittent oscillation) occurs, which cannot maintain the oscillation frequency and causes the oscillation frequency to drop.

In addition, in discontinuous mode, ringing of the switch node occurs. The ringing of the switch node becomes the cause of noise mixing or radiation noise of the output voltage.

Forced PWM control

    In forced PWM control, in order to make the coil current flow in reverse, it becomes a continuous mode even under light load conditions, and the duty is constant from light load to heavy load.

    Unlike normal PWM control, the duty does not decrease even when the load is light, so pulse skipping (intermittent oscillation) does not occur when the load is light.

    In addition, since the discontinuous mode is not entered, ringing of the switch node does not occur, and noise caused by ringing does not occur.

   The disadvantage is that the efficiency at light load is lower than that of normal PWM control.

    In addition, since forced PWM control requires the coil current to flow in reverse, forced PWM control cannot be performed in non-synchronous rectification type DC/DC that requires an external diode.