7.4.2 MODE Control (Skip-Cycle Mode and Pulse-Skipping Mode)

Light load efficiency of the regulator typically drops as the losses associated with switching and bias currents of the converter become a significant percentage of the total power delivered to the load. In order to increase the light load efficiency the LM5122ZA provides two types of light load operation in diode-emulation mode.

The skip-cycle mode integrated into the LM5122ZA controller reduces switching losses and improves efficiency at light-load condition by reducing the average switching frequency. Skip-cycle operation is achieved by the skip cycle comparator. When a light-load condition occurs, the COMP pin voltage naturally decreases, reducing the peak current delivered by the regulator. During COMP voltage falling, the skip-cycle threshold is defined as V_MODE – 20 mV and during COMP voltage rising, it is defined as V_MODE + 20 mV. There is 40 mV of internal hysteresis in the skip cycle comparator.

When the voltage at PWM comparator input falls below V_MODE – 20 mV, both HO and LO outputs are disabled. The controller continues to skip switching cycles until the voltage at PWM comparator input increases to V_MODE + 20 mV, demanding more inductor current. The number of cycles skipped depends upon the load and the response time of the frequency compensation network. The internal hysteresis of skip-cycle comparator helps to produce a long skip cycle interval followed by a short burst of pulses. An internal 700-kΩ pullup resistor and 100-kΩ pulldown resistor sets the MODE pin to 0.15 V as a default. Because the peak current limit threshold is set to 750 mV, the default skip threshold corresponds to approximately 17% of the peak level. In practice the skip level is lower due to the added slope compensation. By adding an external pullup resistor to SLOPE or VCC pin or adding an external pulldown resistor to the ground, the skip cycle threshold can be programmed. Because the skip cycle comparator monitors the PWM comparator input which is proportional to the COMP voltage, skip-cycle operation is not recommended when the bypass operation is required.

Conventional pulse-skipping operation can be achieved by connecting the MODE pin to ground. The negative 20-mV offset at the positive input of skip-cycle comparator ensures the skip-cycle comparator does not trigger in normal operation. At light or no load conditions, the LM5122ZA skips LO pulses if the pulse width required by the regulator is less than the minimum LO on-time of the device. Pulse skipping appears as a random behavior as the error amplifier struggles to find an average pulse width for LO in order to maintain regulation at light or no load conditions.