SKIP CYCLE OPERATION

During light load conditions, the efficiency of the switching

power supply typically drops as the losses associated with

switching and operating bias currents of the converter be-

come a significant percentage of the power delivered to the

load. The largest component of the power loss is the switch-

ing loss associated with the gate driver and external MOS-

FET gate charge. Each PWM cycle consumes a finite amout

of energy as the MOSFET is turned on and then turned off.

These switching losses are proportional to the frequency of

operation. The Skip Cycle function integrated within the

LM5021 controller reduces the average switching frequency

to reduce switching losses and improve efficiency during

light load conditions.

When a light load condition occurs, the COMP pin voltage is

reduced by the voltage feedback loop to reduce the peak

current delivered by the controller. Referring to Figure 6, the

PWM comparator input tracks the COMP pin voltage through

a 1.25V level shift circuit and a 3:1 resistor divider. As the

COMP pin voltage falls, the input to the PWM comparator

falls proportionately. When the PWM comparator input falls

to 125mV, the Skip Cycle comparator detects the light load

condition and disables output pulses from the controller. The

controller continues to skip switching cycles until the power

supply output falls and the COMP pin voltage increases to

demand more output current. The number of cycles skipped

will depend on the load and the response time of the fre-

quency compensation network. Eventually the COMP volt-

age will increase when the voltage loop requires more cur-

rent to sustain the regulated output voltage. When the PWM

comparator input exceeds 130mV (5mV hysteresis), normal

fixed frequency switching resumes. Typical power supply

designs will produce a short burst of output pulses followed

by a long skip cycle interval. The average switching fre-

quency in the Skip Cycle mode can be a small fraction of the

normal operating frequency of the power supply.

20144221

FIGURE 5. Waveform at SS and COMP Pin due to Brief Overload

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