In a switch-mode power supply, when pulses are ignored...

Switching regulators used for voltage conversion typically employ an adjustable or fixed switching frequency, a value typically listed on the first page of the switching regulator IC data sheet. The choice of switching frequency is crucial for power supply circuits because it affects the size and cost of external passive components. It also influences achievable conversion efficiency. The switching frequency is also crucial for the entire circuit—not just the power converter but also other circuit components in the system. The switching frequency is typically selected within a frequency range that minimizes interference across the entire system. Due to printed circuit board parasitics, the power supply's switching frequency is often coupled to many other circuit components through capacitive and inductive coupling.

After selecting the correct switching frequency, circuit designers often encounter surprising results when evaluating their actual circuits. The designed circuit often fails to switch as expected at the selected switching frequency. This is typically due to two reasons.

Many applications require very high conversion efficiency, even at low output loads. If the required output power is only a few milliwatts, the supply current of the switching regulator itself is significantly disproportionate. This is particularly evident when efficiency is expressed as a percentage. To improve efficiency in these situations, switching regulator ICs are often configured with a special Burst Mode function. Figure 1 shows the voltage variation over time in a switching regulator during Burst Mode®. The switch node switches once before switching to a longer pause phase. During this pause phase, many functions of the switching regulator IC enter sleep mode, consuming minimal power. Figure 1 shows the switch node voltage, inductor current, and output voltage.

When operating in Burst Mode, the output voltage ripple is larger. This frequency is much lower than the voltage ripple set by the switching frequency under normal operating conditions. Depending on the voltage converter IC and circuit conditions, operating in Burst Mode typically involves a very small number of pulses, such as one pulse or a large number of pulses. Typically, as many pulses as possible are generated until the output voltage reaches the set upper threshold. This is followed by a pause until the output voltage drops below the lower threshold. In this case, switching still occurs at the selected switching frequency during the pulses, but the lower frequency and pause defined by the burst phase appear in the spectrum.

Compared to Burst Mode, pulse-skipping mode only maintains the output voltage within a specific range, which doesn't save significant energy. Therefore, conversion efficiency is only marginally improved. Therefore, if a switching regulator switches at a frequency different from the programmed frequency, it could be because the circuit is operating in Burst Mode or pulse-skipping mode. However, there could be other reasons for discontinuous pulses at the switch node. These include general control loop instability, reaching the existing current limit, or exceeding the thermal shutdown limit.

Switched-mode power supplies can operate at pulses that differ from the intended switching frequency. This typically occurs under low-load conditions. Understanding the mechanism behind this behavior is extremely useful when evaluating switched-mode power supply circuits. This allows designers to accurately infer whether the power supply is operating reliably.