8.3.3 Power-Save Mode Operation (PSM)

To increase efficiency across a wide range of operating conditions, the device automatically changes from pulse-width modulation (PWM) at medium and high output currents to pulse-frequency modulation (PFM) at low output currents.

• During PWM operation, the device switches continuously and adjusts the duty cycle of each switching cycle to regulate the output voltage.
• During PFM operation, the device switches in bursts of a few switching cycles, separated by periods when the device does not switch (see Figure 13). PFM operation increases efficiency at low output currents because when the device does not switch, there are no switching losses and most of the internal circuitry is disabled, which reduces quiescent power consumption. A comparator with hysteresis compares the output voltage of the error amplifier to a predefined PFM threshold voltage. When the output voltage of the error amplifier is greater than the burst threshold voltage, the device starts switching. When the output voltage of the error amplifier is less than the burst threshold voltage, the device stops switching. This scheme automatically adjusts the frequency and the duration of the switching bursts to regulate the output voltage. During PFM operation, the output voltage ripple can be higher and the transient response is not as good as during PWM operation (see Table 1).

To enable power-save mode, clear the FPWM bit in the Control register to 0.

Figure 13. Pulse-Frequency Modulation

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