5 Lower-ripple power-save mode

While most applications operate a fixed-frequency DCS-Control device in forced PWM mode in order to obtain lower output-voltage ripple at light loads and a better transient response, the topology does support a power-save mode to increase efficiency at light loads. To maintain the target switching frequency and provide lower ripple down to lower load currents, fixed-frequency DCS-Control reduces the on-time in power-save mode, whereas DCS-Control keeps the on-time constant. Both topologies enter power-save mode when the inductor current becomes discontinuous, which creates slightly higher ripple compared to PWM mode.

Instead of reducing the frequency with the same on-time, fixed-frequency DCS-Control’s power-save mode reduces the on-time while maintaining the same frequency. Reducing the on-time delivers less energy to the output, thereby reducing the ripple voltage compared to DCS-Control. Once the on-time reduces to its minimum, skipping pulses reduces the output power further for the lightest loads. Skipping pulses also reduces the frequency.

Figure 4 and Figure 5 show the difference in frequency reduction in power-save mode. The fixed-frequency DCS-Control device reduces its frequency below loads of around 60mA, while the DCS-Control device begins reducing the frequency around 500mA. Although these current values are different for different devices and operating conditions, fixed-frequency DCS-Control maintains its switching frequency down to lower load currents, leading to lower ripple.