Why does low ripple occur when PFM/PWM is switched automatically?
The greatest feature of this IC is its low ripple voltage when transitioning from PFM operation to PWM operation. When transitioning from a light load to a heavy load, the pulse interval of the PFM gradually approaches. When it finally reaches the frequency of PWM operation, it transitions to PWM operation. However, the previous conventional products cannot generate discrete pulses but generate bursts of pulses before reaching PWM operating conditions. (Refer to Figure 1)
Fig. 1 Continuous pulse waveform of the previous product
Switching is not uniform and dispersed, and thus variations occur in ripple voltage. (Previous product PFM switch current is 120 mA)
When the PFM is operating, the load current increases. When the output voltage drops, the switch starts operating. At this time, if the output voltage cannot be restored after one switching operation, the second switching operation will continue. If the second switching operation cannot be resumed, the third restoration operation is continued. Because the switching operation is continued until the recovery output voltage is reached, the coil current locally overlaps and increases, causing a large ripple voltage to be output. Because the PFM intermittently switches, the coil current cannot be evenly distributed, and even if the local current is large, the output current and the output voltage cannot be balanced.
In contrast, when the PWM is operating, it is necessary to evenly disperse the coil current at a constant current because the switching operation is performed within a certain period. The PWM/PFM automatic switching control of the XC9236 series dramatically suppresses this localized burst, maintains an evenly distributed current, and can continuously transition from PFM operation to PWM operation, as shown in Figure 14 Low ripple state within the load range.
Fig. 2 Ripple pattern of XC9236 with respect to load current and coil current
Input voltage: 3.6 V, Output voltage: 1.8 V
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