Low Ripple Design Guide. Learn by case study of DC/DC Converters XC9235/XC9236/XC9237 Series. (4/5)

Why we could lower the ripple during PFM/PWM auto switching?

The key feature of this IC is low ripple voltage when switching from PFM to PWM control. As the load changes from light to heavy, the PFM pulse interval gradually becomes shorter and more frequent. When the frequency of PWM control is finally reached, control changes to PWM. However, with previous products, it is not possible to generate a properly dispersed pulse before switching to PWM control, and a group of pulses is occasionally generated. (See Fig. 13)

Fig 13: Repeated Pulse Waveform in Previous Product

Switching is not uniform and dispersed, and thus variations occur in ripple voltage.
(Previous product PFM switch current is 120 mA)

In PFM control, switching takes place when the load current grows large and the output voltage falls. If the output voltage does not recover after switching is performed once, switching is performed a second time. If the output voltage still does not recover, switching is performed a third time. Switching continues in this way until the output voltage recovers, and as a result localized coil current overlay occurs and may cause a large ripple in the output. Switching is performed intermittently in PFM, and thus even if the current is not dispersed evenly through the coil and localized overlay occurs, balance is maintained between the output current and the output voltage.

By contrast, switching takes place at a constant interval in PWM control, and thus the current in the coil must be balanced to a constant current and dispersed. PWM/PFM auto switching control in the XC9236 Series dramatically reduces the occurrence of this localized grouping, and by effecting a continuous transition from PFM control to PWM control while maintaining a dispersed and balanced current, achieves low ripple across the entire load range as shown in Fig. 14.

Fig. 14: XC9236 Ripple vs. Load Current/Coil Current Graph

Input voltage: 3.6 V, Output voltage: 1.8 V

Output current =10mA
Operates on one switching cycle

Output current =50mA
Load current is large, but there is no continuous switching: operates on 1 uniform switching cycle.

Output current =100mA
Load current is even larger and is close to PWM control frequency, but operates on 1 uniform switching cycle.

Output current =200mA
Switches to PWM control. Coil current overlay occurs.

Upper: Output voltage ripple(10mV/div.,1µs/div.)
Lower: Coil current(100mA/div.,1µs/div.)

Torex Products in this article

XC9235

Driver Tr. Built-In, Synchronous PWM Control, Step-Down DC/DC Controller, Maximum Output Current 600mA

Driver Tr. Built-In, Synchronous PWM Control, Step-Down DC/DC Controller, Maximum Output Current 600mA

XC9236

Driver Tr. Built-In, Synchronous Step-Down DC/DC Converters , PWM/PFM auto switching, Maximum Output Current 600mA

Driver Tr. Built-In, Synchronous Step-Down DC/DC Converters , PWM/PFM auto switching, Maximum Output Current 600mA

XC9237

Driver Tr. Built-In, Synchronous Step-Down DC/DC Converters , PWM/PFM manual switching, Maximum Output Current 600mA

Driver Tr. Built-In, Synchronous Step-Down DC/DC Converters , PWM/PFM manual switching, Maximum Output Current 600mA

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