WM8326GEFLSLASHV datasheet(91/255 Pages) WOLFSON

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Production Data

WM8326

PD, June 2012, Rev 4.0

during this cycle. (Note that the load current corresponds to the average inductor current.) The

negative portion of the cycle corresponds to inefficient operation, as the output capacitor is

discharged unnecessarily by the converter circuit. Accordingly, this mode is not optimally efficient for

light load conditions.

This mode offers excellent performance under transient load conditions. It exceeds the performance

of the other operating modes in the event of a decreasing current demand or a decreasing voltage

selection. This is because FCCM mode can actively pull down the output voltage to the required level,

whilst other modes rely on the load to pull the converter voltage down under these conditions.

Another important benefit of this mode is that the switching pattern is fixed, regardless of load

conditions. This provides best compatibility with noise-sensitive circuits where the noise frequency

spectrum must be well-defined.

Although this mode is not optimally efficient for light loads, it delivers the best possible transient load

performance and fixed frequency switching. This mode should be selected when best performance is

required, delivering minimum output voltage ripple across all static or transient load conditions.

Auto Mode: Continuous / Discontinuous Conduction with Pulse-Skipping (CCM/DCM with PS)

This is an automatic mode that selects different control modes according to the load conditions. The

converter supports the full range of load conditions in this mode, and automatically selects power-

saving mechanisms when the load conditions are suitable. Under light load conditions, the efficiency

in this mode is superior to the FCCM mode. The transient load performance may be slightly worse

than FCCM mode.

The converter operates in Continuous Conduction Mode (CCM) for heavy load conditions, and

Discontinuous Conduction Mode (DCM) under lighter loads. Discontinuous conduction is when the

inductor current falls to zero during the discharge phase, and the converter disables the synchronous

rectifier transistor in order that the inductor current remains at zero until the next charge phase.

Negative inductor current is blocked in this mode, eliminating the associated losses, and improving

efficiency.

The transient response in this mode varies according to the operating conditions; it differs from FCCM

in the case of a decreasing current demand or a decreasing voltage, as the converter uses the load to

pull the output voltage down to the required level. A light load will result in a slow response time.

A minimum inductor charge time is applied in DCM mode; this leads to a minimum average inductor

current when operating as described above. Under very light load conditions, pulse skipping is used

to reduce the average inductor current to the level required by the load. In pulse-skipping mode, the

charge phase of selected cycles is not scheduled, and the load is supported by the output capacitor

over more than one cycle of the switching frequency. As well as supporting very light load current

conditions, this mechanism offers power savings, as the switching losses associated with the skipped

pulses are eliminated. A disadvantage of this is that the transient response is degraded even further

with respect to DCM. When the pulse-skipping behaviour is invoked, an increased output voltage

ripple may be observed under some load conditions.

This mode is suitable for a wide range of operating conditions. It supports the full range of load

currents, and offers efficiency savings under light load conditions.

Hysteretic Mode

Hysteretic mode is a power-saving mode. It does not support the full load capability of the DC-DC

converter, but offers efficiency improvements over the FCCM and Auto (CCM/DCM with PS) modes.

The control circuit in Hysteretic mode operates very differently to the Pulse-Skipping mode that is

available in Auto mode. In Pulse-Skipping mode, selected switching cycles are dropped in order to

reduce the output current to match a light load condition, whilst maintaining good output voltage ripple

as far as possible. In Hysteretic mode, the converter uses switched operation on an adaptive

intermittent basis to deliver the required average current to the load.

In the switched operation portion of the Hysteretic mode, the converter drives the output voltage up;

this is followed by a power-saving period in which the control circuit is largely disabled whilst the load

pulls the output voltage down again over a period of many switching cycles. The duration of the fixed

frequency bursts and the time between bursts is adapted automatically by the output voltage

monitoring circuit.

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