WM8311 datasheet(96/302 Pages) WOLFSON

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WM8311

Pre-Production

PP, May 2012, Rev 3.1

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.

In this mode, the power dissipation is reduced to a very low level by disabling parts of the control

circuitry for the duration of selected switching cycles. This improves the overall efficiency, but also

leads to output voltage ripple and limited performance. This mode produces a larger output voltage

ripple than the Pulse-Skipping mode. In order to limit the degradation of the DC-DC converter

performance in Hysteretic mode, the control circuit is designed for a restricted range of load

conditions only. Note that the irregular switching pattern also results in degraded EMI behaviour.

Hysteretic mode and Pulse Skipping mode are both Pulse Frequency Modulation (PFM)-type modes,

where the switching pulse frequency is adjusted dynamically according to the load requirements. A

consequence of this frequency modulation is that the circuit’s EMI characteristics are less predictable.

In Hysteretic mode in particular, the EMI effects arising from the DC-DC switching are present across

a wider frequency band than is the case in CCM and DCM. It is more difficult to effectively suppress

the wide band interference, and this factor may result in Hysteretic mode being unsuitable for some

operating conditions.

WM8311 Datasheet(PDF) 96 Page - Wolfson Microelectronics plc

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