WM8321
Production Data
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.
PD, February 2012, Rev 4.0
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
