Production Data
WM8321
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.
Hysteretic mode is suitable for light load conditions only, and only suitable for operating modes that
are not sensitive to wide band RF/EMI effects. The output voltage ripple (and frequency) is load
dependent, and is generally worse than Pulse-Skipping operation in the Auto mode. Provided that the
EMI and voltage ripple can be tolerated, the Hysteretic mode offers an efficiency advantage over the
Auto (CCM/DCM with PS) mode.
LDO Mode
In this mode, there is no FET switching at all, and the converter operates as a Low Drop-Out (LDO)
regulator. In this mode, the FET switching losses are eliminated, as is the power consumption of the
DC-DC control circuit. Under suitable operating conditions, this provides the most efficient option for
light loads, without any of the EMI or voltage ripple limitations of Hysteretic mode.
As with any LDO, the output voltage is constant, and there is no internal source of voltage ripple.
Unlike the switching modes, the power efficiency of the LDO mode is highly dependent on the input
and output voltages; the LDO is most efficient when the voltage drop between input and output is
small. The power dissipated as heat loss by an LDO increases rapidly as the input - output voltage
difference increases.
LDO mode is suitable for light loads, and provides a ripple-free output. The LDO mode features a very
low start-up current; this mode can be used to avoid the higher in-rush current that occurs in the
switching converter modes. The efficiency is dependent on the input - output voltage configuration;
the LDO mode can be highly efficient, but may also be unacceptably inefficient. If an improvement in
power efficiency is required, then Hysteretic mode may be the preferred choice or, for better EMI and
voltage ripple, the Auto (CCM/DCM with PS) mode may be the optimum selection.
Operating Mode Summary
MODE
DESCRIPTION
APPLICATION
Forced Continuous Conduction
Mode (FCCM)
Buck converter operation where inductor
current is continuous at all times.
High performance for all static and transient
load conditions. Fixed frequency switching
offers best compatibility with sensitive
circuits.
Auto Mode: Continuous /
Discontinuous Conduction with
Pulse-Skipping Mode (CCM/DCM loads; pulse-skipping also enabled under
Buck converter operation where inductor
current may be discontinuous under reduced
High efficiency for all static and transient
load conditions. Performance may be less
than FCCM mode for heavy load transients.
with PS)
lighter loads.
Hysteretic Mode
The converter uses a hysteretic control
scheme with pulsed switching operation. The
control circuitry is disabled intermittently for
power saving.
High efficiency for light static and light
transient loads only. Maximum load current is
restricted; output voltage ripple is increased.
LDO Mode
No FET switching at all; linear regulator
operation.
Power saving mode for light loads only. High
efficiency for ultra light loads. Low current
soft-start control.
Table 42 DC-DCSynchronous Buck Converter Operating Modes Summary
PD, February 2012, Rev 4.0
91
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
