WM8321 datasheet(91/253 Pages) WOLFSON

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Production Data

WM8321

PD, February 2012, Rev 4.0

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

with PS)

Buck converter operation where inductor

current may be discontinuous under reduced

loads; pulse-skipping also enabled under

lighter loads.

High efficiency for all static and transient

load conditions. Performance may be less

than FCCM mode for heavy load transients.

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

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