WM8321 datasheet(89/253 Pages) WOLFSON

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Production Data

WM8321

PD, February 2012, Rev 4.0

15.14 DC-DC SYNCHRONOUS BUCK CONVERTER OPERATION

15.14.1 OVERVIEW

The WM8321 provides four DC-DC switching converters. Each of these is a synchronous buck (step-

down) converter. The principal characteristics of each DC-DC converter are shown below. Converters

DC-DC3 and DC-DC4 can either be operated as single converters, or may be ganged together in

‘dual’ mode to provide an increased current capability.

DC-DC1 / DC-DC2

DC-DC3 / DC-DC4

(SINGLE MODE)

DC-DC3 / DC-DC4

(DUAL MODE)

Converter Type

Buck (step-down)

Input Voltage Range

2.7V to 5.5V

Output Voltage Range

0.6V to 1.8V

0.85V to 3.4V

Load Current Rating

Up to 1250mA

Up to 1000mA

Up to 1600mA

Switching Frequency

2MHz or 4MHz

2MHz

Table 41 DC-DC Converter Overview

15.14.2 DC-DC SYNCHRONOUS BUCK CONVERTERS

DC-DC Converters 1, 2, 3 and 4 are synchronous buck converters which deliver high performance

and high efficiency across a wide variety of operating conditions.

The high switching frequency, together with the current mode architecture, delivers exceptional

transient performance suitable for supplying processor power domains and similar applications

requiring high stability through fast-changing load (or line) conditions.

The current mode architecture enables extended bandwidth of the control loop, allowing the DC-DC

converter to adapt for changes in input or output conditions more rapidly than can be achieved using

other feedback mechanisms. This improves the converter’s performance under transient load

conditions.

The flexible design of the DC-DC Converters allows a selection of different operating configurations,

which can be chosen according to the performance, efficiency, space or external component cost

requirements.

The DC-DC Converter design achieves high performance with a small inductor component. This is

highly advantageous in size-critical designs for portable applications. In the case of DC-DC1 and DC-

DC2, the switching frequency is selectable (2MHz or 4MHz). The higher frequency supports best

transient performance and the smallest external inductor, whilst the lower rate supports best power

efficiency. It should be noted that the supported output voltage range is restricted in the 4MHz mode;

for output voltages greater than 1.4V, the 2MHz mode must be used.

The DC-DC Converters are compatible with a range of external output capacitors. A larger capacitor

(eg. 47

F) will deliver best transient performance, whilst a smaller capacitor (eg. 4.7F) may be

preferred for size or cost reasons.

Four different operating modes can be selected, allowing the user to configure the converter

performance and efficiency according to different demands. This includes power-saving modes for

light load conditions and a high performance mode for best transient load performance. A low power

LDO regulator mode is also provided. The DC-DC Converters maintain output voltage regulation

when switching between operating modes.

Forced Continuous Conduction Mode (FCCM)

This mode delivers the best load transient performance across the entire operating load range of the

converter. It also provides the best EMI characteristics due to the fixed, regular switching pattern.

For normal DC-DC buck converter operation, there is an inductor charging phase followed by a

discharging phase. Under light load conditions, the inductor current may be positive or negative

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.

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