Production Data
WM8321
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)
Buck (step-down)
2.7V to 5.5V
DC-DC3 / DC-DC4
(DUAL MODE)
Buck (step-down)
2.7V to 5.5V
Converter Type
Buck (step-down)
2.7V to 5.5V
Input Voltage Range
Output Voltage Range
Load Current Rating
Switching Frequency
0.6V to 1.8V
0.85V to 3.4V
Up to 1000mA
2MHz
0.85V to 3.4V
Up to 1600mA
2MHz
Up to 1250mA
2MHz or 4MHz
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. 47F) will deliver best transient performance, whilst a smaller capacitor (eg. 4.7F) 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.
PD, February 2012, Rev 4.0
89
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