WM8961
Pre-Production
The Charge Pump can also be enabled by running the default Start-Up sequence as described in the
“Control Write Sequencer” section. (Similarly, it will be disabled by running the Shut-Down sequence.)
In these cases, the user does not need to write to the CP_ENA bit.
CHARGE PUMP CLOCKING
The charge pump clock is derived from SYSCLK, using a clock divider to generate a nominal 1MHz
clock, as shown in Figure 43. The exact frequency depends on MCLK and the charge pump clock
division ratio, which is based on the SAMPLE_RATE[2:0] and CLK_SYS_RATE[3:0] register settings.
For example, with MCLKDIV=0, SAMPLE_RATE[2:0]=000:
256fs: CLK_SYS_RATE[3:0]=0011 gives a charge pump clock division ratio of 12, hence
•
•
MCLK=12.288MHz gives a charge pump frequency of 1.024MHz at full output power.
MCLK=11.2896MHz gives a charge pump frequency of 940.8kHz at full output power.
128fs: CLK_SYS_RATE[3:0]=0001 gives a charge pump clock division ratio of 6, hence
•
•
MCLK=6.144MHz gives a charge pump frequency of 1.024MHz at full output power.
MCLK=5.6448MHz gives a charge pump frequency of 940.8kHz at full output power.
CHARGE PUMP CONTROL
In order to minimise power consumption, both the output voltages (VPOS and VNEG) and the
switching frequency of the charge pump are automatically adjusted according to the operating
conditions. This can take two forms:
1. Charge pump settings only depend on volume register settings (
CP_DYN_PWR[1:0] = 0b00): the maximum of LOUT1VOL and ROUT1VOL volume
register settings (excluding disabled or muted outputs) controls the charge pump
mode of operation.
2. Dynamic (Class W) power saving: Charge pump settings depend on dynamic signal level
(CP_DYN_PWR[1:0] = 0b11) and volume register settings: the audio signal input to the DAC,
multiplied by the maximum of LOUT1VOL and ROUT1VOL volume register settings, is used
to control the charge pump mode of operation. This is the Wolfson ‘Class W’ mode, which
allows the power consumption to be optimised in real time.
The Charge Pump operating mode defaults to Register control. Dynamic (Class W) power saving may
be selected by setting the CP_DYN_PWR[1:0] bits to 0b11. If dynamic power saving is to be selected,
this register write must take place before the charge pump is enabled. If the write sequencer is used
to configure the DAC to HP path, this register write must take place before the write sequencer is
enabled and triggered. Dynamic power saving results in a very slight reduction in headphone driver
performance.
When the ADC sidetone is used for the headphone output, and the charge pump settings depend on
dynamic signal level CP_DYN_PWR[1:0] = 0b11), the charge pump will monitor the DAC output level
rather than the ADC sidetone level. If the ADC sidetone signal is likely to exceed the DAC output
level, to avoid clipping it is recommended to set the charge pump settings to depend on volume
register settings (CP_DYN_PWR[1:0] = 0b00). See also the DAC Digital Sidetone section.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
Enable dynamic (Class W) power
saving
R82 (52h)
Charge
1:0
CP_DYN_PWR[1:0]
0
00: dynamic power saving
disabled
Pump B
01: Reserved
10: Reserved
11: dynamic power saving
enabled
Table 42 Charge Pump Configuration Registers
PP, August 2009, Rev 3.1
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
