Production Data
WM8945
LDO REGULATOR
The WM8945 provides an internal LDO which provides a regulated voltage for use as in internal
supply and reference, which can also be used to power external circuits.
The LDO is enabled by setting the LDO_ENA register bit. The LDO supply is drawn from the
LDOVDD pin; the LDO output is provided on the LDOVOUT pin. The LDO requires a reference
voltage and a bias source; these are configured as described below.
The LDO bias source is selected using LDO_BIAS_SRC. Care is required during start-up to ensure
that the selected bias is enabled; the master bias will not normally be available at initial start-up, and
the fast bias should be selected in the first instance.
The LDO reference voltage can be selected using LDO_REF_SEL; this allows selection of either the
internal bandgap reference or one of the VMID resistor strings. When VMID is selected as the
reference, then LDO_REF_SEL_FAST selects either the Normal VMID reference or the Fast-Start
VMID reference. Care is required during start-up to ensure that the selected reference is enabled; the
VMID references are enabled using VMID_ENA and VMID_FAST_START as described in Table 39
and Table 40 respectively.
The internal bandgap reference is nominally 1.5V. Note that this value is not trimmed and may vary
significantly (+/-10%) between different devices. When using this reference, the internal bandgap
reference must be enabled by setting the BG_ENA register, as described in Table 36. The bandgap
voltage can be adjusted using the BG_VSEL register as described in Table 38.
The LDO output voltage is set using the LDO_VSEL register, which sets the ratio of the output
voltage to the LDO reference voltage. See Table 37 for LDO output voltages.
Example1:
How to generate an LDOVOUT voltage of 3.0V from a 3.3V LDOVDD supply voltage.
LDO_REF_SEL = 0 (VMID as the reference voltage)
VMID = 1.5V (VMID_REF_SEL = 0, VMID_CTRL = 0)
LDOVOUT = Vref * 1.97 = 1.5V * 1.97 = 2.96V (see Table 37)
Example2:
Generating an LDOVOUT voltage of 2.4V from a 3.0V LDOVDD supply.
For maximum signal swing the VMID voltage should be half of the LDOVOUT voltage. For LDOVOUT
of 2.4V the optimum VMID voltage is 1.2V. Select the VMID source voltage as LDOVOUT
(VMID_REF_SEL = 1) and the VMID ratio as 1/2 (VMID_CTRL = 1). This gives VMID = 1.2V.
VMID cannot be used as the LDO reference voltage so use the Bandgap voltage as the LDO
reference voltage (LDO_REF_SEL = 1, BG_ENA = 1). The default Bandgap voltage is 1.467V. For
LDOVOUT of 2.4V LDOVSEL should be set to 2.4V / 1.467V = 1.636. Referring to Table 37
LDO_VSEL = 03h will give LDOVOUT = 1.467 * 1.66 = 2.435V.
Note that the Bandgap voltage is not trimmed so if required the Bandgap voltage can be changed
(BG_VSEL – see Table 38) to get closer to the required voltage.
By default, the LDO output is actively discharged to GND through internal resistors when the LDO is
disabled. This is desirable in shut-down to prevent any external connections being affected by the
internal circuits. The LDO output can be set to float when the LDO is disabled; this is selected by
setting the LDO_OP_FLT bit. This option should be selected if the LDO is bypassed and an external
voltage is applied to LDOVOUT.
The LDO output is monitored for voltage accuracy. The LDO undervoltage status can be read at any
time from the LDO_UV_STS bit, as described in Table 36. This bit can be polled at any time, or may
output directly on a GPIO pin, or may be used to generate Interrupt events.
PD, May 2011, Rev 4.1
59
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
