WM8948ECSSLASHR datasheet(64/189 Pages) WOLFSON

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WM8948

Production Data

PD, May 2011, Rev 4.1

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 40

and Table 41 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 37. The bandgap

voltage can be adjusted using the BG_VSEL register as described in Table 39.

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 38 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 38)

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 38

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 39) 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 37. This bit can be polled at any time, or may

output directly on a GPIO pin, or may be used to generate Interrupt events.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R17 (11h)

Status Flags

LDO_UV_STS

LDO Undervoltage status

0 = Normal

1 = Undervoltage

R53 (35h)

LDO

LDO_ENA

LDO Enable

0 = Disabled

1 = Enabled

LDO_REF_SEL_

FAST

LDO Voltage reference select

0 = VMID (normal)

1 = VMID (fast start)

This field is only effective when

LDO_REF_SEL = 0

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