WM8948ECSSLASHR datasheet(66/189 Pages) WOLFSON

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WM8948

Production Data

PD, May 2011, Rev 4.1

BG_VSEL [4:0]

BG Voltage (V)

BG_VSEL [4:0]

BG Voltage (V)

00h

1.200

08h

1.414

01h

1.227

09h

1.440

02h

1.253

0Ah

1.467

03h

1.280

0Bh

1.494

04h

1.307

0Ch

1.520

05h

1.334

0Dh

1.547

06h

1.360

0Eh

1.574

07h

0.387

0Fh

1.600

Table 39 Bandgap Voltage Control

REFERENCE VOLTAGES AND MASTER BIAS

This section describes the analogue reference voltage and bias current controls. It also describes the

VMID soft-start circuit for pop suppressed start-up and shut-down.

The analogue circuits in the WM8948 require a mid-rail analogue reference voltage, VMID. This

reference is generated via a programmable resistor chain. Together with the external decoupling

capacitor (connected to the VMIDC pin), the programmable resistor chain results in a slow, normal or

fast charging characteristic on the VMID reference. This is enabled using VMID_ENA and

VMID_SEL. The different resistor options controlled by VMID_SEL can be used to optimise the

reference for normal operation, low power standby or for fast start-up as described in Table 40.

The VMID resistor chain can be powered from the LDO output (LDOVOUT) or from the LDO supply

(LDOVDD). This is selected using VMID_REF_SEL.

Note that when VMID is selected as the LDO reference voltage, VMID cannot be generated from the

LDOVOUT supply voltage (VMID_REF_SEL = 1) and must be generated from the LDOVDD supply

voltage (VMID_REF_SEL = 0).

The VMID ratio can be selected using VMID_CTRL. This selects the ratio of VMID to the supply

voltage that has been selected by VMID_REF_SEL. VMID should be half of the LDOVOUT supply

voltage for maximum voltage swing. In the case where VMID_REF_SEL has selected the LDOVOUT

supply voltage output, then VMID_CTRL should select the ratio “1/2”. In the case where

VMID_REF_SEL has selected the LDOVDD supply voltage, then the alternate ratio “5/11” may be

preferred provided LDOVDDD = 3.3V and LDOVOUT = 3.0V.

Note that the “5/11” ratio is designed for the case where LDOVDD = 3.3V and LDOVOUT = 3.0V.

This results in a VMID = 3.3V x (5/11) = 1.5V which is half of the LDOVOUT voltage.

If these conditions are not being used or the LDO has been bypassed then VMID_REF should be set

to select LDOVOUT as the VMID source and VMID_CTRL should be set to select the ratio “1/2”.

The analogue circuits in the WM8948 require a bias current. The normal bias current is enabled by

setting BIAS_ENA. Note that the normal bias current source requires VMID to be enabled also.

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