34. Voltage reference buffer (VREFBUF)

34.1 VREFBUF introduction

The devices embed a voltage reference buffer which can be used as voltage reference for the on-chip ADCs, and also as voltage reference for external components through the VREF+ pin.

34.2 VREFBUF implementation

The table below describes the VREFBUF voltages typical values:

Table 271. VREFBUF typical values

Note: Refer to the product datasheet for more details.

34.3 VREFBUF functional description

Figure 320. VREFBUF block diagram

The internal voltage reference buffer is an operational amplifier, with programmable gain. The amplifier input is connected to the internal voltage reference \( V_{REFINT} \) . The VREFBUF supports two voltages ^(a) , which are configured with VRS bits in the VREFBUF_CSR register:

• • VRS = 000: VREFBUF0 voltage selected
• • VRS = 001: VREFBUF1 voltage selected

The internal voltage reference can be configured in four different modes depending on ENVR and HIZ bits configuration. These modes are provided in the table below:

Table 272. VREF buffer modes

After enabling the VREFBUF by setting ENVR bit and clearing HIZ bit in the VREFBUF_CSR register, the user must wait until VRR bit is set, meaning that the voltage reference output has reached its expected value.

34.4 VREFBUF trimming

The VREFBUF output voltage is factory-calibrated by ST. At reset, and each time the VRS setting is changed, the calibration data is automatically loaded to the TRIM register.

Optionally user can trim the output voltage by changing the TRIM register bits directly. In this case, the VRS setting has no more effect on the TRIM register until the device is reset.

a. The minimum \( V_{DDA} \) voltage depends on VRS setting, refer to the product datasheet.

34.5 VREFBUF power sequence

While VREFBUF is activated, \( V_{DDA} \) supply changing from 1.8 V to 0 V can cause large current flowing from the \( V_{REF+} \) output capacitor to the \( V_{DDA} \) power domain through on-chip parasitic resistance. To avoid this current, the following procedure is required:

• • While \( V_{DDA} \) is available, configure VREFBUF in buffer off mode.
• • Wait for \( V_{REF+} \) to discharge (through internal pull-down) to less than 100 mV (typically 1 ms, depending on the capacitor value).
• • Switch off \( V_{DDA} \) supply to 0 V.

34.6 VREFBUF registers

34.6.1 VREFBUF control and status register (VREFBUF_CSR)

Address offset: 0x00

Reset value: 0x0000 0002

Bits 31:7 Reserved, must be kept at reset value.

Bits 6:4 VRS[2:0] : Voltage reference scale

These bits select the value generated by the voltage reference buffer.

VRS = 000: VREFBUF0 voltage selected.

VRS = 001: VREFBUF1 voltage selected.

Others: Reserved

Note: Refer to the product datasheet for each VREFBUFx voltage setting value.

The software can program this bitfield only when the VREFBUF is disabled (ENVR=0).

Bit 3 VRR : Voltage reference buffer ready

0: the voltage reference buffer output is not ready.

1: the voltage reference buffer output reached the requested level.

Bit 2 Reserved, must be kept at reset value.

Bit 1 HIZ : High impedance mode

This bit controls the analog switch to connect or not the V _REF+ pin.

0: V _REF+ pin is internally connected to the voltage reference buffer output.

1: V _REF+ pin is high impedance.

Refer to Table 272: VREF buffer modes for the mode descriptions depending on ENVR bit configuration.

Bit 0 ENVR : Voltage reference buffer mode enable

This bit is used to enable the voltage reference buffer mode.

0: Internal voltage reference mode disable (external voltage reference mode).

1: Internal voltage reference mode (reference buffer enable or hold mode) enable.

34.6.2 VREFBUF calibration control register (VREFBUF_CCR)

Address offset: 0x04

Reset value: 0x0000 00XX

Bits 31:6 Reserved, must be kept at reset value.

Bits 5:0 TRIM[5:0] : Trimming code

The TRIM code is a 6-bit unsigned data (minimum 000000, maximum 111111) that is set and updated according the mechanism described below.

Reset:

TRIM[5:0] is automatically initialized with the VRS = 0 trimming value stored in the OTP memory during the production test.

VRS change:

TRIM[5:0] is automatically initialized with the trimming value (corresponding to VRS setting) stored in the OTP memory during the production test.

Write in TRIM[5:0]:

User can modify the TRIM[5:0] with an arbitrary value. This is permanently disabling the control of the trimming value with VRS (until the device is reset).

Note: If the user application performs the trimming, the trimming code must start from 000000 to 111111 in ascending order.

34.6.3 VREFBUF register map

The following table gives the VREFBUF register map and the reset values.

Table 273. VREFBUF register map and reset values

Refer to Section 2.3: Memory organization for the register boundary addresses.