AN4206 datasheet(11/34 Pages) STMICROELECTRONICS

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AN4206

Power supplies

Figure 5. Schottky diode connection

1.1.2

Sigma Delta supply voltages (only on F37x)

To improve Sigma delta ADC (SDADC) peripherals performance the device implements two

independent power supplies used to power SDADC peripherals. There are two power

supply pins with common ground pin (VDDSD12, VDDSD3, VSSSD).

Those power supply sources also defines voltage levels on digital GPIO pins which are

sharing SDADC analog inputs for given SDADC peripheral. Refer to device datasheet which

GPIO pins are powered from VDDSD12 and which from VDDSD3.

Sigma delta power supplies must be always less or equal to the analog supply: VDDSDx <

VDDA but they can be lower or higher than VDD.

If no SDADC is used in application then VDDSDx must be connected externally to VDD.

VSSSD must be always connected to VSS.

There are some next important restrictions to VDDSD12 and VDDSD3 with relation to the

reference voltage used for the SDADCs:



If the VREFSD+ pin is selected as the external reference voltage for the SDADCs:

–

If SDADC1 or SDADC2 are enabled in the PWR controller (ENSD1, ENSD2 bits)

then: VDDSD12 > VREFSD+, VDDSD3 > VREFSD+

–

If SDADC1 and SDADC2 are disabled in the PWR controller then: VDDSD3 >

VVREFSD+



If the VDDSDx power supply is selected as the reference voltage for the SDADCs:

–

If SDADC1 or SDADC2 are enabled in PWR controller (ENSD1, ENSD2 bits)

then: VDDSD12 = VDDSD3



If SDADC1 and SDADC2 are disabled and SDADC3 is enabled in the PWR controller

(ENSD1, ENSD2, ENSD3 bits) then: VDDSD12 <= VDDSD3.

1.1.3

Battery backup

To retain the content of the Backup registers when VDD is turned off, the VBAT pin can be

connected to an optional standby voltage supplied by a battery or another source.

The VBAT pin also powers the RTC unit, allowing the RTC to operate even when the main

digital supply (VDD) is turned off.

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