AN4488 datasheet(7/44 Pages) STMICROELECTRONICS

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AN4488

Power supplies

2.1

Introduction

The operating voltage supply (VDD) range is 1.8 V to 3.6 V, which can be reduced down to

1.7 V with some restrictions, as detailed in the product datasheets. An embedded regulator

is used to supply the internal 1.2 V digital power.

The real-time clock (RTC) and backup registers can be powered from the VBAT voltage

when the main VDD supply is powered off.

2.1.1

Independent A/D converter supply and reference voltage

To improve conversion accuracy, the ADC has an independent power supply that can be

filtered separately, and shielded from noise on the PCB.

•

the ADC voltage supply input is available on a separate VDDA pin

•

an isolated supply ground connection is provided on the VSSA pin

In all cases, the VSSA pin should be externally connected to same supply ground than

VSS

On packages with 100-pins and above

To ensure a better accuracy on low-voltage inputs, the user can connect a separate external

reference voltage ADC input on VREF+. The voltage on VREF+ may range from (VDDA- 1.2 V)

to VDDA with a minimum of 1.7 V.

When available (depending on package), VREF– must be externally tied to VSSA.

On packages with less than 100-pins

The VREF+ and VREF- pins are not available, they are internally connected to the ADC

voltage supply (VDDA) and ground (VSSA).

2.1.2

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. The switch to the VBAT supply is controlled by the power

down reset (PDR) circuitry embedded in the Reset block.

If no external battery is used in the application, it is highly recommended to connect VBAT

externally to VDD.

AN4488 Datasheet(PDF) 7 Page - STMicroelectronics