
AN2586 - Application note Power supplies
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On packages with 64 pins or less
The VREF+ and VREF- pins are not available, they are internally connected to the ADC
voltage supply (VDDA) and ground (VSSA).
1.1.2 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, VBAT must be connected externally to VDD.
1.1.3 Voltage regulator
The voltage regulator is always enabled after reset. It works in three different modes
depending on the application modes.
●in Run mode,the regulator supplies full power to the 1.8 V domain (core, memories and
digital peripherals)
●in Stop mode, the regulator supplies low power to the 1.8 V domain, preserving the
contents of the registers and SRAM
●in Standby mode, the regulator is powered off. The contents of the registers and SRAM
are lost except for those concerned with the Standby circuitry and the Backup domain.
1.2 Power supply schemes
The circuit is powered by a stabilized power supply, VDD.
●Caution:
– If the ADC is used, the VDD range is limited to 2.4 V to 3.6 V
– If the ADC is not used, the VDD range is 2 V to 3.6 V
●The VDD pins must be connected to VDD with external stabilization capacitors (five
100 nF ceramic capacitor + one Tantalum capacitor (min. 4.7 µF typ.10 µF).
●The VBAT pin must be connected to the external battery (1.8 V < VBAT < 3.6 V). if no
external battery is used, this pin must be connected to VDD with a 100 nF external
ceramic stabilization capacitor.
●The VDDA pin must be connected to two external stabilization capacitors (10 nF
ceramic + 1 µF Tantalum).
●The VREF+ pin can be connected to the VDDA external power supply. If a separate,
external reference voltage is applied on VREF+, two 10 nF and 1 µF capacitors must be
connected on this pin. In all cases, VREF+ must be kept between 2.0 V and VDDA.