RM0033 Rev 9 65/1381
RM0033 Power control (PWR)
83
4.1.1 Independent A/D converter supply and reference voltage
To improve conversion accuracy, the ADC has an independent power supply which can be
separately filtered and shielded from noise on the PCB.
• The ADC voltage supply input is available on a separate V
DDA
pin.
• An isolated supply ground connection is provided on pin V
SSA
.
To ensure a better accuracy of low voltage inputs, the user can connect a separate external
reference voltage ADC input on V
REF
. The voltage on V
REF
ranges from 1.8 V to V
DDA
.
4.1.2 Battery backup domain
Backup domain description
To retain the content of the RTC backup registers, backup SRAM, and supply the RTC when
V
DD
is turned off, V
BAT
pin can be connected to an optional standby voltage supplied by a
battery or by another source.
To allow the RTC to operate even when the main digital supply (V
DD
) is turned off, the V
BAT
pin powers the following blocks:
• The RTC
• The LSE oscillator
• The backup SRAM when the low-power backup regulator is enabled
• PC13 to PC15 I/Os, plus PI8 I/O (when available)
The switch to the V
BAT
supply is controlled by the power-down reset embedded in the Reset
block.
Warning: During t
RSTTEMPO
(temporization at V
DD
startup) or after a PDR
is detected, the power switch between V
BAT
and V
DD
remains
connected to V
BAT
.
During the startup phase, if V
DD
is established in less than
t
RSTTEMPO
(Refer to the datasheet for the value of t
RSTTEMPO
)
and V
DD
> V
BAT
+ 0.6 V, a current may be injected into V
BAT
through an internal diode connected between V
DD
and the
power switch (V
BAT
).
If the power supply/battery connected to the V
BAT
pin cannot
support this current injection, it is strongly recommended to
connect an external low-drop diode between this power
supply and the V
BAT
pin.
If no external battery is used in the application, it is recommended to connect the V
BAT
pin to
V
DD
with a 100 nF external decoupling ceramic capacitor in parallel.
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