Power supply AN2752
8/40 AN2752 Rev 6
2.2 Main operating voltages
STM8S and STM8AF devices are made in 0.13 µm technology. The STM8S and STM8AF
core and I/O peripherals need different power supplies. In fact, STM8S and STM8AF
devices have an internal regulator with a nominal target output of 1.8 V.
Stabilization for the main regulator is achieved by connecting an external capacitor C
EXT
to
the VCAP pin. Refer to the STM8S or STM8AF datasheets (available on www.st.com) for
more information on the VCAP characteristics. Take care to limit the series inductance to
less than 15 nH.
Figure 2. External capacitor
2.3 Power-on/power-down reset (POR/PDR)
The input supply to the main and low power regulators is monitored by a power-on/power-
down reset circuit. The monitoring voltage range is 0.7 V to 2.7 V.
During power-on, the POR/PDR keeps the device under reset until the supply voltages (V
DD
and V
DDIO
) reach their specified working area.
At power-on, a defined reset must be maintained below 0.7 V. The upper threshold for a
reset release is defined in the electrical characteristics section of the product datasheet.
Hysteresis is implemented (POR > PDR) to ensure clean detection of voltage rise and fall.
The POR/PDR also generates a reset when the supply voltage drops below the V
POR/PDR
threshold (isolated and repetitive events).
C
Rleak
ESR ESL
Where:
ESR is the equivalent series resistance
ESL is the equivalent inductance
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