Power Supply Routing - Fujitsu F2MC-16LX Series Application Note

4. Power supply routing

One topic our noise reduction technology is lowering internal power supply voltage on 3V
level to reduce the current flow. Fig. 2 shows the structure of 5V and 3V power supply.
Figure 3: Structure of power supply for MCU core and IO-Port
Only the right placement and the value of decoupling capacitor (DeCap) guaranty the
function of decoupling capacitors. The high speed current (di/dt) will be supported from
DeCap only. The exactly use of DeCap is important for the noise reduction on the PCB.
VCC
DeCap
GND
a) VCC and GND lead to supply
noise current flows not via
DeCap, DeCap has not effect
VCC
DeCap
GND
GND
d) VCC and GND lead to supply
noise current flows not via
DeCap, DeCap has not effect
Figure 4: The exactly use of the DeCap (decoupling capacitor)
Vcc3
3V Regulator
Vss
RAM
AVcc
VCC
µC
DeCap
GND
b) GND lead noise to system GND
noise current flows partly via
DeCap, DeCap has hardly effect
µC
VCC
DeCap
e) GND is not short connected to
DeCap. between GND and
DeCap flows a loop current
DeCap has hardly effect
I/O-PORT
FLASH
CPU
.
SCI / TIMER / etc
A/D+D/A
AVss
µC
DeCap
c) GND lead noise to System GND
noise current flows partly via
DeCap, DeCap has hardly effect
µC
DeCap
GND
f) DeCap correct connected to µC
and power supply.
high speed current will be
supported from DeCap
Vcc
Vss
VCC
µC
GND
VCC
µC
GND
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