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Recommended Layout Practices
25.2 Printed Circuit Board Layout Practices
25.2.1 Power Supply Decoupling
The SoCs have separate power and ground pins for different internal circuitry sections.
The VDD and VSS pins supply power to I/O buffers, while VDDC and VSSC supply power
to the core logic.
Each of the VDD and VDDC pins must be provided with a low impedance path to the cor-
responding board power supply. Likewise, the VSS and VSSC pins must be provided with
a low impedance path to the board ground.
Each power supply must be decoupled to ground using at least one 0.1 µF high frequency
capacitor located as close as possible to a VDDx, VSSx pin pair on each of the four sides
of the chip. If room on the circuit board allows, add one 0.01 µF high frequency capacitor
near each VDDx, VSSx pair on the chip.
To be effective, the capacitor leads and associated circuit board traces connecting to the chip
VDDx, VSSx pins must be kept to less than half an inch (12.7 mm) per capacitor lead. There
must be one bulk 10 µF capacitor for each power supply placed near one side of the chip.
25.2.2 Required VDDA_PLL, VSSA_PLL Filter
The VDDA_PLL pin supplies power to the chip PLL circuitry. VSSA_PLL is the ground
return path for the PLL circuit. If the internal PLL circuit will be used, these pins must have
a low-pass filter attached as shown in Figure 25-1.
The power pin VDDA_PLL path must be a single wire from the IC package pin to the high
frequency capacitor, then to the low frequency capacitor, and finally through the series
resistor to the board power supply. The distance from the IC pin to the high frequency
capacitor must be kept as short as possible.
Similarly, the VSSA_PLL path is from the IC pin to the high frequency capacitor, then to
the low frequency capacitor, keeping the distance from the IC pin to the high frequency cap
as short as possible.
25-2
100 Ω
V+1.8
(from board)
+
22 µF
Figure 25-1. VDD_PLL, VSSA_PLL Filter Circuit
Note that the VSSA_PLL pin specifically does not have a connection to
the circuit board ground. The PLL circuit has an internal DC ground
connection to VSS (GND), so the external VSSA_PLL pin must NOT be
connected to the circuit board ground, but only to the filter components.
6/17/03
LH75400/01/10/11 (Preliminary) User's Guide
PIN 85
VDDA_PLL
0.1 µF
LH754xx
VSSA_PLL
PIN 84
CAUTION
LH754xx-76
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