Altera Device-Specific Power Delivery Network - Manual (Page 26)

1–22 Chapter 1: User Guide for the Device-Specific Power Delivery Network (PDN) Tool
Design PCB Decoupling Using the PDN Tool
Device-Specific Power Delivery Network (PDN) Tool User Guide September 2012 Altera Corporation
The PDN tool calculated that Z
TARGET
is 0.0077  and F
EFFECTIVE
is 24.91 MHz.
Figure 1–16 shows one of the capacitor combinations that you can select to meet the
design goal. Figure 1–16 is the enlarged view of the Z
EFF
plot. As shown in the plot,
Z
EFF
remains under Z
TARGET
up to F
EFFECTIVE
. There are many combinations, but the
ideal solution is to minimize the quantity and the type of capacitors needed to achieve
a flat impedance profile below the Z
TARGET
.
Derive Decoupling in the Power-Sharing Scenarios
It is a common practice that several power rails in the FPGA device share the same
power supply. For example, you can connect VCCIO, VCCPD, VCCPGM, and
VCC_CLKIN rails that require the same supply voltage to the same PCB power plane.
This can be required by the design, such as in the memory interface case. This can also
come from the needs to reduce BOM cost. In the power sharing scenario, the noise
within a power supply is generated by transient current from multiple rails. The
design flow is different comparing to the single power rail scenario.
Step 1: Derive Z
TARGET
To derive Z
TARGET
, follow these steps:
1. Enter the power supply voltage.
2. Choose the smallest allowable voltage ripple of all shared power rails as the
allowable voltage ripple.
3. Use the total current of all rails as I
MAX.
4. Derive the percentage of transient current by dividing the total transient current of
all rails by the total current of all rails. Select the setting in the Transient current
column that is the closest to derived number.
Figure 1–16. Enlarged Plot of Zeff Using the Figure 1-16 Design