Altera Device-Specific Power Delivery Network User Manual
Page 7
Chapter 1: User Guide for the Device-Specific Power Delivery Network (PDN) Tool
1–3
PDN Decoupling Methodology Review
September 2012
Altera Corporation
Device-Specific Power Delivery Network (PDN) Tool User Guide
Z
target
is defined using the maximum allowable voltage ripple and transient current
and is calculated as follows:
For example, to reliably decouple a 3.3-volt power rail that allows 5% of AC ripple
and a maximum 2 A current draw, 50% of which is transient current, the desired
target impedance is:
To accurately calculate the Z
target
for any power rail, the following information must
be known:
■
The maximum transient current requirements for all devices in the system that are
powered by the power rail under consideration. You can obtain this information
from manufacturers of the respective devices. You can calculate the maximum
transient current of a device using the maximum total current and the transient
current percentage.
1
The percentage of transient current is signal-pattern dependent. It changes
as the output signal pattern varies for drivers using the power rail. You
need to choose the value that represents the worst-case scenario of the
power rail. For information about recommended settings, refer to the table
in the Introduction tab of the PDN tool.
Table 1–1 on page 1–4
is from the
PDN tool for a Stratix IV GX device. It lists the Stratix IV GX default power
supply voltage, the recommended settings of the transient current
percentage, and the allowable voltage ripple for power rails.
1
You can obtain accurate estimations on the maximum total current for
Altera devices using the Altera PowerPlay Early Power Estimator (EPE)
tool or the Quartus II PowerPlay Power Analyzer tools. You can download
the EPE tool for your target Altera device from the
Equation 1–1.
Equation 1–2.
Z
TARGET
VoltageRail
%Ripple
100
--------------------
MaxTransientCurrent
----------------------------------------------------------
=
Z
TARGET
3.3
0.05
2 x 0.5
----------------------------
0.165
=
=