AN2586 Freescale Semiconductor / Motorola, AN2586 Datasheet - Page 5

• Manufacturer Part Number: AN2586
• Description: MPC8260 PowerQUICC II Family Power Distribution Trends
• Manufacturer: Freescale Semiconductor / Motorola

This excessive amount of current change definitely creates a huge stress on the power delivery network,
especially if it was not properly designed.
So far we have seen some description of load current consumption and have had a rough estimation of its
rate of change. Imagine the current demand curve as it is shown in Figure 3.
Note that the current exact profile depends on the code. If I(t) is the exact current profile, our assumption is
the following:
The first technique to roughly estimate the bypass capacitors is based on the following set of equations:
In this case:
Based on our example, if Vcc = 2.0V and tolerance is 5% (also the current assumed is 1.2A), it could be
stated as follows:
MOTOROLA
Current
MPC8260 PowerQUICC II Family Power Distribution Trends: A Survey
clock cycles
Freescale Semiconductor, Inc.
For More Information On This Product,
N
Go to: www.freescale.com
∆V
∆I
----- -
Cbypass
∆t
∆V
dI t ( )
------------ -
=
dt
=
Fpsw
=
1.2 200e6
---------------------------- -
2.0 0.05
Xmax
Figure 3.
VCC tolerance
≤
×
=
×
max I t ( )
----------------------------------- -
=
2
------------------------------------ -
2πFpswXmax
-------------------- -
2πLpsw
×
=
Xmax
(
∆
------- -
N
I ∆
=
V
=
1
100mV
) F
1.2e8
×
Previous Work Study
max(I(t))
Time
5