AN2586 Freescale Semiconductor / Motorola, AN2586 Datasheet - Page 6

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

Previous Work Study
Previous Work Study
The Xmax that is maximum common path impedance we can tolerate at the consumed current is the
following:
As shown in the following equation, Fpsw is the frequency below which the power supply wiring is
adequate, and Lpsw is the power supply wiring inductance. The supply inductance value is a function of the
power track’s geometry. Assume a system with 100nH:
Thus, the bypass first bypass capacitor to add is the following:
Because bypass capacitors have inductance in series, we must estimate a frequency Fbypass after which this
bypass capacitor is not adequate. In the following equation, L is the series inductance of the bypass
capacitor.
Now, let L = 5nH.
This result tells us that the 14.5pF capacitor is adequate from 132 KH to 2.6 MH.
The next step is to add more capacitors to take care of the higher frequency range. In this analysis, the
frequency that is proportional to a digital signal rise time frequency is called Fknee, and is defined as the
following:
In our case, let Trise be as follows:
Therefore:
If we want to add m small capacitors with L inherent inductance per each, then:
6
MPC8260 PowerQUICC II Family Power Distribution Trends: A Survey
Cbypass
Freescale Semiconductor, Inc.
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Fpsw
Fknee
--- -
m
L
=
=
=
Fbypass
------------------------------------ -
2πFpswXmax
-------------------- -
2πLpsw
---------------------- -
2πFknee
Xmax
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Xmax
=
Xmax
---------------- -
2Trise
Trise
1
1
Fbypass
=
=
=
=
100mV
----------------- -
Fknee
=
Xmax
-------------- -
=
------------------------ -
2π 100n
2πL
1.2
----------------------------- -
2 π
------------- -
200M
0.083
200M
------------- -
×
2 2
×
=
2
0.083
=
×
=
-------------------------------------------- -
2π 0.083 132k
×
=
=
Xmax
-------------- -
2πL
50M
---------------- -
2Trise
=
×
----------------------- -
2π 5nH
=
83mOhms
0.01µs
=
1
0.083
50Mhz
×
132165 132kH
=
1
0.2643nH
×
=
≅
2643312 2.6Mhz
=
14.5uF
≅
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