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Philips Semiconductors
Application note
AN212
Package lead inductance considerations
in high-speed applications
Authors: Stephen C. Hinkle, Jeffrey A. West
1
1987 June
INTRODUCTION
A circuits become faster, more concern needs to be focused on
packaging and interconnects in order to fully utilize device
performance. One area of concern is with the package leads
between the chip and the board environment. The current flowing
into or out of an integrated circuit is conducted through a lead frame
trace and bonding wire connecting the integrated circuit to outside
circuitry. these leads are circuit elements, inductors, and have a
definite effect on the circuit performance because they generate
noise in high-speed applications.
Inductance is the measure of change in the magnetic field
surrounding a conductor resulting form the variation of the current
flowing through the conductor. The change in current through the
inductor induces a counter electromotive force, EMF, which opposes
that change in current.
An example is a buffer driver discharging a 50pF load. At a
switching rate of about 3V in 2ns, the current generated by
discharging that capacitor at that rate is:
I C
dV
dt
50pF
3v
2ns
75mA.
All this current flows through the ground lead of the package.
Changing the current through this lead generates a ground lead
voltage or ground bounce. A typical lead inductance has been
measured to be about 10nH. Switching 75mA through a ground lead
with an inductive value of 10nH causes a ground bounce of about:
V L
dI
dt
10nH
75mA
1ns
750mV.
Figure 1 illustrates the current surge and ground bounce during
switching. This was modeled using the equations:
V(t)
3V
1 E
(tt
O
)K
I
C
(t) C
dV(t)
dt
V
L
(t) L
dI
C
(t)
dt
LC
D
2
V(t)
dt
2
If more than one output is switched at a time, this ground bounce
can get very large. Changing the ground reference on the chip can
have significant effects on circuit performance. A V
CC
bounce can
also be calculated when the 50pF load capacitors are being charged
and can also have serious effects on circuit performance.
Some of the problems caused by package lead inductance are:
1. Adding delay through buffer parts
2. Changing the state of flip-flop parts
3. Output glitching on unswitched outputs
4. Circuit oscillations.
4V
400mA
–100mA
–1V
0.0 1.0E–00
SECONDS
GROUND BOUNCE
CURRENT SURGE
OUTPUT WAVEFORM
GRAPHICAL OUTPUT–GROUND BOUNCE
SF01318
Figure 1.
