CM1215
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5
APPLICATION INFORMATION
Design Considerations
In order to realize the maximum protection against ESD pulses, care must be taken in the PCB layout to minimize parasitic
series inductances on the Supply/ Ground rails as well as the signal trace segment between the signal input (typically
a connector) and the ESD protection device. Refer to Figure 1, which illustrates an example of a positive ESD pulse striking
an input channel. The parasitic series inductance back to the power supply is represented by L1 and L2. The voltage VCL on
the line being protected is:
V
CL
= Fwd voltage drop of D
1
+ V
SUPPLY
+ L1 x d(I
ESD
) / dt+ L2 x d(IESD) / dt
where IESD is the ESD current pulse, and VSUPPLY is the positive supply voltage.
An ESD current pulse can rise from zero to its peak value in a very short time. As an example, a level 4 contact discharge
per the IEC61000−4−2 standard results in a current pulse that rises from zero to 30 Amps in 1ns. Here d(IESD)/dt can be
approximated by d(
ESD
)/dt, or 30/(1x10−9). So just 10 nH of series inductance (L1 and L2 combined) will lead to a 300 V
increment in VCL!
Similarly for negative ESD pulses, parasitic series inductance from the V
N
pin to the ground rail will lead to drastically
increased negative voltage on the line being protected.
As a general rule, the ESD Protection Array should be located as close as possible to the point of entry of expected
electrostatic discharges. The power supply bypass capacitor mentioned above should be as close to the V
P
pin of the Protection
Array as possible, with minimum PCB trace lengths to the power supply, ground planes and between the signal input and the
ESD device to minimize stray series inductance.
Additional Information
See also ON Semiconductor Application Note, “Design Considerations for ESD Protection”, in the Applications section.
Figure 3. Application of Positive ESD Pulse between Input Channel and Ground
POSITIVE SUPPLY
PATH OF ESD CURRENT
PULSE (IESD)
SYSTEM OR
CIRCUITRY
BEING
PROTECTED
CHANNEL
IMPUT
LINE BEING
PROTECTED
ONE
CHANNEL
D1
D2
C1
L1
GROUND RAIL
CHASSI‘S GROUND
