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CM1293A
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 5, 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 L 1 and L 2 . The voltage V CL on the
line being protected is:
V CL = Fwd voltage drop of D 1 + V SUPPLY + L 1 x d(I ESD ) / dt+ L 2 x d(I ESD ) / dt
where I ESD is the ESD current pulse, and V SUPPLY 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 1 ns. Here d(I ESD )/dt can be
approximated by D I ESD / D t, or 30/(1x10 ? 9 ). So just 10 nH of series inductance (L 1 and L 2 combined) will lead to a 300 V
increment in V CL !
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.
The CM1293 has an integrated Zener diode between V P and V N . This greatly reduces the effect of supply rail inductance
L 2 on V CL by clamping V P at the breakdown voltage of the Zener diode. However, for the lowest possible V CL , especially when
V P is biased at a voltage significantly below the Zener breakdown voltage, it is recommended that a 0.22 μ F ceramic chip
capacitor be connected between V P and the ground plane.
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.
V P
L 2
POSITIVE SUPPLY RAIL
V CC
PATH OF ESD CURRENT PULSE I ESO
0.22 m F
D 1
D 2
ONE
CHANNEL
L 1
CHANNEL
INPUT
25 A
LINE BEING
PROTECTED
V CL
SYSTEM OR
CIRCUITRY
BEING
PROTECTED
0A
V N
GROUND RAIL
CHASSIS GROUND
Figure 5. Application of Positive ESD Pulse between Input Channel and Ground
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