SRDA3.3-6 and SRDA05-6
PROTECTION PRODUCTS
Applications Information
(continued)
approximation, the clamping voltage due to the charac-
teristics of the protection diodes is given by:
V =V +V
C
CC
C
F
F
(for positive duration pulses)
(for negative duration pulses)
V =
Descriptions
PIN
-V
However, for fast rise time transient events, the
effects of parasitic inductance must also be consid-
ered as shown in Figure 2. Therefore, the actual
clamping voltage seen by the protected circuit will be:
V = V + V + L di
C
CC
F
P
ESD
/dt (for positive duration pulses)
(for negative duration pulses)
Figure 1 - “Rail-To-Rail” Protection Topology
(First Approximation)
V = -V - L di
C
F
G
ESD
/dt
ESD current reaches a peak amplitude of 30A in 1ns
for a level 4 ESD contact discharge per IEC 1000-4-2.
Therefore, the voltage overshoot due to 1nH of series
inductance is:
V = L di
P
ESD
/dt = 1X10
-9
(30 / 1X10
-9
) = 30V
Example:
Consider a V = 5V, a typical V of 30V (at 30A) for the
CC
F
steering diode and a series trace inductance of 10nH.
The clamping voltage seen by the protected IC for a
positive 8kV (30A) ESD pulse will be:
V = 5V + 30V + (10nH X 30V/nH) = 335V
C
This does not take into account that the ESD current is
directed into the supply rail, potentially damaging any
components that are attached to that rail. Also note
the high V of the discrete diode. It is not uncommon
F
for the V of discrete diodes to exceed the damage
F
threshold of the protected IC. This is due to the
relatively small junction area of typical discrete compo-
nents. It is also possible that the power dissipation
capability of the discrete diode will be exceeded, thus
destroying the device.
The RailClamp is designed to overcome the inherent
disadvantages of using discrete signal diodes for ESD
suppression. The RailClamp’s integrated TVS diode
helps to mitigate the effects of parasitic inductance in
the power supply connection. During an ESD event,
Figure 2 - The Effects of Parasitic Inductance When
Using Discrete Components to Implement Rail-To-Rail
Protection
Figure 3 - Rail-To-Rail Protection Using
RailClamp TVS Arrays
2008 Semtech Corp.
6
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