RClamp0544M
PROTECTION PRODUCTS
Applications Information
(continued)
ESD Protection With RailClamps
RailClamps are optimized for ESD protection using the
rail-to-rail topology. Along with good board layout, these
devices virtually eliminate the disadvantages of using
discrete components to implement this topology. Con-
PIN Descriptions
sider the situation shown in Figure 4 where discrete
diodes or diode arrays are configured for rail-to-rail
protection on a high speed line. During positive duration
ESD events, the top diode will be forward biased when
the voltage on the protected line exceeds the reference
voltage plus the V
F
drop of the diode. For negative
events, the bottom diode will be biased when the voltage
exceeds the V
F
of the diode. At first approximation, the
clamping voltage due to the characteristics of the protec-
tion diodes is given by:
V
C
= V
CC
+ V
F
V
C
= -V
F
(for positive duration pulses)
(for negative duration pulses)
PRELIMINARY
“Rail-T
Pro
Topology
Figure 4 - “Rail-To-Rail” Pro tection Topology
(First Approximation)
However, for fast rise time transient events, the effects of
parasitic inductance must also be considered as shown
in Figure 5. Therefore, the actual clamping voltage seen
by the protected circuit will be:
V
C
= V
CC
+ V
F
+ L
P
di
ESD
/dt (for positive duration pulses)
V
C
= -V
F
- L
G
di
ESD
/dt
(for negative duration pulses)
ESD current reaches a peak amplitude of 30A in 1ns for
a level 4 ESD contact discharge per IEC 61000-4-2.
Therefore, the voltage overshoot due to 1nH of series
inductance is:
V = L
P
di
ESD
/dt = 1X10
-9
(30 / 1X10
-9
) = 30V
Example:
Consider a V
CC
= 5V, a typical V
F
of 30V (at 30A) for the
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
C
= 5V + 30V + (10nH X 30V/nH) = 335V
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
that it is not uncommon for the V
F
of discrete diodes to
exceed the damage threshold of the protected IC. This is
due to the relatively small junction area of typical dis-
crete components. It is also possible that the power
2006 Semtech Corp.
6
Figure 5 - The Effects of Parasitic Inductance
When Using Discrete Components to Implement
Rail-T
Pro
Rail-To-Rail Pr o t ection
Rail-T
Pro
Figure 6 - Rail-To-Rail Protection Using
RailClam p T V S Arrays
RailClamp TV Arrays
dissipation capability of the discrete diode will be ex-
ceeded, 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, the
current will be directed through the integrated TVS diode
to ground. The maximum voltage seen by the protected
IC due to this path will be the clamping voltage of the
device.
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