CM1223
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7
BACKDRIVE PROTECTION
Backdrive protection is needed to block against backdrive current flowing from a high potential voltage node toward a lower
potential voltage node through the interface cable.
For example, consider a DVD player connected to a TV via an HDMI interface. If the DVD player is switched off and the
TV is left on, there is a possibility of reverse current flow back into the main power supply rail of the DVD player. Typically,
the DVD’s power supply has some form of associated bulk supply capacitance, and it is possible over time to charge that bulk
supply capacitance to some intermediate level.
If that level rises above the power−on−reset (POR) voltage level of some of the integrated circuits, the DVD player may not
reset properly when the DVD player is turned back on. This is largely because all CMOS logic exhibits a very high impedance
on the power rail node even when ”off”.
To avoid this situation, the CM1223 with integrated backdrive protection diode was designed to block backdrive current,
guaranteeing no more than 5
mA on any I/O pin when the I/O pin voltage is greater than the CM1223 supply voltage.
APPLICATION INFORMATION
Design Considerations
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 segments between the signal input (typically a connector)
and the ESD protection device. Application of Positive ESD Pulse between Input Channel and Ground illustrates an example
of a positive 8 kV ESD pulse striking an input channel. The 8 kV ESD current pulse will divert along the path as indicated in
Application of Positive ESD Pulse between Input Channel and Ground, through the D1 diode and the Zener diode back to the
ground rail.
An ESD current pulse can rise from zero to its peak value in a very short time. For 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. The CM1223 has a fast response
time of less than 1ns and low clamp voltage to handle this pulse scenario.
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 CM1223 also has an integrated backdrive diode between V
P
and V
N
to prevent backdrive current flow from the powered
sources.
As a general rule, the ESD Protection Array should be located as close as possible to the point of entry of expected
electrostatic discharges.
