CM1235 Datasheet, PDF (5/14 Pages) ON Semiconductor – Small Footprint PicoGuard XS® ESD Clamp Array For High Speed Data Line Protection

CM1235
CM1235 Inductor Elements
In the CM1235 PicoGuard XS architecture, the inductor elements and ESD protection diodes interact differently
compared to the standard ESD model.
In the standard ESD protection device model, the inductive element presents high impedance against high slew
rate strike voltage, i.e. during an ESD strike. The impedance increases the resistance of the conduction path
leading to the ESD protection element. This limits the speed that the ESD pulse can discharge through the ESD
protection element.
In the PicoGuard XS architecture, the inductive elements are in series to the conduction path leading to the
protected device. The elements actually help to limit the current and voltage striking the protected device.
First the reactance of the inductive element, L1, on the connector side when an ESD strike occurs, acts in the
opposite direction of the ESD striking current. This helps limit the peak striking voltage. Then the reactance of the
inductive element, L2, on the ASIC side forces this limited ESD strike current to be shunted through the ESD
protection diodes. At the same time, the voltage drop across both series element acts to lower the clamping
voltage at the protected device terminal.
Through this arrangement, the inductive elements also tune the impedance of the ESD protection element by
cancelling the capacitive load presented by the ESD diodes to the signal line. This improves the signal integrity
and makes the overall ESD protection device more transparent to the high bandwidth data signals passing
through the channel.
The innovative PicoGuard XS architecture turns the disadvantages of the parasitic inductive elements into useful
components that help to limit the ESD current strike to the protected device and also improves the signal integrity
of the system by balancing the capacitive loading effects of the ESD diodes. At the same time, this architecture
provides an impedance matched signal path for 50Ω loading applications.
Board designs can take advantage of precision internal component matching for improved signal integrity, which
is not otherwise possible with discrete components at the system level. This helps to simplify the PCB layout
considerations by the system designer and eliminates the associated passive components for load matching that
is normally required with standard ESD protection circuits.
Each ESD channel consists of a pair of diodes in series that steer the positive or negative ESD current pulse to
either the Zener diode or to ground. This embedded Zener diode also serves to eliminate the need for a separate
bypass capacitor to absorb positive ESD strikes to ground. The CM1235 protects against ESD pulses up to ±8kV
contact per the IEC 61000-4-2 standard.
Rev. 3 | Page 5 of 14 | www.onsemi.com