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CM1209-08MS Datasheet(PDF) 6 Page - California Micro Devices Corp |
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CM1209-08MS Datasheet(HTML) 6 Page - California Micro Devices Corp |
6 / 9 page © 2004 California Micro Devices Corp. All rights reserved. 6 430 N. McCarthy Blvd., Milpitas, CA 95035-5112 L Tel: 408.263.3214 L Fax: 408.263.7846 L www.calmicro.com 01/09/04 CM1209 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 4, which illus- trates an example of a positive ESD pulse striking an input channel. The parasitic series inductance back to the power supply is represented by L1 and L2. The volt- age VCL on the line being protected is: VCL = Fwd voltage drop of D1 + VSUPPLY + L1 x d(IESD ) / dt + L2 x d(IESD ) / dt where IESD is the ESD current pulse, and VSUPPLY 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 1ns. Here d(IESD)/dt can be approximated by ∆IESD/∆t, or 30/(1x10-9). So just 10nH of series induc- tance (L1 and L2 combined) will lead to a 300V incre- ment in VCL! Similarly for negative ESD pulses, parasitic series inductance from the VN pin to the ground rail will lead to drastically increased negative voltage on the line being protected. The CM1209 has an integrated Zener diode between VP and VN. This greatly reduces the effect of supply rail inductance L2 on VCL by clamping VP at the breakdown voltage of the Zener diode. However, for the lowest possible VCL, especially when VP is biased at a voltage significantly below the Zener breakdown voltage, it is recommended that a 0.22 µF ceramic chip capacitor be connected between VP 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 VP 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. Additional Information See California Micro Devices Application Note AP209, “Design Considerations for ESD Protection", under Applications at www.calmicro.com. Figure 4. Application of Positive ESD Pulse between Input Channel and Ground N L 2 L1 VP V PATH OF ESD CURRENT PULSE I ONE CHANNEL OF CM1209 CHANNEL INPUT GROUND RAIL CHASSIS GROUND POSITIVE SUPPLY RAIL SYSTEM OR CIRCUITRY BEING PROTECTED LINE BEING PROTECTED ESD 0.22µF D1 2 D 0A 20A VCL |
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