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# [FAQ] TPD1E01B04-Q1: How do you correlate capacitance needed in ESD protection device with protocol data rate?

Part Number: TPD1E01B04-Q1
Other Parts Discussed in Thread: ESD401, TPD1E10B06,

Hi Erika,

Could you help to recommend the ESD diode for the UB933 and UB954?  What’s the maximum IO cap for the FPD-link for these two parts?

Customer want to know what’s the relationship between the data line-rate and Cap.

For example, 4Gbps need something around smaller 20pF?

Do you have any formula or calculation for this

Thanks!

BR,

James

• In general, the bit rate does not paint the complete picture for the capacitance requirement of the system. You need to know other things like rise time, bits per cycle, and number of lines being used to share the data transfer. For example, a 1 Gb Ethernet interface can handle much more capacitance per line than a 1 GHz single ended line since Ethernet splits the data into 4 twisted pairs. You can read more about capacitance requirements for high speed signals like USB and HDMI in this app note: https://www.ti.com/lit/an/slva793/slva793.pdf?ts=1639156799830

For FPD-Link, 20 pF seems way too large for a 4 Gbps signal. I’m not exactly sure how FPD-Link encodes their signals, but general rule of thumb is that the bit rate is equal to 2x the signal frequency (4Gbps/2 = 2 GHz). TPD1E10B06, which has only 12 pF of capacitance, has a -3 dB insertion loss of 400 MHz (600 Mbps). If you refer to the eye diagrams from the ESD401 datasheet below, the 3 Gbps signal takes ~666 ps to complete one cycle. Since frequency = 1/period, the frequency for this signal is 1/666ps = 1.5 GHz which is half of 3 Gbps. Therefore, since the -3 dB insertion loss of ESD401 is 6 GHz, it can support this 3 Gbps signal.

With all of that being said, line/junction capacitance is not the only thing that will attenuate the signal. Another factor could be the line-to-line capacitance for multi-channel didoes. While the line capacitance is a good starting point, its best to refer to the eye diagram to see exactly how the diode will perform. For 4 Gbps, I would recommend staying below 0.8 pF.

For for automotive serdes ESD protection, TPD1E01B04-Q1 is always a good choice due to its low capacitance and small package size. You can refer to this app note for more details: www.ti.com/.../slvaf60a.pdf