SN65HVD32: SN65HVD32 Transient Protection

Rajesh,
I'm afraid SN65HVD32 might not be able to handle 1.25A for 100us. The only integrated surge protection device now we have is SN65LBC184, which can take about 500W surge energy. You can refer to this app note www.ti.com/.../tida-00731 for some protection schemes and test results. Please let me know if you have more questions.
Regards,
Hao

Hi Hao,

Thanks for your quicker response!!!

We are providing front level of transient protection by having external TVS diode which is sufficient for 100usec pulse. However, during 100usec transient event, TVS diode clamps to a voltage of max. 14.5V. So to take care of this clamping voltage, we have put 10 ohm pulse proof thick film resistors (as recommended in the device datasheet, see page no. 29 / Table 9) to limit the current through transceiver pins.

Also as I could see Transceiver equivalent Input Output schematic diagram (Figure 33 / Page no. 25) in the datasheet. For Y and Z outputs, there are clamping circuits provided internal to the device. Please have a look and let us know your opinion about the limitations of handling secondary level of transients by the IC SN65HVD32.

Similarly if you look at the attenuator circuit of the transceiver inputs A and B pins, I feel it will be sufficient to handle the residual transient as the R3 value listed ( in Table 7/ page no. 25) is very high 180Kohm. Please correct me if I am wrong in my assumption.

Hope the above rephrasing of the issue will make you understand better. Please feel free to ask me more explanations if required.

Regards,
Rajesh

Rajesh,
Thanks for your detailed explanation. The clamping diode can handle some overstress transient current. Unfortunately 100us surge event is much more severe than HBM/CDM events, which is beyond the cell's clamping capability. A high impedance like R3 can limit the current flowing into that pin. But we still need a path for diverting the surge current. I agree with most of your analysis. But unfortunately this IC has very limited capability in surge protection due to its physical restriction. Please let me know if you have more questions.
Regards,
Hao

Hi Hao,

I could see the limitations of SN65HVD32 in handling surge transients.

Could you please elaborate on below application circuit provided in the datasheet:

Is above protection circuit applicable only for SN65HVD33, what about its transient handling capabilities? 

Will this external TVS, R1 and R2 components not sufficient for protecting SN65HVD32 from surge transient?

Please clarify.

Thanks & Regards,

Rajesh Kumar

Hi Hao,

Please find below RS422 transceiver protection circuit for our application:

Signalling speed requirement: 500Kbps

Surge waveform duration: 100usec

Residual transient current flowing into the IC: 1.25A

Note: As below circuit is already implemented and under evaluation, so we will not able to change the device to different package. If you have better transient handling capability IC in same package compared to SN65HVD32, we shall be able to replace it.

Regards,

Rajesh 

Rajesh,
You can consider SN65HVD1471, which has better ESD protection but might not be enough for the surge current. What's your schedule of product release? We're developing some RS485 transceivers with surge protection feature. You could evaluate them when we have samples.
Regards,
Hao

Hi Hao,

SN65HVD1471 can not be used, as we require minimum 500kbps signalling.

Currently we are in testing phase, by August it will be completed and then production shall start accordingly.

Did you get a chance to have a look on the circuit which I had shared in previous mail. For your reference I am attaching the circuit diagram once again:

I am also curious to know that what is the maximum surge current and duration SN65HVD32 will able to tolerate with good safety of margin.

Thanks & Regards,

Rajesh

Rajesh,
In that case, you can consider SN65HVD1474 that can support 20Mbps data rate. Unfortunately we don't have any characteristic data of surge current for SN65HVD32 or SN65HVD1474. The only data we have is from SN65LBC184. The schematic looks good except I think we need find a current path for the residue surge voltage internally or externally. Please let me know if you have more questions.
Regards,
Hao