DP83848K: short distance PCB connections

Hi Daisuke,

Two DP83848 devices can be connected directly with cap isolation.

Please see the following application note for more details on transformer-less operation:

www.ti.com/.../snla088a.pdf

Regards,

Hung Nguyen

Hi Hung-san,

Thank you for your reply.

If two DP83848 devices are directly connected without isolation on a board, what are the risks?

Best regards,

Daisuke

Hi Hung-san,

Should the isolation capacitors be placed closest to the terminal on the TX side?

Best regards,

Daisuke

Hi Hung-san,

Thank you for your reply.

If two DP83848 devices are connected via PCB connectors, should the isolation caps be placed on both the RX side and the TX side?

Best regards,

Daisuke

Hi Hung-san,

Thank you for your reply.

Best regards,

Daisuke

Hi Hung-san,

Each of two DP83848 devices is supplied from a different power source. While only one DP83848 device is powered-on, another DP83848 device can be powered-off, so some pulse signals may be input to the PMD pins of the DP83848 which is powered off.

Can such a situation be allowed?

If not, a protection circuit will be implemented to prevent this.

Best regards,

Daisuke

Hi Daisuke,

There should not be any issue with such situation.

Regards,

Hung Nguyen

Hi Hung-san,

Thank you for your reply.

Why is there no issue with such situation?

I guess that it is because the PMD pins can tolerate such situation. If so, what is the absolute maximum rating for the PMD pins?

In the threads listed below, it is mentioned that the transient peak voltage is tolerated in the case of DP83848I or DP83848-EP, but the power-off situation is not discussed.

http://e2e.ti.com/support/interface/f/138/p/485944/1764770#1764770

http://e2e.ti.com/support/legacy_forums/hirel/f/935/p/710427/2618612#2618612

On the other hand, the absolute maximum rating for the PMD pins is defined in the case of DP83822I. The input voltage for the PMD pins seems to be tolerated in the range of -0.3V to 6V regardless of whether the DP83822I device is powered-on or not.

Best regards,

Daisuke

Hi Daisuke,

I hope you correctly followed the app note I recommended in my first response. The cap coupling method is recommended for forced 100Base-TX, with auto-negotiation disabled.

The swing for 100Base-TX is small enough and meet the absolute maximum rating input voltage VCC + 0.5V.

Regards,

Hung Nguyen

Hi Hung-san,

Thank you for your reply.

The 100M Transmit voltage (VTPTD_100) is defined its maximum of 1.05V. It can swing in the range -0.525V to +0.525V. If the maximum swing is input, it will not meet the absolute maximum rating input voltage.

My understanding is not correct?

Best regards,

Daisuke

Hi Hung-san,

I checked the transformerless application note. Figure 8 shows the Sample 100 Mb/s Waveform With No Transformer. It seems to swing beyond the range -0.5V to +0.5V of the absolute maximum rating input voltage.

http://www.ti.com/lit/pdf/snla088

Best regards,

Daisuke

Hi Daisuke,

The absolute maximum input voltage rating is -0.5V MIN and VCC+0.5V MAX

Regards,

Hung Nguyen

Hi Hung-san,

Thank you for your reply.

Our customer is concerned about the situation in which the device is not powered-on (VCC = 0V or VCC < 3.3V).

Best regards,

Daisuke

Hi Hung-san,

As I posted earlier, the absolute maximum rating for the PMD pins is defined in the case of DP83822I. The input voltage for the PMD pins seems to be tolerated in the range of -0.3V to 6V regardless of whether the DP83822I device is powered-on or not.

Is the same in case of DP83848K? Is the absolute maximum input voltage rating the range of -0.3V to 6V?

Best regards,

Daisuke

Hi Daisuke,

Unfortunately, there is no absolute maximum input voltage rating separately for MDI pins on DP83848.

DP83822 absolute maximum input voltage rating: -0.3V to 6V
DP83848 absolute maximum input voltage rating: -0.5V to 3.8V

For 100M application, the swing is from -0.5V to 0.5V which is well within the input voltage range. There should not be any issue for this application.

Regards,

Hung Nguyen

Hi Daisuke,

I haven’t heard back from you, I’m assuming you were able to resolve your issue. I will go ahead and close this thread.

If you need further support, kindly open a new thread.

Regards,

Hung Nguyen

Hi Hung-san,

Daisuke Maeda said:

The 100M Transmit voltage (VTPTD_100) is defined its maximum of 1.05V. It can swing in the range -0.525V to +0.525V. If the maximum swing is input, it will not meet the absolute maximum rating input voltage.

I am concerned that -0.525V will be applied to the PMD pins of the DP83848K which is powered off.

Best regards,

Daisuke

Hi Hung-san,

Thank you for your reply.

Is your suggestion needed in case of connecting with cap isolation?
Is it also needed in case of connecting with trans isolation?

I am concerned that there is the stress with typical cable connection.

Best regards,

Daisuke