AM5718: HW Design Checklist - ferrite beads on unused interface power pins?

David M. Alter

Guru 61235 points

Part Number: AM5718

Sitara Team,

In the HW Design Checklist document, SPRACK7, sections 2.7.4 and 2.7.6 say:

2.7.4 If HDMI is not Used

• VDDA_HDMI should be connected through a ferrite bead to V1_8D

2.7.6 If SATA is not Used

• VDDA_SATA should be connected to V1_8D through a ferrite bead.

What are the purpose of the ferrite beads, and are they really necessary?

Regards,

David

over 5 years ago

0 Biser Gatchev-XID over 5 years ago

TI__Guru**** 393215 points

Hi David,

AM57xx devices contain multiple analog power pins that provide power to sensitive analog circuitry like PLLs, DLLs and SERDES buffers and terminations. These must be attached to filtered power sources. These filter solutions must match the recommendations in the Hardware Design Guide. Each of the VDDA_xxxxx power input pins should be connected to V1_8D through a filter circuit.

For unused power rails it's specified that VDDA_USBx does not require filtering. VDDA_HDMI, VDDA_PCIEx and VDDA_SATA do require filtering even if the respective interface is not used.

0 David M. Alter over 5 years ago in reply to Biser Gatchev-XID

TI__Guru 61235 points

Hello Biser,

I have a follow-up question. Connecting the module VDDA's (e.g., VDDA_HDMI, VDDA_PCIEx, VDDA_SATA) to V1_8D using a ferrite bead would seem to violate Figure 5-2 timing diagram in the datasheet SPRS957G, p.177. Does it not?

The Figure indicates that the module VDDA's should ramp AFTER Vdds18V (V1_8D).

How do we justify using the ferrite beads with respect to the diagram?

Or perhaps I am misidentifying V1_8D in the figure?

Thank you,

David

0 Ahmad_Rashed over 5 years ago in reply to David M. Alter

TI__Genius 13567 points

Hi David,

It is best to stick with the assignments as shown in the sequencing diagram you posted and detailed in Table 3 of the PMIC user guide.

The supply name "V1_8D" really should not have been used here.

0 David M. Alter over 5 years ago in reply to Ahmad_Rashed

TI__Guru 61235 points

Hi Ahmad,

I guess this leaves me confused. The design checklist SPRACK7 says for example:

2.7.4 If HDMI is not Used

• VDDA_HDMI should be connected through a ferrite bead to V1_8D

So what should be do with VDDA_HDMI (for example) if the HDMI is unused? What signal is V1_8D referring to?

Regards,

David

0 Ahmad_Rashed over 5 years ago in reply to David M. Alter

TI__Genius 13567 points

David,
I'm in agreement with you. The VDDA's should power on after VDDS18V so using the "V1_8D" supply (LDO2 on the PMIC) would be incorrect. That supply name comes from the AM5718 IDK EVM. The VDDA supplies that belong to a PHY should be on one of the two LDOs dedicated to PHY supplies. This is detailed in Table 3 of the PMIC user guide. EDIT: It doesn't matter if the PHY is used or not.

Regards,
Ahmad

0 David M. Alter over 5 years ago in reply to Ahmad_Rashed

TI__Guru 61235 points

Ahmad,

I think I understand what you're saying. For example, the 5718 IDK schematic shows VDDA_PCIE and _HDMI connected to LDO4 on the PMIC using a capacitive filter NFM21PC474 from Murata.

We can use this design whether or not the individual module is used. Alternately, if one of the modules is not used, we could replace the NFM21PC474 with a ferrite bead (presumably cheaper). But we still leave it connected to the same LDO4.

Am I correct above?

If so, can we also remove the decoupling caps for that unused module (i.e., the 1uF and two 0.01 uF caps)?

Thank you,

David

0 Ahmad_Rashed over 5 years ago in reply to David M. Alter

TI__Genius 13567 points

Hi David,
You can change to the ferrite bead but it is not recommended to remove the decoupling capacitors.

Regards,
Ahmad

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AM5718: HW Design Checklist - ferrite beads on unused interface power pins?