AM5728: Unexpected behavior on PMIC LDO's

matusan

Guru 10235 points

Part Number: AM5728
Other Parts Discussed in Thread: TPS659037

Hello, TI Experts,

Our customer sent us some questions about AM5728 with PMIC(TPS6590377ZWST).

They designed PMIC connection to AM5728 based on Figure 2 of TPS659037 User's Guide(SLIU011D).

And they found strange phenomena on LDO3 signal and LDO4 signal.

Please look attached pdf.

Strange phenomena:

- If SMPS7 is ramped up, LDO3/LDO4 signals seems to be slightly changed.

Connection detail between AM5728 and PMIC is as follows;

- SMPS7 is connected to vdd1-23 and vdd_rtc.

- LDO3 is connected to vdda_sata,vdda_usb1,vdda_usb2 and vdda_usb3.

- LDO4 is connected to vdda_hdmi,vdda_pcie,vdda_pcie0 and vdda_pcie1.

Question:

- Is there same report or experience?

- Is there any clue to solve this problem?

They are worried that some unexpected weak connection is existing from SMPS7 through AM5728 to LDO3/LDO4 from their trial as below.

- If SMPS7 connection is cut, LDO3/4 signal changing seems to be disappeared.

We would appreciate if you tell us how to solve this problem.

Best regards,

Problem_LDO3_4.pdf

over 8 years ago

0 Biser Gatchev-XID over 8 years ago

TI__Guru**** 393215 points

Hi,

According to Figure 2 in SLIU011D, SMPS7 should be connected to VDD_CORE. Why isn't the user guide followed?

0 matusan over 8 years ago in reply to Biser Gatchev-XID

Guru 10235 points

Hi,

Thank you for your prompt reply.

Our understandings are as follows;
- SMPS7 should be connected to vdd-pins of AM5728 which stands for VDD_CORE in Figure 2 in SLIU011D.
- From AM57-EVM shchematics (page7 of TI_AM572XEVM_REV_A3a.pdf) , VDD_CORE is connected to vdd-pins.

So we think the signal name "VDD_CORE" is used as same as vdd-pins.

Is this understanding correct?

If this understanding correct, please tell us how to solve this problem.

Best regards,

0 Biser Gatchev-XID over 8 years ago in reply to matusan

TI__Guru**** 393215 points

matusan said:
- SMPS7 is connected to vdd1-23 and vdd_rtc.

OK, about vdd1-23 I agree, the notation is not clear, but vdd_rtc must be connected to LDO9.

0 matusan over 8 years ago in reply to Biser Gatchev-XID

Guru 10235 points

Hi,

Thank you for your prompt reply.
I really appreciate your help.

I will send the answer to the customer.

Best regards

0 matusan over 8 years ago in reply to matusan

Guru 10235 points

Hello, TI Experts,

Our customer sent us the detail as follows;

- They designed based on the note(4) of datasheet(SPRS953B) "Figure 5-2. Power-Down Sequencing" as below;

(Because they don't use RTC-mode.)

(4) If RTC-mode is not used then the following combinations are approved:

- vdda_rtc can be combined with vdds18v

- vdd_rtc can be combined with vdd

So, their understanding and implementation is as follows;

For power domain connection between AM5728 and PMIC,

- SMPS7 can be connected to vdd1-23 and vdd_rtc.

- LDO2 can be connected to vdds18v and vdda_rtc.

Is this understanding correct?

If this understanding correct, please check the attatched pdf before and tell us how to solve this problem, again.

Best regards,

0 Biser Gatchev-XID over 8 years ago in reply to matusan

TI__Guru**** 393215 points

Yes, this is correct. See processors.wiki.ti.com/.../AM57xx_Schematic_Checklist

0 matusan over 8 years ago in reply to Biser Gatchev-XID

Guru 10235 points

Hi,

Thank you for your prompt reply.
I really appreciate your help.

We understand "If RTC-mode is not used, vdd_rtc can be connected to SMPS7."

I will send the answer to the customer including the checklist as below.
processors.wiki.ti.com/.../AM57xx_Schematic_Checklist

Again, please continue to check the attatched pdf which we sent before and tell us how to solve this problem.

Best regards,

0 Biser Gatchev-XID over 8 years ago in reply to matusan

TI__Guru**** 393215 points

From this PDF I cannot see how much the voltage is increased. Please provide this information.

0 matusan over 8 years ago in reply to Biser Gatchev-XID

Guru 10235 points

Hi,

Thank you for your prompt reply.
We will ask the detail to the customer.

Best regards,

0 matusan over 8 years ago in reply to matusan

Guru 10235 points

Hi,

Thank you for your help.

Our customer sent us the waveform chart in detail .

Please check the attatched pdf.

We would appreciate to continue to investigate the problem.

Best regards,

LDO_wave.pdf

0 Biser Gatchev-XID over 8 years ago in reply to matusan

TI__Guru**** 393215 points

That's < 200mV. No damage should occur from this.

0 matusan over 8 years ago in reply to Biser Gatchev-XID

Guru 10235 points

Hi,

Thank you for your prompt reply.
This information is very helpful!

Best regards

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AM5728: Unexpected behavior on PMIC LDO's