LMX2820: Lock Detect pin status

Hi Ishiwata, 
That does not make sense. 
To ensure I understand, what you're saying is you have a reference at OSCIN_N and the PLL locks which results in Lock detect being high as expected, but once you remove the reference from OSCIN_N lock detect stays high instead of going low? 

Regards, 

Vicente

Hi Vicente,

Thank you for your reply.
I agree with your understanding.

If the power is input without a reference clock being input to 8 pin of OSCIN_N, 38 pin (LD pin) will remain in the High state.

Best Regards,
Ishiwata　

Hi Vicente,

Thank you for your reply.

Confirmed with customer.
LD_Type = Continuous

Is there anything additional to confirm with the customer?

Best Regards,
Ishiwata

Hi Shuji, 
That is very odd, if LD_TYPE is set to continuous I definitely expect it to go low once reference to OSCIN is removed. 

Can you please share customer config ( preferably .tcs) file with me? 

Regards, 

Vicente 

Hi Vicente,

Thank you for your support.

Can I send you the .tcs file via private message?
May contain customer information.
I have sent you a private message, so please check it.

Best Regards,
Ishiwat

Hi Shuji, 
I have reviewed the config and see no issue with it. 
I checked the config on bench and had no issue with LD. If i removed the reference from OSCIN, I saw LD go LOW. 

Can you share schematic? 

Regards, 

Vicente 

Hi Vicente,

Thank you for checking the .tcs file.
I understand that there is no particular problem.

I have sent the circuit diagram to your private message, please check it.

Best Regards,
Ishiwat

Hi Shuji,

After reviewing the diagram I have some questions... 
Is customer only using one output & single-ended?

Also I cannot see where many of the nets go including LD...
Please note - you can also post on the internal forum where only we have access. 

Regards, 

Vicente 

Hi Vicente,

I will answer your questions.
1ch single-ended output.
Also, the connection destination of the LD terminal is shown in the circuit diagram, but it is connected to the MCU on the same board.

I have a request.
Can I check the operation using the following startup sequence using the EVM of LMX2820?
Check whether the LD terminal transitions to High.

1. Power on
2. Register settings using SPI (without reference input)
3.Issue VCO Cal command
4.PLL Lock

Normally, reference input is performed at step 2, but please check whether the LD terminal transitions to High without reference input.

If the reference clock is not input at startup, I think the LD terminal is in an undefined (Hi-Z) state. Is my idea correct?

Best Regards,
Ishiwata

Hi Vicente,

Thank you for your support.
I understand that it did not reproduce with EVM.

If the reference clock is not input at startup, I think the LD terminal is in an undefined (Hi-Z) state. Is my idea correct?

Best Regards,
Ishiwata

Hi Shuji,

No, the logic level stayed low.

Regards,

Vicente

Hi Vicente,

Thank you for your validation.

The customer confirmed the waveform and sent it to a private message.
Do you know why LD becomes High?

Best Regards,
Ishiwata

Hello Shuji,

Due to the holidays, please expect a delay in our response.

Hi Shuji,
In the image you sent via PM, is the reference signal not available? On my end I only see noise. 
Nevertheless if that's the case, I certainly don't expect LD to have gone high regardless. 

Regards, 

Vicente 

Hi Vicente,

I sent a waveform to the PM where LD is High even though there is no reference input.

Although it is not the correct input sequence, it is locked because LD goes high.

Is LD an open drain output? Is it push-pull?

Best Regards,
Ishiwata

Hi Ishiwata,

let me follow up with design for this.

regards,

vicente 

Hi Vicente,

Thank you for your support.
Is there anything you would like the customer to confirm?

Best Regards,
Ishiwata

Hi Ishiwata,
Can you ask customer to quantify this? 
Is this seen on various units, one unit? 
On the units this issue is being seen do they see this randomly, every time? 

Regards, 

Vicente 

Hi Vicente,

We will check with the customer regarding the number of units that have occurred.
We've heard from our customers that this happens with a high degree of frequency.

Best Regards,
Ishiwata

Hi Shuji, 
LD is push-pull. 

I will wait for you to check with customer. 

Regards, 

Vicente 

Hi Vicente,

Thank you for your answer.

In the case of push-pull, I think it is necessary to connect a load resistor, is my thinking correct?

Best Regards,
Ishiwata

Hi Vicente,

Thank you for your answer.

I understand that installing an LED as an indicator means that it is better to install a load resistor, but is my understanding correct?
If you check the EVM schematic, there is an LED through a 330Ω resistor. Is there no problem with LMH2820 even if about 10mA flows?
I would like the customer to install a 330Ω resistor like EVM and check the operation, but do you think it will be effective?

Best Regards,
Ishiwata

Hi Vicente,

Does "correct" in your answer mean that a pulldown resistor is effective in solving the problem?

Best Regards,
Ishiwata

Hi Ishiwata-san,

The forward voltage of a LED is typical 1.8V. 

LD pin output high voltage is 3.3V. We must use a series resistor to limit the current flow otherwise there will be huge current flow in the LED.

Hi Noel-san,

Thank you for your answer.

I do think that LEDs are necessary to confirm abnormalities, but they are not a means to solve this problem.
Please suggest a way to resolve the issue.

Best Regards,
Ishiwata

Hi Ishiwata-san, 
By adding a 330 Ohm resistor, I don't expect the issue of LD going high without a reference at OSCIN to be resolved. 
Have you heard back from the customer regarding how frequent this occurs, on how many units, is it repeatable/predictable or random, etc.? I am very puzzled given I have tested this on the bench and never see this issue of LD going high without a reference clock occur. 

Is the .tcs file you sent previously still the most up to date configuration or has it changed? 

Regards, 

Vicente 

Hi Vicente,

Sorry for the late reply.

I confirmed with the customer about the frequency of occurrence.

There are a total of 4 boards created by the customer. In two of them, the LD terminal becomes High without reference input.
Also, the probability that the LD terminal goes high is 2 out of 10 times.

The previously sent .tcs file will be the latest version.
There are no changes to the register settings.

Best Regards,
Ishiwata

Hi Shuji, 
So this fault appears to be stuck to two ICs. 
Can customer solder the 2 good units onto the 2 boards with the faults and see if the issue persists and vice versa, can they solder the known two bad ICs onto the 2 known good boards and see if the fault follows the IC? Essentially an ABA swap. 

Regards, 

Vicente