LMK00105: Using VDDO as Output Enable

Don Schelle

Part Number: LMK00105

Hello,

The LMK00105 features two independent banks of output clocks, but only one output enable pin. Would it be possible to control the clock outputs by connecting independent load-switches to the corresponding VDDO?

There's a statement on page 9 of the datasheet that sort of addresses this question, but the reasons behind why aren't exactly clear.

Don

over 5 years ago

0 Timothy T over 5 years ago

TI__Mastermind 28530 points

Hello Don,

My take is that using a load switch would qualify as disconnecting the Vddo pins which is not allowed by that note on page 9.

I'll assign this to another engineer in case he has a better understanding. But this is my current understanding based on the datasheet.

73,
Timothy

0 Don Schelle over 5 years ago in reply to Timothy T

TI__Expert 4150 points

Thanks Timothy,

Appreciate the timely response. Looking forward the response from the other engineer as to why this won't work.

Don

0 Shawn Han over 5 years ago in reply to Don Schelle

TI__Genius 9920 points

It seems the note "DO NOT DISCONNECT OR GROUND ANY OF THE Vddo PINS because the Vddo pins are internally connected within an output bank."need be clarified, especially for "within an output bank". Our AE would do it when he come back in next week.

Regards,
Shawn

0 Don Schelle over 5 years ago in reply to Shawn Han

TI__Expert 4150 points

Folks,

Appreciate the responses. Looking forward to some clarification now that everyone is back from holidays.

Don

0 Alan O over 5 years ago in reply to Don Schelle

TI__Mastermind 32575 points

Don,

I will confirm if this is doable with the IC designer. Thanks for your patience.

Alan

0 Alan O over 5 years ago

TI__Mastermind 32575 points

Don,

VDDO only goes to the Output buffer stages for the corresponding output bank. So, yes, the VDDO for the given output bank could be used to effectively enable the output bank.

Alan

0 Don Schelle over 5 years ago in reply to Alan O

TI__Expert 4150 points

Thanks Alan,

When VDDO is disconnected via a load switch, what is the state of the outputs? High impedance?

Don

0 Alan O over 5 years ago in reply to Don Schelle

TI__Mastermind 32575 points

The LVCMOS output supply goes to 0V, along with the gate voltages, etc. So effectively the impedance at 0V will be very high. If you were to curve trace the output in this condition though, you will see a diode starting to turn on once you start going above or below ground, so the high impedance is only right around when the output is at 0V. If the output is pulled up with an external resistive termination in this mode, it will clamp at roughly 1V or so.

Alan

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LMK00105: Using VDDO as Output Enable