LSF0108: CIO and Total Cl

Michael Balourdas

Part Number: LSF0108

Hello,

Can you help clarify what this C_io value is? Does this mean that this part has 10.5pF typ load not taking into account the PCB trace loading?

The delay values in the datasheet are given for C_L = 15pF, 30pF and 50pF. Is this value taking into consideration Cio on/off values?

In the IBIS model, the series-switch C_comp value is 0.

I want to calculate the total C_Lseen by this part. Should I be using only the PCB loading or do I need to add PCB+LSF part loading?

Thanks,

Michael

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 317060 points

The LSF essentially works as an analog switch; it can be modelled as a resistor and a capacitor. C_io is the capacitance of the switch itself; C_L is the capacitance of whatever load you have connected to the LSF. See [FAQ] How do I Approximate Propagation Delay and Channel to Channel Skew in an Analog Switch/Multiplexer?

0 Michael Balourdas over 2 years ago in reply to Clemens Ladisch

TI__Expert 5455 points

Hi Clemens,

In the IBIS model for the switch, for 1.8 and 3.3, the C die is specified as ~3pF. How does this correlate to Cio value of 12pF? For sims, I’ll be using the IBIS model and want to make sure I’m modelling it correctly.

Thanks,

Michael

0 Michael Ikwuyum over 2 years ago in reply to Michael Balourdas

TI__Guru 51316 points

Hi Michael,

Please help clarify the 3pF referred by highlighting where you reading the 3pF from, thanks.

Best Regards,

Michael.

0 Michael Balourdas over 2 years ago in reply to Michael Ikwuyum

TI__Expert 5455 points

This is where we see the 3pF, is this referring to something else?

Thanks,

Michael

0 Michael Ikwuyum over 2 years ago in reply to Michael Balourdas

TI__Guru 51316 points

Hi Michael,

C_comp is the total silicon die capacitance as seen at the die pad and do not include package capacitance.

So, basically C_comp is a 3-state capacitance. For the 2-state output model, because there is no way to disable the output, C_comp is determined by taking the average of the capacitance determined in the high and low states, thanks.

Best Regards,

Michael.

0 Michael Balourdas over 2 years ago in reply to Michael Ikwuyum

TI__Expert 5455 points

Thank you Michael!

Basically I am trying to determine the loading induced by the switch on the IO’s as the IO’s are very sensitive . Can you please help me fill out the below table for LSF0108?

Up Translation – 1.8V – 3.3V

Min loading (pF) –

Max loading (pF) -

Down Translation – 3.3V – 1.8V

Min loading (pF) –

Max loading (pF) -

Thanks,

Michael

0 Michael Ikwuyum over 2 years ago in reply to Michael Balourdas

TI__Guru 51316 points

Hi Michael,

Unfortunately, we do not have the min value since it was not characterized or specified. However, you can use the datasheets max 12.5 pF as the Cio of the LSF device for both the up and down translation cases as this is the value per the device's design, process and package, thanks.

Best Regards,

Michael.

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LSF0108: CIO and Total Cl