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LSF0108: will be able to drive the capacitive load of the TPM & the SIO?

Part Number: LSF0108

Hi Sirs,

Sorry to bother you.

As title, we will use LSF0108 on Intel LPC interface.

Could you help us check  LSF0108 is able to drive the capacitive load of the TPM & the SIO??

Thanks for your help.

  • I'm afraid I am unfamiliar with the signal requirements of TPM and SIO, however I can tell you that the LSF0108 is a passive translator with no drive strength at all.  The maximum up-translation rate will be determined by the high-side pull-up resistor selection, capacitive load on the line, and the drive strength of the connected devices.  Please see the application report Voltage-Level Translation With the LSF Family (Rev. B) for details on how this device works.

    I noticed that some signals on the LPC operate at 33.3 MHz.  If this is the case, the LSF0108 would require a fairly strong driver (>10mA sink current) and low capacitance on the line (~5pF is best).  As load capacitance increases, the driver's strength requirement increases significantly.

    If you can give me details about how these signals operate, I can help you select a translation solution.

    What voltages are being translated to/from?

    What is the maximum data rate?

    How much current can the devices at each end source/sink?

    Is the data being sent only in one direction on a single wire (unidirectional), or is it switching directions during operation (bidirectional)?

    If a signal is bidirectional, is there an indicator signal for which direction it is transmitting (direction controlled) or is there no way to tell (auto-direction sensing)?

  • Hi Emrys

    Thanks for your reply.

    Questions update as below, could you help check could we use lsf0108 do it?

    What voltages are being translated to/from?

    1.8V to 3.3V

    What is the maximum data rate?

    33MHz

    How much current can the devices at each end source/sink?

    We didn’t know the current of source/sink. The attached file is datasheet of TPM. Do you know how to check the current of LPC interface??

    Is the data being sent only in one direction on a single wire (unidirectional), or is it switching directions during operation (bidirectional)?

    Bidirectional

    If a signal is bidirectional, is there an indicator signal for which direction it is transmitting (direction controlled) or is there no way to tell (auto-direction sensing)?

    No, it is auto-direction sensing

    Both of the Capacitance of TPM/SIO are 10pF.

  • The TPM datasheet didn't come through, but I'll try to answer with what I know.

    Assuming that you can minimize trace capacitance, I would expect to see ~0.5pF from each input and we will say 1pF for the trace.  This means a total of 12pF on the 3.3V side of the LSF0108.

    At 33MHz (assuming a 33 MHz square wave), the pulse width is 15.2ns.  In order to have a successful up-translation, the output voltage should rise to 3.3V within ~1/3 of the pulse width, so ~5ns.

    The time to rise to 98% of Vcc is ~4 time constants, so:  tau = 4*R*C

    Rearranging for the pull-up resistor value yields: R = tau/(4*C) = 5ns / (4*12pF) = 104.1667 ohms (maximum)

    Assuming standard resistor values, if you use a 100 ohm resistor for the 3.3V side pull-up, the current sink requirement of the low-side device will be ~ I = V/R = 3.3V/100ohm = 33mA

    While this isn't a completely unreasonable amount of current, it's likely that the driver won't be able to handle that amount of sink current.

    So, to summarize - the LSF0108 can be used for your application assuming the low side driver (1.8V device) can sink 33mA and that you can limit the high side capacitance to 12pF total.

    For completeness in this answer, the low-side (1.8V side) pull-up resistor should be a relatively large value to minimize current requirements.  I would likely use 1kohm, or if the system can tolerate it, no pull-up resistors on the 1.8V side.