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12 LVDS Outputs

amnon pomerants
Intellectual 780 points
Other Parts Discussed in Thread: LMK03806

Hello

I am trying to calculate the LMK03806 current consumption at my design.

I used the datasheet information of a 445mA total consumption in case of 12 LVDS clock outputs with DIV=16.

I did the following math:

a. Core + Base Clock = 139.3mA

b. Clocks group = 6x2.8mA = 16.8mA

c. LVDS x12 = 12x 14.3m = 171.6mA

d. there is also 25.5mA which I was not sure how to use

Total Sum: 327.7mA  --> not the expected 445mA.

1. Can you explain please how to reach the 445mA from the spec?

2. In case I have 3 PECL @ 240 ohm and 5 LVCMOS, using 5 out of the 6 clocks group, what will be my current consumption?

Thanks

Amnon

  • 0
    TI__Mastermind 24205 points

    Hi Anmon,

    Base + Core = 139.3mA

    CLKout Pairs = 3 *  2.8 = 8.3mA (There are 3 CLKout pairs active) -> CLKout0/1 and 10/11 is treated as one pair, CLKout2/3 & 4/5 as the second one and CLKout 6/7 & 8/9 as the third for power consumption calculations

    12 x LVDS outputs = 171.6mA

    6 Output Dividers (Divide < 25) = 6 * 25.5 = 153.2mA

    Total for LVDS = 472.4mA

    I expect the base current is overstated by ~20mA which should account for the difference. Actual measured should be around 455mA as stated in the electrical spec table.

    Total current consumption for 3 LVPECL 1600mV (with 240 ohms emitter resistor to ground) = 31mA (102.3mW) out of which ~67mW is power dissipated in device and 35mA is power dissipated externally (resistors)

    Total current consumption for 5 LVCMOS outputs (better to operate them as 3 pairs) = ~3* 26mA (@30.72 MHz) = ~262.3mW

    For both the LVPECL and LVCMOS cases above, the Base+Core+CLKout pair current consumption will be the same as the LVDS case.

    Regards

    Arvind Sridhar

  • 0 in reply to Arvind Sridhar
    Intellectual 780 points

    Hi

    Thanks for the prompt and elaborated answer.

    I looked at the LVDS example and one thing I didn't understand is the six output dividers (6 * 25.5 = 153.2mA).

    Why six? I can use a different divider for each output , can I ?

    If so, this will make 12 output dividers... what am I missing here?

    Thanks

    Amnon

  • 0 in reply to amnon pomerants
    Intellectual 780 points
    Hi
    I looked only now on the second part of your answer.
    Regarding LVPECL, you wrote about 3 LVPECL but the numbers as I understand are for only one LVPECL.
    Am I right? Shoud I do for 3 LVPECL: Itotal_lvpecl=3x 31mA => Pd= 3x (67mW + 35mW)

    Thanks
    Amnon
  • 0 in reply to amnon pomerants
    TI__Mastermind 24205 points

    Hi Amnon,

    The 6 dividers comes from the figure below. 2 differential outputs share a divider.

    For the LVPECL use-case power dissipation calculation, you will need to multiply the numbers in my previous response by 3 for the 3 outputs.

    Regards

    Arvind Sridhar

  • 0 in reply to Arvind Sridhar
    Intellectual 780 points
    Hello Arvind
    I just want to make sure, although CLKout0 andCLKout1 (for example) share the same "Divide" unit, they still can be programmed to totally different output frequencies ?
    Or in other words, is there any type of limitation on the freedom to select independent output frequency to CLKout0 and separately to CLKout1 ?

    Thanks
    Amnon
  • 0 in reply to amnon pomerants
    TI__Mastermind 24205 points

    Hi Anmon,

    CLKout0 and CLKout1 share one divider so they cannot be set independently to different frequencies.

    Regards

    Arvind Sridhar

  • 0 in reply to Arvind Sridhar
    Intellectual 780 points

    Hello Arvind

    I was trying to implement the calculation of the LVPECL output power dissipation on a different PECL device I am using and I noticed that

    what I thought to be the way you separated the power dissipating over the external resistors and the internal section of the LMK

    doesn't provides me with the expected results at all the rows specified at Table 41.

    For example see last row: I thought 0.7Vpp over two resistors with R=240ohm should dissipated: 2x(0.7^2)/240= 4mW <> 40mW

    Please explain .

    Thanks

    Amnon

  • 0 in reply to amnon pomerants
    TI__Mastermind 24205 points

    Hi Amnon,

    LVPECL 1.6 Vpp with 240 ohms to ground has a DC common votlage of ~2.0V.

    2.0*(2.0^2)/240 ohms = ~33.33mW

    Regards

    Arvind Sridhar

  • 0 in reply to Arvind Sridhar
    Intellectual 780 points

    Hello Arvind

    Many many thanks for your great help

    Amnon