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LMX2541: Current consumption versus mode

NTN
Prodigy 100 points
Part Number: LMX2541
Other Parts Discussed in Thread: LMX2571, , LMX2572, LMX2485, LMX2491

I went through the data sheet, but I am not clear on the power consumption of the device. I plan to use the device in the external VCO mode with divider ratio up to 8.

1. The data sheet page 7 shows the current consumption. Ipll=72mA and Idiv=84mA. Why Ipll is lower current than Idiv? The divider only mode should turn off the PLL, so Ipll should be higher than Idiv.

2. Page 35 Table 17 shows Change is current consumption in div mode as a function of DIVGAIN and OUTTERM, and the reference is when each parameter set to 12. but because of above question, I can't set the absolute current as the reference.

3. Does Idiv vary depending on the divider ratio?

What I want to know is that the actual (approximate) current under these conditions:

Mode: external VCO

External VCO frequency: 1000MHz ~ 2000MHz

PD frequency: ~ 4.8MHz

Divider: ~ 8

DIVGAIN and OUTTERM: 12

Thank you

  • 0
    TI__Mastermind 27831 points

    Hi,

    Here's my reasoning:

    PLL = 60 mA

    VCO = 30 mA

    Divide = 40 mA

    Output = 32 mA

    ExtVCOIn Pin = 12 mA

    I think that you are not accounting for the output buffer.   Here's my calculations:

    Let's get the inputs.  Look not only at the electrical specs, but also table 9.3.2 on page 31.

      

    Electrical Specs Say

    PLL+VCO + Divide + Output = 170 mA

    PLL + VCO + Output = 130 mA

    This implies that:

    Divide = 40 mA

    Other specs are:

    PLL + ExtVCOIn = 72 mA

    ExtVCOin+Divide + Output = 84 mA

    Page 31 says:

    Divide + Output = 72 mA

    I know it says divide =32 and output = 40 mA, but this contradicts electrical specs.  Maybe it's swapped.  So I'll use the sum instead

    Therefore it follows that

    ExtVCOin = 12 mA

    Output = 32 mA

    PLL = 60 mA

    VCO = 38 mA

    In other words, I think that it does all work out.

    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 100 points
    Dean,

    Thank you for your quick response. it makes sense to me.
    It seems that the current, even lower the output power, is too high to use the device for my application.
    LMX2571 was a candidate, but the maximum RF frequency is limited up to 1400MHz if you use the higher divider ratio.
    Do you have any recommendations?

    Regards,
    Neil
  • 0 in reply to NTN
    TI__Mastermind 27831 points
    Neil,

    For integrated VCO devices, consider the LMX2572 that is lower current, better phase noise, and lower spurs compared to the LMX2541. However it does not support external VCO.

    If you want external VCO, consider the LMX2491 that supports higher VCO frequency. If you want lower current (at the expense of phase noise), consider the LMX2485 family of devices.

    Regards,
    Dean