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LMX2571EVM: LMX2571EVM

Yerach Krupnik
Prodigy 40 points
Part Number: LMX2571EVM
Other Parts Discussed in Thread: LMX2571, USB2ANY,

Hi,

PLL Synthesizer LMX2571 is specified for operation with 3.3V. Control  and programming is through SPI/Microwire digital interface.

Based on the datasheet it has no problem to work with SPI digital interface at 3.3V:

Then why in the LMX2571EVM Schematic they use resistor dividers (10K/12K; R42-R51) to convert 3.3V logic from USB2ANY Interface Adapter down to 1.8V at LMX2571 SPI/Microwire inputs?

Thanks, Yerach

  • 0
    TI__Guru* 88030 points

    Hi Yerach,

    This was copied from old design. In the past, the EVM supports parallel LPT port, which is a 5V interface. The resistor dividers are used to divided down 5V to 3.3V. If you interface is 3.3V, you don't need the resistor divider, simply put a series 33Ω resistor to limit the current is good enough.

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Hi Noel,

    Thanks for your prompt reply.

    There are 2 issues with what you wrote:

    1. The 10K/12K resistive divider will bring 3.3V down to 1.8V but will not bring 5V down to 3.3V (rather it will bring 5V down to ~2.72V).

    2. In the LMX2571EVM User Guide it explicitly says (under the schematic in Figure 7):

    Notes:
    1. Voltage dividers divide down 3.3V to 1.8 V.
    2. CE pin can be configured with pull-up resistor
    3. ......

    So please re-assure again that there is no need for resistive dividers if our logic is 3.3V.

    Also, what is the concern with current limiting (the suggested 33Ohms...)? We connect the chip directly to MCU.

    Regards, Yerach

  • 0 in reply to Yerach Krupnik
    TI__Guru* 88030 points

    Hi Yerach,

    As I said before, the resistor divider is added to support legacy 5V LPT port. With 3.3V logic, our suggestion is a series 33Ω resistor. 

    It is a common practice to add a series resistor to limit the current flow in cases the 3.3V logic is ready while the synthesizer is not up and has the input pin connected to ground. This situation may not happen, but just in case. 

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Noel,Thank You very much.

    As far as I'm concerned the case is closed.

    Best Regards, Yerach