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LMX2572: Register write/read issues

KB
Prodigy 180 points
Part Number: LMX2572
Other Parts Discussed in Thread: LMX2594,

I am using the LMX2572 as a replacement for the LMX2594 in a custom design.  We are able to successfully use the LMX2594 to generate the required frequency, but when we drop in the LMX2572, we are not able to get a locked signal.  Also, when we read the registers over SPI upon power up, all registers are reading values of '0' instead of the default values in the datasheet.  After we write values to the registers, we are able to write and read back all registers successfully except for R0.  I have included our TICS Pro setup as well as our schematic.  I'm particularly concerned with the capacitor values since they are so different from the suggested values.  If there are other avenues to try first before replacing caps, I'd prefer that since these components are all under a shield on the PCB and are very difficult to get to.  Or at the very least, point to particular caps that might more likely be the issue.

   

  • 0
    TI__Guru** 112720 points

    Hi KB,

    LMX2572 and LMX2594 are pin compatible but not 100% drop-in replaceable, additional jobs need to do to make your system works for both devices.

    On the hardware side, the following circled areas need pay attention.

    Check datasheet for the recommended bypass capacitors, some capacitor values are different between these synthesizers, you can find the information in the Pin Configuration section of the datasheet. These bypass capacitors are important, they will affect start up operation.

    RFout, LMX2572 does not require external pull-up, you should not populate those resistors with LMX2572. The value of the DC-blocking capacitor should match your application need. We put a broadband 0.1µF in our EVMs because we don't know what frequency customers will be using it.

    Since the VCO characteristics of these synthesizers are different, you need to re-design the loop filter for LMX2572. You can use PLL Sim (www.ti.com/.../PLLATINUMSIM-SW) to make the design and simulate the loop performance. 

    TICS Pro configuration, your PLL configuration is fine. My recommendation is use a higher fpd, either 10MHz or 20MHz (with the Doubler). 

    For below register in LMX2572, you may need to make it =1. See datasheet section 8.1.12 for details.