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LMX2594: Power up sequence after powerdown

Andrew Polyakov
Prodigy 20 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMX2572

Hi!

In section 7.3.9 of LMX2594 datasheet it's said that "When the device comes out of the powered down state, either by resuming the POWERDOWN bit to zero or by pulling back CE pin HIGH, register R0 must be programmed with FCAL_EN high again to re-calibrate the device." In similar device LMX2572 this sentence has been removed. Is this really required (i.e. recalibration) for LMX2594 after power up provided that the device was fully calibrated before powerdown?

  • 0
    TI__Guru** 107300 points

    Hi Andrew,

    This is true. When the devices are power down, some internal LDOs will be shut down as well. It takes time for them to wake up after coming out of power down state. in cases when these LDOs are not up and stable before the last calibration data is applied, you may not get the same performance as it was before power down. As a result, we will need to calibrate it again.

    In LMX2572, we introduced the LDO_DLY register to adjust the LDOs wake up time in order to make sure the LDOs is up before calibration data is applied.

  • 0 in reply to Noel Fung
    Prodigy 20 points

    Hi Noel,

    Thank you for the answer!

    As I can understand, calibration algorithm implemented in LMX2594 waits for the LDOs settling after power up and guarantees that all internal voltages are OK before calibration procedure. Will the performance be the same as before power down if I don't recalibrate after power up, but wait for some time (enough for LDOs settling)? Are all internal calibration registers (VCO sel, CAPs, current) are retained in powerdown mode?

  • 0 in reply to Andrew Polyakov
    TI__Guru** 107300 points

    Hi Andrew,

    LMX2594 uses a higher current to charge up the LDOs, as a result, the settling time is short. However, the settling time still depends on the external bypass capacitors. So the settling time will vary, especially over temperature. We have seen that some devices cannot wake up with proper phase noise and require a second calibration to resume the same performance as before.