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LMX2595: About power down mode

user1656626
Prodigy 70 points

Part Number: LMX2595

After powering up, I am using power down mode without OSCin input. is there a problem. I am in trouble because the frequency is sometimes not locked at high temperatures. The set frequency is 12GHz. 

  • 0
    TI__Guru** 115290 points

    Hi There,

    You can program the device without a valid OSCin signal. However, if you need to calibrate the VCO, a valid OSCin signal must exist before R0 (with FCAL_EN = 1) is programmed.

    Make sure you have VCO_SEL = 4, VCO_DACISET_STRT = 300, and VCO_CAPCTRL_STRT = 1.

  • 0 in reply to Noel Fung
    Prodigy 70 points

    Hi Noel, thank you for your reply.

    VCO_SEL = 4 VCO_DACISET_STRT VCO_CAPCTRL_STRT = 300, VCO_CAPCTRL_STRT = 1
    Is set to.

    After inputting OSCin, the frequency is always locked when the power is turned on.
    After turning on the power, when I input OSCin, the frequency does not lock sometimes.

    After power is turned on, the power down mode register is sent after a waiting time of 2ms.

    Should I send a power down register after a wait time of reset = 1, reset = 0, 10ms when using the power off mode when the power is started?

    Many thanks in advance

  • 0 in reply to user1656626
    TI__Guru** 115290 points

    Hi There,

     you keep saying power down, what is the intention of power down?

    The recommended power up sequence is as follows:

    1. Apply power to device.
    2. Program RESET = 1 to reset registers.
    3. Program RESET = 0 to remove reset. (this step is optional)
    4. Program registers as shown in the register map in REVERSE order from highest to lowest. That is, the last register to write is R0 (with FCAL_EN = 1).
    5. Wait 10 ms.
    6. Program register R0 one additional time to ensure that the VCO calibration runs from a stable state.

  • 0 in reply to Noel Fung
    Prodigy 70 points

    Thank you for your answer.

    The power down mode is adopted in order to reduce the inrush current of a DCDC converter.

    When the following setting sequence is performed, it sometimes does not lock at high temperature (+ 70 ℃ or higher).
    1, Vcc input
    2, Set to power down mode (send register R0: 2515)
    3, OSCin input
    4, all register transmission
    5,10ms (waiting time)
    6, register R0: 251C transmission
    Will be.

    The problem was solved after implementing the recommended power-on sequence.
    I misunderstood that I didn't need a reset signal because I was powering down at the register.

    What happens if you ignore the recommended power-on sequence? (Does the device hang up?)

  • +1 in reply to user1656626
    TI__Guru** 115290 points

    Hi There,

    The key step in the recommended sequence is step 2.

    The device has a built-in POR (power on reset) circuitry that is triggered by the Vcc rail. If this POR failed due to poor Vcc edge rate, step 2 can make sure the device is reset to a known state before programming. 

    Programming the device from a known state is important as the logic might get sucked otherwise.