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LM46002 Sync and RT priority?

BX27
Genius 3900 points
Other Parts Discussed in Thread: LM46002

Hi there, some questions about LM46002, could you please help have a look? 

Suppose we use RT=80.6K to set a default Fs of 500KHZ. When power up, the IC works at 500khz . After some time delay, an external clock of 1MHZ is sent into SYNC pin, then IC will change to work at 1MHZ, which means the SNYC clock overrides the RT setting. If the external clock is turned off later, the IC will switch back to frequency set by RT .

Questions:

1. I think SYNC pin has a higher priority than RT setting. right?

2. What will happen during the frequency changing process? Will the output oscillate or be unstable?

3. If I get off the 80.6K RT resistor( let this RT pin float),the default Fs is still 500khz. Will the process still be the same as I wrote above ?

Thanks a lot :-)

  • 0
    Genius 3900 points
    Hi , anybody give some advice? Thanks
  • 0 in reply to BX27
    TI__Genius 15810 points

    Hello! Sorry for the delay in response.

    The device will always try to sync if a sync input is present. The internal PLL takes a few cycles before transferring over to the sync input. There will be a small transient like behavior on the output voltage, but the output should not oscillate. If you are changing between two far ends of the allowable frequency range, then I would suggest that you make sure that your BOM can handle both ends of the frequency operation. 

    The same will happen if you float the RT pin.

    Regards,

    Akshay

  • 0 in reply to Akshay Mehta
    Genius 3900 points
    Hi, Akshay,
    Our sync clock will be 500KHZ or 1MHZ precisely, but the frequency set by RT may not be very accurate because of resistor tolerance, with 80.6K resistor we will get about 498khz. So when transfering clock source, there will be small transient on Vout, but we are not sure how many voltage will this trasient be. Is there any similar test by TI? or is there method to estimate this transient value? If not, we have to make test by ourself before schematics release.
    Thanks!
  • 0 in reply to BX27
    TI__Genius 15810 points
    Hello,
    There will be a small transient on VOUT if you go from an Rt setting of 500kHz to a sync clock of 1MHz. The transient should be minimal and only of about tens of milli-volts. For a clock change from Rt setting of 500kHz to synch clock of 500kHz, you would not notice the transient at all.

    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    Genius 3900 points
    Thanks, Akshay.
    If I go from an Rt setting of 500kHz to a sync clock of 1MHz, generally how long will the transient last? I care about this because the IC is internally compensated, not much thing could user do with control loop, except for adding a feed-forward Capacitor. I think it might be a few clocks, but not sure.
  • 0 in reply to Akshay Mehta
    Genius 3900 points

    Hi , Akshay.

    If I go from an Rt setting of 500kHz to a sync clock of 1MHz, generally how long will the transient last? I care about this because the IC is internally compensated, not much thing could user do with control loop, except for adding a feed-forward Capacitor. I think it might be several clocks, but not sure.

    Thanks!