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Clock solution providing in phase and 90 / 180 degrees out of phase outputs?

Tom Smith
Expert 2930 points
Other Parts Discussed in Thread: SN7414, LMK04906, LMK04806

Customer request;

I have two (or three) power supplies that I would like to sync to an external clock.  This way the power supplies can be run 180 degrees (or 90 degrees) out of phase from each other to reduce ripple current in the power system. 

Is there a synchronizer chip that puts out two (or three) clocks that are 180 degree (or 90 degrees) out of phase with each other to take care of this?

The switcher runs at 300KHz but the SYNC pin will take 200KHz – 400KHz.  The input logic levels on the SYNC pin are TTL.  Commercial temperatures should be okay here.

Comments.....

-Tom-

  • 0
    TI__Mastermind 28540 points

    Hello Tom,

    What is the phase tolerance for 180 degree inversion?  300 kHz has a period of 3.33 us.  Is it possible to use a simple inverter?  Like SN7414 with a propagation delay max of 22 ns.

       - An LVCMOS input clock could drive multiple inputs of the SN7414 at the cost of increased capacitive loading.  The outputs could be direct through 1 inverter for a 180 degree shift or through 2 inverters for the 0 degree shift.

    73,

    Timothy

  • 0
    TI__Mastermind 32575 points

    LMK04906 (to be released in May 2012) is a lower-cost version of the LMK04806 (released) with fewer outputs and can provide 2 to 12 multi-phase CMOS/TTL outputs at 300 kHz for synchronizing multiple switchers.  Any unused outputs / features may be shut-down to reduce power.  Note that this device was not designed specifically for this application and may be overkill in terms of performance/power/size, but does provide a lot of flexibility/programmability if this is important for system performance optimization.

    LMK04906 Example Use Case for Generating Multiphase SYNC Outputs at 300 kHz

    • Use single-loop PLL2 mode with a low-cost 10 MHz crystal to lock the on-chip VCO to 2.4 GHz (yes, Giga).  The 2.4 GHz VCO can be divided to 300 MHz (distribution bus frequency) using VCO divider=8 and ultimately 300 kHz (output frequency) using output divider=1000. 
    • Programmable output drivers allows you to independently configure each output as 3.3 V CMOS with normal or inverted phase (0 or 180 degrees).  To achieve phases between 0 degrees to +/-180 degrees (or slightly beyond), use the programmable digital delay (per channel) and sync features to adjust the delay range from 0 us to 1.723 us (i.e. >180 degrees offset at 300 kHz) with delay step size of 3.333 ns (1 period of 300 MHz bus frequency) or 0.36 degrees/step. 

    LMK04906 operates over the industrial temp range and uses a Microwire programming interface (subset of SPI).  Note that minimum VCO tuning range of this device (2.37 GHz) limits the lowest output frequency to 284 kHz (= 2.37 GHz / VCO_DIV / CLKout_DIV, where VCO_DIV=8 (max) and CLKout_DIV=1045 (max) ).

    Best regards,

    Alan

  • 0 in reply to Alan O
    Prodigy 130 points

    Hi Tom,

    The LMK should work but might be overkill for a 300KHz quadrature clock. Like Timothy said the 7414 can produce the complementary outputs easily.

    Now for the quadrature phases, I would use the standard positive feedback CMOS oscillator which has 1 inverter,  and  3 RC phase shifters with a buffer in a loop.

    Hartley oscillator for example with 90 phase shifter>