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LM21215A can SYNC input be driven by the switch node of another LM21215A

Gabor Szakacs
Intellectual 420 points
Other Parts Discussed in Thread: LM21215A, LM555, SN74LVC1G14

I have a design that uses multiple LM21215A's to generate a number of low voltages from 5.0V input.  I'd

like to reduce the worst-case ripple on the 5V by synchronizing these supplies in pairs using the SYNC

pin of one tied to the switch node of another.  The sum of the duty cycle of the pair plus the 200 ns delay

from SYNC to SW is less than 100% so at least in steady state, this should provide no overlap on the

high-side drive between the pair of supplies.

While this seems like it would work for the steady-state case, I'm not sure if I will run into problems

under start-up or load-change conditions.  Is this a bad idea?  And if so is there an appropriate part

I should be looking at to drive the SYNC pins at about 500 KHz with different phases?

  • 0
    TI__Intellectual 1135 points

    Hi Gabor,

    In order to minimize the input voltage ripple, you might want to interleave the two phases (synchronize the two phases to two clock signals with 180 degree phase-shift ).  I would recommend you to use external circuit (such as LM555 timer along with inverter SN74LVC1G14) to generate the clock signals, these signals does not have the noise/voltage rings on the SW pin and will provide predictable and stable performance.

    Haifeng Fan

  • 0 in reply to Haifeng Fan
    Intellectual 420 points

    Haifeng,

      Thanks for the reply.  As this would seem to be something that makes sense in general when

    using multiple point of load supplies, I was hoping that there was some sort of device that gives

    more control of the phase between outputs (not just 0 and 180 degrees).  At this point I think a better

    choice in my application, since I have six of these supplies, is to use a small PIC micro with an

    internal 32 MHz oscillator (PIC16F1823) to generate six outputs with programmable phase.  These

    parts have internal oscillators trimmed for +/- 5% over temperature and voltage with no external

    parts, and cost less than $1 even in low quantities.  With the 555 timer, I'd get a much wider

    frequency range even with tight tolerance external components, and even with inverters I'd only

    get two usable phases.  Adding more 555 timers as one-shots to generate additional phases

    would end up with a higher BOM cost and footprint.

    As a thought for future products with SYNC inputs, it might be nice to allow a programmable

    delay or phase (from SYNC in to switch assertion) using a single resistor.  Then any number

    of these devices could use the same SYNC input and the resistors could be tuned to minimize

    drive overlap.

    Regards,

    Gabor

  • 0 in reply to Gabor Szakacs
    TI__Intellectual 1135 points

    Hi Gabor,

    I agree that the micro controller makes more sense in terms of the cost, solution size, and flexibility when more than two phases of power supplies are required. We also appreciate your valuable inputs and will keep it in mind for future product definition.

    Haifeng Fan