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LM22677 Master/Slave

Stephen Gross
Prodigy 20 points
Other Parts Discussed in Thread: LM22677, LM5576

Hi,

I would like to operate two LM22677's driving separate loads both at the same voltage. One would be on continuously and the other would be commandable ON/OFF as needed. When both were on, the slave should operate at the same frequency as the master. I believe I can do this but Figure 18 on Page 11 of the data sheet shows both Enables tied together (not separate) and also indicates that the FET gate should be tied to the higher voltage regulator. In my application, both would be at the same voltage.

Can I operate one as a slave to the other? Do I need to have an external clock?

Thank you for your time in this matter.

Regards,

Steve

  

  • 0
    TI__Expert 3870 points

    Hi Steve,

    The self-sync setup described on Figure 18 for the LM22677 will not work if you turn ON/OFF one of the devices as needed.  The RT/SYNC pin will command 3 different modes of operation, depending on the setup of this pin during power-up.  Once the mode is determined after intialization (latched) the device will not change modes (i.e. go into external clock sync) unless is it turned OFF and back ON.

    Scenario 1: Let's assume you start both regulators ON at the same time during system start-up.  Both regulators will latch the SYNC mode.  When you turn-off one of the regulators (slave), then the master regulator will see a ~500 Ohm resistor to GND (assuming the gate of the MOSFET has been tied to the Vout of the master) and the master regulator will try to operate at the max frequency setting (given by Fig 17).  Your inductor and output capacitor values might not be proper for the new frequency setting.

    Scenario 2: Let's assume you start only the master regulator ON during system start-up.  The master regulator will latch the frequency setting mode, once again with an operating frequency outside the graph given by Fig 17.  When you turn ON the slave regulator you will also have to power-cycle the master regulator to make sure it goes into SYNC mode.

     

    The LM5576 SIMPLE SWITCHER has independent RT and SYNC pins so it allows the user to tie the SYNC pins together, without the need of a MOSFET and a pull-down (1 Kohm) resistor.  In this case your independent EN pins are okay.

    Best Regards,

    -JP

  • 0 in reply to Juan Paulo Fung
    Prodigy 20 points

    Juan,

    Thank you very much for the reply and the recommendation of the LM5576. The '5576 is a 3A regulator, though, and I think insufficient for our current needs. I'll keep looking at other solutions.

    Regards,

    Steve