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LM5122 Sync pulses voltage level

Other Parts Discussed in Thread: LM5122, PMP7969

Hello,

In LM5122 datasheet, section 8.1.3, figures 32, 33, 34, the sync pulses into SYNC IN/RT pin are required to be 5Vp-p, even though the SYNC thresholds data seems to be compatible with 3.3V.

In reference design PMP7969 schematic the Sync pulses are derived from 3.3V powered CMOS circuit that directly drive the SYNC/RT pins.

Is it possible to drive the SYNC pins of for example 4-phase interleaved boost converter directly from a 3.3V power micro-controller?

There is a requirement in the datasheet for a low impedance source for those pulses, but no specific numbers. Does a totem pole microcontroller output pin with 7mA current capability provide low enough impedance to directly drive the pins?

Thanks.

  • Hi Kiril

    In PMP7969 design,U1 is configured as a 'MASTER2'mode and the other three slave channels are configured as a 'SLAVE1' mode.(Please refer the Table 1 in the datasheet)
    In the 'SLAVE1' , 'MASTER2' and 'SLAVE2' modes, RT regulator is off and the SYNC/RT pin can be driven by using 3.3V power micro-controller.

    Regards,
    EL
  • Hi Eric,

    Thank you for the clarification. 

    I have two more questions in regards to PMP7969 schematic:

    1. The SS pins of the slaves are driven by BJTs, controlled by the master SS pin. Is that a recommended configuration for 4 phase interleaved boost application? What is the reasoning behind this circuit? Does it improve the start up?

    2. The COMP pins of the slaves are connected to the Master's COMP pin through 4.7K resistor? Is that required? There is no mentioning of that in the datasheet.

    And one more general question. How does the circuit behave if the input voltage is applied with, let say, a circuit breaker and there is no external soft starting? The datasheet kind of mentions that external soft start is recommended. In my application, input voltage is 24V and I am wondering if I need to soft start the converter externally.

    Thanks,

    Kiril

  • Hi Kiril
    A1) The circuit will allow customers to shutdown channels independently. It allows a Phase Shedding externally. I guess you don't need.
    A2) You don't need. Connect COMP pins directly.
    A3) A huge inrush current may flow because boost converters have no inrush current limit capability due to the pass path through the high side MOSFET body diode.
    Regards,
    EL
  • Hi Eric,
    On A1) I think you misunderstood me. My question was about Q15, Q13 and Q14, connected to SS pins of the slaves.
    Your answer describes Q9-Q12, which are tied to UVLO pins and can shut down the controllers.
    Why the SS pins are not connected directly together?
    Thanks,
    Kiril
  • Hi Eric,

    You didn't answer my question about Q13, Q14 and Q15 transistors used to buffer the SS pin of the master controller. Is that required for 4 phase configuration?

    Thanks,

    Kiril

  • Hi Kiril

    Looks like PMP7969 designer worried about too much. Based on the PMP7937 design http://www.ti.com/tool/pmp7937, connecting all SS pins together is enough for 4-phase configuration.

    Regards,
    EL
  • Thanks, Eric!