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LM5118: Control Mode during Boost Mode

Carolus Andrews
Mastermind 22275 points
Part Number: LM5118

Team,

I have a customer designing with LM5118. According to the datasheet, it appears current sensing is not used when the device is in Boost mode. Does that imply that the device only has voltage mode control during boost operation? Could you elaborate on how the devices control structure works in boost mode, as well as transitions?

  • 0
    TI__Guru* 78060 points
    Hi Carolus,

    I think it is a misunderstanding of the LM5118 operating modes. It only has two modes: Buck, or Buck-Boost. There is no pure boost mode. In buck-boost, both FETs are switched together, and both diodes are conducting at the same time. So there is always current sense signal.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    TI__Mastermind 22275 points

    Youhao,

    Thanks for the response. The main issue I'm trying to discern here is this: In buck-boost mode, when Q1 and Q2 are on, inductor current ramps up based on Vin. As per the datasheet, it sounds as though there is no current sensing to limit this inductor current. Is this the case? If so, if the inductor gets saturated for some reason, could Q1 and Q2 become damaged? Also, if that is the case, is it a good idea to tie source of the Q2 and D1 anode to the CS pin? 

  • 0 in reply to Carolus Andrews
    TI__Mastermind 22275 points

    Youhao,

    I never heard anything back here. Could you comment on my last question?

  • 0 in reply to Carolus Andrews
    TI__Guru* 78060 points

    Hi Carolus,

    I now see where the confusion comes from. The LM5118 is an Emulated Current Mode controller.  It sense the current when main switch is off, and the freewheeling diode current (on the buck low side) is sensed through a Sample & Hold circuit, and reconstructed with the RAMP to emulate the main switch current. See Figure 13 of the datasheet.  This is also why the RAMP pin capacitor must be selected according to the inductor (equation 4).  The cap is required to produce a ramp proportional to the inductor ramp current.

    Hope this clarifies.

    Thanks,

    Youhao Xi