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Effect of IOC in Inverting Buck Boost Topology

Dylan Colaco
Prodigy 40 points

Hello,

While using a device in IBB configuration, I noticed that there is a bypass capacitor(IOC) from input to output. (Capacitor C4 in Figure 3 in http://www.ti.com/lit/an/slva257a/slva257a.pdf)

I have a couple of questions regarding this:

1) As per my understanding, when the device starts up, if VIN is ramped up from 0 to 18V, VOUT should jump to some positive value due to charge sharing between C4 and C3 before the device starts ramping VOUT down to a negative value. How is the initial rise prevented? I do not see it in the bench results(figure 6 in http://www.ti.com/lit/ug/slvu243/slvu243.pdf

2) Won't line transients cause severe overshoot and undershoot as the transient is passed through the IOC straight to VOUT? 

  • TI__Guru**** 191445 points
    The circuit seems to work best with some capacitance from Vin to -Vout. Remember -Vout is the IC internal ground. If you look at the EVM users guide here:

    www.ti.com/.../slvu243.pdf

    You can see example load transient and start up waveforms. I don't have a convenient way to test line transients unfortunately. But I have never heard any complaints about it. I suppose it would depend greatly on the slew rate.
  • in reply to JohnTucker
    Prodigy 40 points

    Hi John,

    Thanks for your reply. I understand that -VOUT is the IC's internal ground. However I am not able to figure out what prevents it from rising above 0V during the initial startup(when VIN ramps up from 0V).