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CCS/LMZM23601: Calculating sense resistor values in inverting mode.

Part Number: LMZM23601

Tool/software: Code Composer Studio

I am confused on the derivation for the feedback sense resistors. In non-inverting mode it seems simple enough where the output is driven such that the FB node equals to V_ref and V_out = (V_ref / R_FBB) (R_FBT + R_FBB). From this you can calculate the resistor values for the given V_ref. However, in inverting model (since V_out is now negative) I am confused on what the FB node is being driven toward. If it were still trying to make FB equal to (positive) V_ref then the FB node would have to source current which doesn't really make sense and its no longer a simple resistor divider. However, looking at different datasheets on changing buck converters to inverting bb converters, the feedback equations seem to mostly stay the same, just V_out becomes negative http://www.ti.com/lit/an/slva317b/slva317b.pdf  . Can anyone help clear up this confusion

  • Hi Jason,

    The FB node of any switching converter will try to regulate to it's internal configured Vref(1V between VFB and system reference point GND). When you configure a buck converter for inverting application, the"nVout" is now considered the system's reference point. When you connect your load to the configured inverting supply, you would connect positive terminal of the load to GND and negative terminal of the load to nVout. The load will drawn current from the switching supply similarly to a regular buck converter. However now the output is simply on the nVout and VFB will want to see -1V between VFB and nVOUT to satisfy the "VFB = 1V".

    Regards,
    Jimmy