2 Concept of Operation

Methods to lower a converter's output voltage below the reference are explored in the application notes SLVA216 and SLUAAO0. When these methods are implemented in a feedback divider of the regulator, an external voltage source (V_EXT) is connected to the feedback node through a resistor (R_EXT). By varying the voltage of V_EXT and the values of the feedback resistors, the voltage at the output of the regulator can be set. With set feedback resistor values, when V_EXT is raised, V_OUT drops, and when V_EXT is lowered, V_OUT rises.

Figure 2-1 Three Resistor Simplified Schematic

When V_EXT is greater than V_REF, current from V_EXT flows to GND through R_EXT and R_B1. The additional current from V_EXT though R_B1 causes a voltage rise at VSNS. The control loop of the regulator senses this voltage rise and decreases V_OUT in response, causing current flow through R_B1 from R_T1 to decrease. As the current through R_B1 decreases, the voltage at VSNS decreases and returns to the reference voltage, but now with V_OUT at a lower level. If enough current is allowed to flow through R_B1 such that the current through the top feedback resistor goes to zero, V_OUT is equal to the reference voltage of the regulator. If the current through R_EXT is increased beyond this point, then V_OUT drops below the reference voltage. During this condition, current no longer flows from V_OUT to GND through the feedback divider but rather flows from V_EXT to V_OUT and to GND through R_T1 and R_B1.

Figure 2-2 Simplified Schematic With Current Flow

In this operating mode R_B1 is no longer required, as this mode no longer contributes to the feedback functionally and only serves as a source of error. The bottom resistor can then be removed and replaced with R_EXT which then becomes the new R_B1 in the divider.

Figure 2-3 Two Resistor Simplified Schematic

In this new configuration, the feedback resistor divider is now formed between V_EXT, and V_OUT. When the voltage of V_EXT is greater than V_REF, to maintain VSNS at the reference voltage, V_OUT must be lower than V_REF to accommodate the voltage drop across R­_T1. Stable regulation is still maintained, as when V_OUT drops or rises, the voltage at VSNS still drops or rises proportionally, as with a standard implementation. V_OUT in this configuration is now determined by the values of R_T1, R_B1, V_REF, and V_EXT.

The configuration shown in Figure 2-3 is already useful, as it allows for V_OUT to be regulated below the reference with only the addition of an external voltage source. However, the problem it introduces can be seen in the equation for V_OUT. Since the output voltage is dependent on both V_REF and V_EXT, errors in both terms must now be considered when calculating output accuracy of the regulator. Since V_EXT is not an accurate voltage source, in this configuration the accuracy of the regulator is permanently degraded by the accuracy of whatever external voltage source is used for V_EXT.