Applications sometimes require the generation of a voltage that is below the reference rail. Many design engineers would just use a second regulator to generate this voltage. However, a simple change in the feedback path of a switching regulator allows a single regulator to perform this task.
In switching power converters, a pulse-width modulation (PWM) IC regulates the output voltage by comparing a fraction of the output voltage with a reference voltage using the chip's error amplifier. The signal representing the output voltage is developed by connecting a voltage divider between the output-voltage rail and ground.
Any deviations from the output-voltage set point are corrected by the converter's feedback network. With this arrangement, shown in Fig. 1, the output voltage can be arbitrarily high but cannot be lower than the error amplifier's reference voltage (VREF). The output voltage of this configuration is given by the equation:
However, there are cases when the output voltage required is lower than the reference voltage. At first sight, this might imply the need to change to a PWM controller with a lower reference voltage. But, in fact, such instances can be easily accommodated with the original PWM controller and the same voltage divider circuit.
By making a slight modification in the way the divider is connected in the circuit, the second lower source can be developed. Its bottom leg is now connected not to ground but to a second reference voltage, which is higher than the PWM controller's reference (Fig. 2).
Using the principle of superposition, the output voltage of this configuration is found to be:
where VREF < VREF2 in order to ensure that VOUT < VREF. It is clear that Eq. 1 is a particular case of the more general Eq. 2. In other words, Eq. 2 is exactly equal to Eq. 1 when VREF2 = 0.
To illustrate the application of this scheme, take a typical example where the desired output voltage is VOUT =1.20 V and the PWM IC's VREF = 1.24 V. The second reference is typically in the 3-V to 5-V range. Let it be VREF2 = 5 V. Now pick R2 = 1 kΩ. Then from Eq.2, R1 = 94 kΩ.
VREF contributes 1.253 V to the 1.2-V output voltage and VREF2 contributes only -0.053 V. This lopsidedness between the two references in contributing to the output voltage will be the case in all instances where the output voltage is only slightly lower than VREF, which will be the majority of cases. Therefore, VREF2 will not have to be tightly specified and can come from, say, the output of a linear regulator or from another switching power supply.