Switching Regulators

Step-Up and Inverting Switching Regulators

             The switching regulator shown in Figure 9-3 is a step-down regulator—Vo is smaller in value than VIN. Figure 9-5 shows two other types of regulators, a step-up and an inverting regulator. The step-up regulator produces a regulated voltage VO that is greater in value than VIN, while the inverting regulator produces a Vo that is inverted in polarity from VIN. A positive VIN produces a negative Vo.

Switching Regulator Design

 

                The design of switching regulators can be accomplished in a number of ways, but they all include the inductor as the temporary energy storage element and large storage capacitors. The inductor and capacitor(s) cannot be integrated into ICs; therefore they are external to any ICs used. Any of the other components, depending on the current and voltage requirements, can at least be partially integrated circuits. For example, if the current handling is within the range of 1–2 amperes, all of the error amplifier, PWM circuit, oscillator, and the control element can be one IC. With higher current requirements, external heat-sinked driver packages can be used for the control element. Resistor dividers are always used to sample the output voltage to feed back to the error amplifier.

Transformed PWM Regulators

In a different design than that shown in Figure 9-3, the PWM circuit, which contains the error amplifier, oscillator, voltage reference and some protection circuits, is used as an AC source. This AC source is transformedto the desired voltage, filtered, and fed back to the error amplifier to close the regulation loop. Such a regulator is similar to the ones described because it uses PWM pulses for regulation control, but it does not utilize the inductor as a temporary storage element. An increased pulse width (larger ON time) will increase the voltage out from the transformed source; while a decreased pulse width decreases the voltage output.