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Buck converters are switch-mode step-down converters which can provide high efficiency and high flexibility at higher Vin/Vout ratios and higher load current. Most Buck converters contain an internal high side MOSFET and synchronous rectifier MOSFET, which are in turn switched on and off via internal duty-cycle control circuit to regulate the average output voltage. The switching waveform is filtered via an external LC filter stage.
Due to the fact that the MOSFETs are either ON or OFF, Buck converters dissipate very little power, and the duty-cycle control makes large Vin/Vout ratios possible. The internal MOSFETs RDS(ON) mainly determines the current handling capabilities of the Buck converter, and the MOSFET voltage ratings determine the maximum input voltage. The switching frequency together with the external LC filter components will determine the ripple voltage on the output. Higher switching frequency buck converters can use smaller filter components, but switch losses will increase. Buck converters with Pulse Skipping Mode (PSM) will reduce their switching frequency at light load, thereby increasing light load efficiency, which can be important in applications with low power standby modes.
Buck controllers will step down the input voltage to lower output voltage with external switches. The output current is limited by external MOSFETs which gives designers the flexibility to optimize performance.
Boost converters are used when Vout needs to be higher than Vin. Boost converters will step-up the input voltage to a higher output voltage. This is accomplished by charging an inductor via an internal MOSFET switch and discharging the inductor via a rectifier to the load when the MOSFET switch is off.
The transition from inductor charge to discharge will reverse the voltage across the inductor, thereby stepping up the voltage higher than VIN. The on/off duty-cycle of the MOSFET switch will determine the Vout/Vin boost ratio, and the feedback loop will control the duty-cycle to maintain stable output voltage. The output capacitor is the buffer element to reduce the output voltage ripple.
The current rating of the MOSFET switch together with step-up ratio will determine the maximum load current, and the MOSFET voltage rating will determine the maximum output voltage capability. In some boost converters, the rectifier is integrated as a MOSFET to provide synchronous rectification.
Boost controllers will step up the input voltage to a higher output voltage with external switches. The output current is limited by external MOSFETs which gives designers the flexibility to optimize performance. With Boost controllers, designers can achieve higher levels of output power and operating temperature range that might be not possible with boost converters.
Buck-Boost converters are used in applications where input voltage can vary, either below or above the output voltage. The four internal MOSFET switches will be automatically configured to form a Buck when Vin exceeds Vout and change to Boost operation when VIN falls below Vout.
This makes Buck-Boost ideal for battery-powered applications to extend battery life when the battery voltage falls below the regulated output voltage. The efficiency of the Buck-Boost converter is very good because of the fully synchronous operation. The output current capability in Buck mode is normally higher than in Boost mode. This is because the Boost mode requires higher switching current at the same load conditions compared to the Buck mode. The MOSFET voltage ratings will determine the maximum input and output voltage range. In applications where the output voltage does not need to be referenced to ground (like LED drivers), buck-boost converters with a single switch and rectifier are used. In most cases, the output voltage is then referenced to Vin.