Many industrial and automotive applications have widely varying input voltage (VIN) rails and often need a buck-boost DC/DC converter to regulate the output voltage (VOUT). Buck-boost DC/DC converters can be either cascaded buck and boost stages or single stage. Cascading buck and boost stages result in double conversion, leading to higher size, cost and power losses.
Figure 1 shows the single-stage buck-boost converter choices from TI for different power levels. At relatively low power levels, the non-synchronous buck-boost topology (for example, the LM5118) is the simplest solution. Traditional single-stage buck-boost topologies such as a single-ended primary inductor converter (SEPIC) or flyback use two inductors and therefore become bulky and less efficient as power level increases. For the highest efficiency, the four-switch buck-boost is the topology of choice for 25W to 250W power levels.
Figure 1: Buck-Boost topologies for different power levels
The LM5175 is a four-switch buck-boost controller that uses a single inductor and integrates MOSFET drivers for all four switches. It delivers higher power and high efficiency compared to other topologies because of the following advantages:
- A single-inductor design results in smaller size and lower losses compared to coupled inductor designs.
- It has integrated MOSFET drivers for synchronous MOSFETs. Synchronous rectification results in higher efficiency than diodes.
- SEPIC or flyback converters always operate in buck-boost mode. The LM5175 operates in buck-boost mode only when the input voltage is close to the output voltage. When VIN is not close to VOUT, the LM5175 operates in either buck or boost mode. Buck or boost operation has lower peak and rms currents, resulting in lower losses than buck-boost operation for identical VIN, VOUT and load current.
- The switches in SEPIC (and synchronous flyback with a 1:1 transformer) see a reverse voltage of (VIN + VOUT) in off time. In a four-switch buck-boost, the buck FETs (QH1 and QL1 in Figure 2) see VIN, while the boost FETs (QH2 and QL2 in Figure 2) see VOUT in off time. Lower voltage MOSFETs have better RDS*QG characteristics, resulting in lower losses.
Figure 2: LM5175 four-switch buck-boost converter
In addition to enabling high-power, high-efficiency buck-boost solutions, the LM5175 has additional features to aid designers. The cycle-by-cycle current limit with optional hiccup mode reduces thermal stress in case of an overload (Figure 3).
Figure 3: Hiccup mode protection for reduced thermal stress
In addition, the converter has an average current limit function that can accurately limit either the source or the load current. This feature can be especially useful for battery charging or LED current control.
Figure 4: LM5175 average current loop used to control current in an LED string
The LM5175 has an optional dither feature that can help with EMI by allowing users to modulate the switching frequency around its nominal value. A dither capacitor sets the modulation frequency.
Figure 5: The dithering feature in the LM5175 helps with EMI
The LM5175 four-switch controller enables high-power and high-efficiency buck-boost solutions with applications in automotive (start/stop for infotainment, LED lighting), industrial (industrial PC, Ethernet switches), communication (power amplifier supplies) and other areas. Additional features such as hiccup mode, average current loop and frequency dithering make it a powerful tool in DC/DC designs.