The SEPIC provides step-down conversion until the input voltage equals or falls below the output voltage. It then provides step-up conversion until the batteries have discharged to the minimum allowed input voltage. The main disadvantage of using a SEPIC is that it requires either a single coupled inductor (transformer) or two separate inductors as well as a coupling capacitor (Figure 1): inductors and coils are large and require much space on the PCB. This is unsuitable for applications where conservation of size and board space are major requirements. The battery-lifetime characteristic with the SEPIC were a total about 260min, which was also limited by the device's minimum-input-voltage specification of 1.8V. The efficiency of a SEPIC converter generally fares worse than that of buck and boost regulators. When the input voltage is lower than the output voltage, the efficiency of the regulator is that of a boost regulator.

Boost converter with LDO
Another option that is frequently overlooked is the possibility of using a boost converter to supply a low-dropout regulator (LDO). The output of the step-up converter (Figure 2) exceeds the LDO output voltage by the specified drop-out voltage (0.3V for this device) for the entire battery lifetime. Due to the loss through the LDO regulator, overall efficiency of the device is lower than that of a normal boost converter. The LDO provides a constant efficiency over the total life of the battery, which means that the losses that occur due to the lower efficiency of the LDO are always apparent in the total efficiency of the device. Even with the higher efficiency of the boost converter, which provides the input for the LDO, the total efficiency of this device is calculated as being the product of the LDO and the preceding boost converter, which produces an overall efficiency of about 84%.

Boost converter with down-mode conversion
Down-mode conversion (Figure 3) allows designers to regulate input voltages that are above the expected output voltage without the need of extra components. In this mode, the control circuit changes the behaviour of the rectifying PMOS. It sets the voltage drop across the PMOS as high as needed to regulate the output voltage. The main advantages of using a device with down-mode conversion is that no extra external components are needed (contrary to the SEPIC, for example), there is no need for an LDO to perform the step-down function, and you can have a device with fewer pins. Another advantage of this converter is that, when it is in operation and the battery is being discharged, the device automatically enters the shutdown mode if the voltage on VBAT drops below approximately 0.8V. This undervoltage-lockout function is implemented in order to prevent the malfunctioning of the converter: this is considerably below the specified 1.8V of the SEPIC. In general, the device does not have a higher efficiency in down-mode than the previously mentioned devices. As can be seen in Figure 4, during the first 12 to 15 minutes, while the device is in down-mode, the efficiency achieved is about 70%. In normal mode, the remaining time is spent in the 93 to 95% efficiency range. Of the total battery lifetime, only about 3% are spent in down-mode. This means that if the total efficiency over the full life of the battery is calculated, that total efficiency would be over 90%.

Conclusion
With the consumer market expanding and the concurrent need for extended battery life in portable applications being as they are, manufacturers are continuously on the lookout for converters which offer area-, cost-, and power-efficient solutions for a broad range of battery-driven applications. The devices compared are all used to meet specific needs for different applications. The down-mode conversion methodology of the TPS6102X family from Texas Instruments offers the customer the favorable option of using very few external components and has the added advantage of being able to maintain a high efficiency over wide load-current ranges, while extracting longer operation time from the application due to the extended battery life.

Figure 1: Block diagram of a SEPIC converter.
Figure 2: Block diagram of a boost converter with LDO.

Figure 3: Boost converter with down-mode conversion.

Figure 4: Efficiency of the converter in down mode and normal boost mode.