RP605 Buck-Boost DC/DC Converter

Nisshinbo Micro Devices' RP605 buck-boost DC/DC converter with a unique integrated battery voltage monitor makes it possible to measure the remaining charge left in the battery. It is designed to be used in wearable devices, applications for Internet of Things (IoT), and wireless communications modules requiring a low level of current consumption to extend battery life.

The additional battery monitor circuit is one of the key features of the RP605, a conventional way to measure the remaining battery charge is using an external resistor divider plus MOSFET connected to an A/D converter. However, the input impedance of this solution is typically low resulting in a considerable current flow to ground, draining the battery and limiting the lifetime of the application. The RP605 provides a simple solution with a built-in resistor divider and voltage follower as a buffer. As an advantage, this circuit has a much lower current flow to ground and the output is compatible with the input impedance of the A/D converter. The quiescent current of this circuit is only 100 nA and has an additional chip enable pin (CE2) instead of using a discrete MOSFET. All essential components for this circuit are integrated into the chip, reducing valuable circuit board space and cost. In this way, it is possible to make a simple fuel gauge for primary batteries, for example.

The RP605 is particularly well suited when a supply voltage is required and located somewhere in between the voltage level of a fully charged and fully discharged battery. At a certain moment, the DC/DC converter switches over from buck to boost mode automatically to maintain the output voltage setting. The RP605 is designed for applications that are mainly in sleep mode and wake-up periodically to perform a measurement, transmit data, and then return to sleep mode. For this type of application, the current consumption in sleep mode should remain as low as possible. The RP605 has an impressive low quiescent current and consumes only 300 nA (DC/DC converter part), prolonging battery life or makes it possible for the designer to select a smaller sized battery for the application.

The RP605 is able to provide up to 300 mA in buck mode while in boost mode, the output current is less and related to the input voltage level.

Multiple protection circuits are integrated, including an undervoltage lockout circuit which disables the DC/DC converter in case the input voltage drops below a minimum threshold. A soft-start circuit controls the output voltage to ramp-up smoothly and prevents any output overshoot and undershoot during the start-up period. The Lx current limit circuit prevents the peak current through the inductor to exceed a specific maximum current threshold. An output overvoltage protection turns off both the P-channel and N-channel MOSFETs and thermal protection, shutting down the IC in case the junction temperature increases above +140°C.