Ricoh Electronic Devices Co., Ltd. in Japan has launched two VFM mode Buck DC/DC Converters RP514 and RP515, designed for use in wearable devices and applications for the Internet of Things (IoT) requiring ultra-low current consumption to extend battery life. These products are equipped with a unique integrated battery voltage monitor to make it possible to measure the remaining charge left in the battery.
The additional battery monitor circuit is one of the key features of the RP514 and RP515, a conventional way to measure the remaining battery charge is using external divider resistors + MOSFET connected to an A/D Converter. However, the input impedance of an A/D Converter is generally low, therefore a considerable current flows to the ground, and drains the battery and reduces the lifetime of the application. The RP514 and RP515 provide 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 the ground and the buffer output is compatible with the input impedance of the A/D converter. In addition, all essential components are integrated into the chip, reducing valuable circuit board space and cost. In this way, it is possible for example to make a simple fuel gauge for primary batteries.
Today’s wearable and IoT devices mainly operate in a sleep mode, therefore power consumption is mainly defined by the quiescent current, which refers to a circuit’s quiet state, without any output load. In this mode, the system is idle but ready to wake up at any time to perform a task, transmitting some data and resume to sleep mode again. The RP514 and RP515 have an impressive low quiescent current and consume only 300 nA, prolonging battery life or makes it possible for the designer to select a smaller sized battery for the application. The difference between the two products is their output current capability, they can provide respectively 100mA (RP514) and 300 mA (RP515). Both products offer synchronous rectification and have high and low side MOSFET driver transistors embedded. The DC/DC Converters operate in a VFM mode which is best to achieve a high efficiency performance at low output current demands, the resulting peak efficiency is in the range of 95%.
“Under Voltage Lock Out” circuit is included, which is disabling 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 with a constant slope, 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.
The RP514 and RP515 have an optional auto-discharge function; this feature rapidly discharges the output capacitor to the ground once the CE pin disables the chip. The solution with the WLCSP-9-P2 package occupies the smallest circuit board space whilst the budgetary version is available in a standard DFN (PLP) 2527-10 package.