300nA Quiescent Current DC-DCs With and Without Battery Monitor
Reducing power consumption for wearables and IoT applications is one of the key features to extend battery life. Ricoh introduced four VFM-mode buck dc-dc converters RP514, RP515, RP516 and RP517 as a solution for IoT applications. The products provide various additional features such as a unique integrated battery voltage monitor and optimization for use in applications requiring ultra-low supply voltage.
The dc-dc converters are intended to be used in USB, coin cell, and Li-Ion battery powered equipment. Potential applications include wearable devices such as sports watches, as well as health and fitness monitors. Low power MCU, GPS/GNSS receiver/processor, sensor devices and energy harvesters can also use the converters. RF modules with low power requirements including Bluetooth® LE, Zigbee, WiSun and ANT are other potential applications for the dc-dc converters.
Battery Monitor:
The additional battery monitor circuit is one of the main features of the RP514 and RP515. A conventional way to measure the remaining battery charge is using an external resistor divider connected to an ac-dc converter. The input impedance of an ac-dc converter, however, is generally low and the resistor divider should have similar characteristics. Therefore, a considerable current flow to ground drains the battery and decreases the lifetime of the application.
The RP514 and RP515 offer a simple solution with a built-in resistor divider (with high resistor values) and voltage follower as a buffer. An exceptional advantage of this circuit is that it has a much lower current flow to ground and the buffer output is compatible with the input impedance of the ac-dc converter.
Block diagram of simple fuel gauge (Click on image to enlarge)
In addition, the chip integrates all essential components, reducing valuable circuit board space and cost. In this way, Ricoh says it is possible for example to make a simple fuel gauge for primary batteries (see block diagram above) for measuring the remaining charge left in the battery.
Ultra-Low Output Voltage:
The RP516 and RP517 are similar, but they lack the battery voltage monitor. Instead, they have an extraordinary low output voltage range from 0.3V to 1.2V. This type of power supply is tailored to support the latest generation of low power MCU's, GPS/GNSS receiver/processors and other ICs which are required in IoT applications for wearable devices and a low power wireless sensor network.
Performance RP514 / RP515 & RP516 / RP517:
When the input voltage (Vin) decreases below the set output voltage (Vout), the buck dc-dc converter stops switching and the ON duty becomes 100%. Vout will now follow Vin when decreasing even further until it reaches the UVLO detector voltage.
Both products provide synchronous rectification and embed high and low side MOSFET driver transistors. The dc-dc converters operate in a VFM mode which is best for achieving a high-efficiency performance at low output current demands.
(Click on graphs to enlarge)
The resulting peak efficiency for RP514 & RP515 is in the range of 95%. Even at 10 µA output current, an efficiency performance of about 90% is achieved.(Vout=1.8V, Vin=2.0V).
The efficiency for the RP516 & RP517 at light loads is significantly improved and has a peak at 75%, even at 10µA output current an efficiency performance of about 67% is achieved.(Vout=0.5V, Vin=1.8V).
(Click on graphs to enlarge)
Low Current Consumption:
Today's wearable and IoT devices mainly operate in a sleep mode and therefore power consumption is primarily defined by the quiescent current, which refers to a circuit's quiet state without any output load. In this mode, the system remains idle but ready to wake up at any time to perform a task, transmit some data, and resume to sleep mode again.
The dc-dc converters have a low quiescent current and consume only 300nA, prolonging battery life, allowing the use of smaller-sized batteries for the application.
The principal difference between the two individual products is their output current capability. They deliver 100mA (RP514 & RP516) and 300 mA (RP515 & RP517) respectively.
Thanks to the wide input voltage range, the two dc-dc converters can operate from various power supplies such as a USB port, a single cell Li-Ion, and other regular batteries.
Safety Circuits:
All four dc-dc converters have an array of safety features, protecting the voltage regulator and other parts of an application from possible damage and failure.
An under-voltage lockout circuit is included, which disables the dc-dc converter if 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 start-up.
The Lx current limit circuit prevents the peak current through the inductor from exceeding a specific maximum current threshold.
Optional Features:
An optional auto-discharge function is available by product version. It discharges the output capacitor to the ground rapidly once the CE pin disables the chip.
The new dc-dc converters are intended for use in a wide variety of applications that require long battery lifetimes. They consume only 300 nA at no-load, and have good transient response performance. An attractive battery monitor circuit or ultra-low output voltage range is available.
