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Ultra small quiescent current DC/DC converters for IoT and wearable devices

June 3, 2019

Ultra small quiescent current DC/DC converters for IoT and wearable devices

Reducing power consumption for wearables and IoT applications is one of the key features to extend battery life. Four VFM mode Buck DC/DC Converters RP514, RP515, RP516 and RP517 were recently launched as a solution for this requirement. The products offer various additional features like a unique integrated battery voltage monitor and optimization for use in applications that require an ultra-low supply voltage.

Battery Monitor:

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 an external resistor divider connected to an A/D Converter. The input impedance of an A/D Converter, however, is generally low and the resistor divider should have similar characteristics. Therefore, a considerable current flow to ground drains the battery and reduces the lifetime of the application.

Conventional Solution

Conventional Solution

  1. A/D Converter with low input impedance
  2. Divider resistance must be low, therefore a large current flows to ground, draining the battery

RP514, RP515 Solution

RP514_RP515 Solution

Built-in high resistance divider resistors and buffered output by an internal voltage follower
No external divider resistors needed
No large current flow to ground

The RP514 and RP515 provide a simple solution with a built-in resistor divider (with high resistor values) and voltage follower as a buffer. A great 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 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 (see illustration) to measure the remaining charge left in the battery.

Low Supply Current LDO Regulator with Battery Monitor

Ultra-Low Output Voltage:

The RP516 and RP517 are similar products but they lack the battery voltage monitor. Instead, they have an extraordinary low output voltage range from 0.3 to 1.2 V This kind of power supply is tailored to support the newest generation of low power MCU’s, GPS/GNSS receiver/processors and other ICs which are required in IoT applications for a low power wireless sensor network and wearable devices.

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 more until it reaches the UVLO detector voltage. 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.

RP514x181x

RP514x181x Efficiency vs Output Current

RP515x181x

RP515x181x Efficiency vs Output Current

The resulting peak efficiency for the 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).

RP516x051x

RP516x051x Efficiency vs Output Current

RP517x051x

RP517x051x Efficiency vs Output Current

The efficiency for the RP516 & RP517 at light loads is considerably improved and has a peak at 75%, even at 10 µA output current an efficiency performance of about 67% is achieved.(Vout=0.5V, Vin=1.8V).

Low Current Consumption:

Low Current Consumption:

Today's wearable and IoT devices mainly operate in a sleep mode and 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 DC/DC Converters 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 main difference between the two individual products is their output current capability. They provide respectively 100mA (RP514 & RP516) and 300 mA (RP515 & RP517). Thanks to the wide input voltage range, the two DC/DC Converters are able to operate from various power supplies like 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 the application from possible damage and failure.

  • An Under Voltage Lock Out circuit is included, 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 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.

Optional Features:

An optional auto-discharge function is available by product version and discharges the output capacitor to the ground rapidly once the CE pin disables the chip.

To be used in:

USB, Coin cell and Li-Ion battery powered equipment:

  • Wearable devices: SportsWatch, Health monitoring or Fitness tracker
  • Low Power RF Modules: Bluetooth® LE, Zigbee, WiSun and ANT
  • Low Power MCU, GPS/GNSS receiver/processor, Sensor device and Energy Harvester

Special Contents for IoT Devices

Supporting your IoT system construction

Ultra-low supply current / Low noise / Low electromotive force / Battery monitoring