LDO regulator with 0.3 µA current consumption and integrated battery voltage monitor

Battery voltage monitor:

The additional battery monitor circuit is one of the key features of the RP124, a conventional way to measure the remaining battery charge is using an external resistor divider + 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 RP124 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. In addition, 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. Refer to the image below for details.

Low current consumption:

Specific applications are mainly in a sleep mode and only wake up briefly to perform a measurement, transmits some data and go back to the sleep mode again. So actually, the sleep mode is the dominating operating mode and it is necessary to reduce the current consumption in this mode as much as possible. The RP124 fits perfectly in such requirement with its current consumption of 0.3µA.

There are a few product versions available with options for the CE pin and the automatic discharge function. The CE pin disables the LDO for the B and D-versions. As for the E-version, it has the possibility to disable the battery monitor circuit when not in use. It saves another 0.1µA whilst the LDO remains in operation, resulting in a current consumption of only 0.2µA.

Three operating modes:

The RP124 (similar to the popular RP118 without battery voltage monitor) is a special kind of product offering three operating modes:

1. Low power consumption mode, saving battery power at low output current demand
2. Fast transient response mode, enhanced dynamic performance at high output current demand
3. Off mode, to reduce current consumption to a minimum

The device switches automatically between the low power consumption and the fast transient response mode, based on the output current demand of the application. The performance of the LDO is enhanced in fast transient response mode. In particular, the ripple rejection as well as the response speed to line and load transients demonstrate better results compared to a conventional LDO with low current consumption. This means the RP124 contributes to optimised output voltage stabi­lity and ripple reduction. The Off Mode is controlled by the Chip Enable pin and turns the LDO offline, reducing current consumption to a minimum. Refer to the images below regarding supply current versus output current (left) and performance versus output current (right).

Safety and other functions:

The RP124 features an embedded fold back current limit circuit. When a short circuit occurs at the output, this circuit will decrease the output current to a level of 65mA, thus protecting the LDO and other electronic parts of the application from possible damage. After removing the short, the regulator resumes to normal operation automatically.

Optional features by product version:

Package: Standard SOT-23-5 or compact DFN1212-6 package to minimize the PCB area.
Battery Monitor Output: 1/3 or 1/4 of the input voltage.
CE pin function: Pin to enable/disable the LDO Regulator or Voltage Monitor circuit.
Auto-discharge function: None, auto-discharge available for LDO Regulator or Voltage Monitor output.
The auto-discharge function enables a fast discharge of the output capacitor when the LDO is switched off.

Conclusion:

The RP124 is intended for use in a wide variety of applications that require a long battery lifetime. It has a remarkable current consumption of only 0.3µA and has features to improve its performance when needed. This makes a significant difference compared to other LDOs with low current consumption and is important for battery powered applications such as wearables, medical and IoT devices with RF transmitters, low power CPUs, stand-alone sensors and energy harvesting circuits etcetera. The additional battery monitor is a valuable add-on to this product for which is a popular request from electronics designers.