Traditionally, the lower voltage rails have been supplied by step-down switching regulators or low-dropout regulators. However, these types of ICs do not capitalize on the battery cell’s full operating range, thereby shortening the device’s potential battery run time. Therefore, when a buck-boost converter is used (it can step voltages up or down from a variable input source) it will allow the battery’s full operating range to be utilized. This increases the operating margin and extends the battery run time as more of the battery’s useful capacity is attained, especially as it nears the lower end of its discharge profile.
It is clear that a DC/DC converter solution that solves the primary cell system application requirements, as well as the associated issues already discussed, should have the following attributes:
- A buck-boost power conversion architecture with wide input voltage range to regulate the output voltage through a variety of battery-powered sources and their associated voltage ranges
- Ultralow quiescent current, both in operating mode and shutdown, to increase battery run time
- The ability to efficiently power system rails
- Capably count Coulombs accurately without significantly affecting IC quiescent current (battery consumption), to determine remaining battery state of charge
- Current limiting for attenuating inrush currents thus protecting the cells
- Small, lightweight and low profile solution footprints
- Advanced packaging for improved thermal performance and space efficiency
A recent product introduction from Linear Technology, the nanopower LTC3335 buck-boost converter with integrated Coulomb counter, has all of these attributes already. The device was designed for primary battery applications that need really low quiescent current and also need to know something about remaining battery capacity. Or, where potential battery component or load leakage may be detected by the coulomb counter as a check for system faults. See Figure 1.
The LTC3335 is a nanopower high efficiency synchronous buck-boost converter with an onboard precision Coulomb counter that delivers up to 50mA of continuous output current. With only 680nA of quiescent current and programmable peak input currents from as low as 5mA up to 250mA, the device is ideally suited for a wide variety of low power battery applications, such as those found in battery backed up portable health monitoring systems. Its 1.8V to 5.5V input range and 8 user-selectable outputs between 1.8V and 5V provide a regulated output supply with an input voltage above, below or equal to the output.