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Monitor battery discharge with power supply

27 Jan 2016  | Samuel Nork

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The battery current management problem is further complicated if a DC/DC converter is needed to maintain a stable supply voltage for the downstream sensor and communication electronics. DC/DC converters optimised for low power applications generally operate in a Burst Mode fashion, where the converter remains in a SLEEP state until the output drops below the regulation point, and then large, short duration bursts of current are supplied to the output until regulation is achieved. As previously discussed, such bursty currents are problematic for lithium thionyl chloride as well as other primary battery chemistries, and result in reduced operating life for the system. The ideal IoT power solution would combine a long lifetime battery with a DC/DC converter designed with a battery-friendly current management system.

Nanopower DC-DC converter
The LTC3335 (figure 2) was designed with these exact requirements in mind. The part is a buck-boost DC/DC converter which generates a fixed, pin-programmable regulated output voltage between 1.8V and 5V from an unregulated input voltage between 1.8V and 5.5V. The part may be used with a wide variety of primary battery sources to regulate an output voltage above, below or equal to the input.

Figure 2: LTC3335 Nanopower Buck-Boost DC/DC with Programmable Peak ILIM.

The LTC3335 is unique among buck-boost converters in requiring only 680nA of input quiescent current to maintain a regulated output. In addition, the part includes 8 programmable peak input current settings from as low as 5mA up to 250mA in order to accommodate the input current limitations of a wide range of primary cell batteries, including lithium thionyl chloride, without any external current limiting.

The LTC3335's DC/DC operation is relatively straightforward (figures 2 and 3). If the output voltage is above the regulation point, the part enters a SLEEP mode with only the output monitoring circuitry active. Once the load forces the output voltage to drop below its regulation point, the DC/DC converter is enabled and power is transferred from input to output using a four switch monolithic full bridge converter. Once the DC/DC converter is enabled, switches A and C turn on, allowing current to flow from the battery through an external inductor connected between pins SW1 and SW2. Once the programmed peak current (IPEAK) is reached, switches A and C turn off and switches B and D turn on, allowing the current flowing in the inductor to charge the output capacitor connected to the PVOUT pin.

Figure 3: LTC3335 DC/DC Block Diagram.

Current continues to flow in switches B and D until it reaches zero. If at this point the output is above the regulation point, the part returns to SLEEP mode until the output drops out of regulation. Otherwise, another cycle AC/BD switch cycle commences. With such low quiescent current and synchronous operation, the LTC3335 achieves power conversion efficiency above 80 per cent with load currents as low as 10µA, a common average load level for a wide variety of wireless sensors. In addition, peak input currents may be reduced to the minimum value necessary to support the average power consumption, thereby maximising battery lifetime and capacity.

Additional challenge: Estimating remaining battery capacity
Despite efforts to minimise load currents and maximise battery life, applications are ultimately area constrained, and batteries will need replacement at some point. In a low cost portable device, monitoring the discharge status of the battery and estimating the remaining capacity may be a low priority. Either the battery outlasts the usable life of the product, or the consequences of going off-line to change the battery are minimal. However, in the case of a critical sensor in a factory automation system or rail car safety monitor, unforeseen downtime to replace a dead battery represents an unacceptable expense.

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