SLAA398A September 2008 – August 2018 MSP430F4794 , MSP430F4794
Powering an MSP430™ microcontroller (MCU) from a single 1.5-V battery cell is desirable in a number of applications. These applications require the use of a charge-pump-based dc/dc converter. This application report describes dc/dc converter basics and selection, and presents the implementation of a single-cell thermostat using an MSP430 MCU and a TPS60313 dc/dc converter. Analysis of the single-cell plus dc/dc converter solution includes the current consumption and the expected battery life.
Source code related to this solution is available for download from www.ti.com/lit/zip/slaa398.
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With a properly selected charge pump, the designer can implement a single-cell powered application and still maintain low-power performance and battery life. The thermostat that is described in this application report is based on the MSP430F4794 MCU and the TPS60313 dc/dc charge pump converter, which is optimized for low-power single-cell applications. The MSP430F47x4 is a high-pin-count microcontroller with many peripherals. This application requires only one of the four SD16 modules (used to sample temperature) and the Basic Timer (used for periodic wakeup). It is left to the reader to decide how functionality can be added to take full advantage of the device’s feature set.
It is important to understand the relationship between power in, power out, and efficiency when selecting a converter. For an ideal physical system, the power in is equal to the power out. For the system overview depicted in Figure 1, this can be expressed as:
VBatt × IBatt = VCC × ICC
For example, if the battery voltage VBatt = 1.3 V, the MSP430 current ICC = 10 µA, and the MSP430 voltage VCC = 3.3 V, then, ideally, from the equation above, solving for IBatt yields:
IBatt = (VCC × ICC) / VBatt = 25 µA
This means that in order for an ideal dc\dc converter to supply 10 µA at 3.3 V to the MSP430, the battery would need to supply 25 µA at 1.3 V to the dc/dc converter. Ideal implies 100% efficiency but, of course, no real system is 100% efficient. Sources of inefficiency include power dissipation in the form of heat generation, switching losses, and quiescent current of the dc/dc converter itself. The graph in Figure 2 (taken from the TPS60313 datasheet) shows that when VIN = 1.3 V and VOUT = 3.3 V, the device is approximately 75% efficient when supplying an output current of 10 µA.
If we reconsider the equation PIN = POUT taking into account the efficiency of the dc/dc converter:
VBatt × IBatt = (1 / eff) × VCC × ICC
Then solving for IBatt:
IBatt = (1 / eff) × ((VCC × ICC) / VBatt) = 34 µA
Using the same values for VBatt, ICC, and VCC, IBatt is found to be 34 µA. This means that to provide 10 µA at 3.3 V to the MSP430, the battery must supply a total of 34 µA at 1.3 V to the dc\dc converter.