Using an MSP430™ MCU and TPS60313 to Implement a Single-Battery-Cell Powered Thermostat

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.

Trademarks

MSP430 is a trademark of Texas Instruments.

All other trademarks are the property of their respective owners.

1 Introduction

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.

Figure 1. System Overview

2 DC/DC Converter Basics – P_IN vs P_OUT

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:

V_Batt × I_Batt = V_CC × I_CC

For example, if the battery voltage V_Batt = 1.3 V, the MSP430 current I_CC = 10 µA, and the MSP430 voltage V_CC = 3.3 V, then, ideally, from the equation above, solving for I_Batt yields:

I_Batt = (V_CC × I_CC) / V_Batt = 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 V_IN = 1.3 V and V_OUT = 3.3 V, the device is approximately 75% efficient when supplying an output current of 10 µA.

Figure 2. TPS60313 Efficiency vs Output Current (V_IN = 1.3 V, V_OUT = 3.3 V)

If we reconsider the equation P_IN = P_OUT taking into account the efficiency of the dc/dc converter:

V_Batt × I_Batt = (1 / eff) × V_CC × I_CC

Then solving for I_Batt:

I_Batt = (1 / eff) × ((V_CC × I_CC) / V_Batt) = 34 µA

Using the same values for V_Batt, I_CC, and V_CC, I_Batt 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.

3 TPS60313 Description and Features

For the application described in this document, the TPS60313 inductorless dc/dc charge-pump converter was selected for its low quiescent current and high-efficiency performance at low operating currents. These attributes make it an ideal fit for use with MSP430 microcontrollers in single-cell applications.

The TPS60313 step-up, regulated charge pump generates a 3-V output voltage from a 0.9-V to 1.8-V input voltage. Only five small 1-µF ceramic capacitors are required to build a complete high-efficiency dc/dc charge-pump converter.

In SNOOZE mode, the TPS60313 operates with a typical operating current of 2 µA, while the output voltage is maintained at 3 V ± 10%. Load current in SNOOZE mode is limited to 2 mA. If the load current increases above 2 mA, the device automatically exits the SNOOZE mode and operates in normal mode to regulate to the nominal output voltage with higher output currents.

While the SNOOZE mode enables greater efficiency at low currents, users must recognize that the output voltage ripple is greater than when SNOOZE mode is disabled (see Figure 3).

Figure 3. TPS60313 Load Transient Response

Channel 1 shows the effect of the SNOOZE feature on the regulated output voltage. Channel 2 shows the output current transitions that cause the device to enter and exit SNOOZE mode. This behavior can possibly affect ADC conversion results or other processes that are sensitive to supply ripple, so it is recommended to take the device out of SNOOZE mode before doing any A/D conversions. Once SNOOZE mode is disabled, the output voltage is regulated with greater accuracy, but the quiescent current is higher.