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Battery monitor also enables constant-power-boost converter

Some microcontrollers permit operation below a supply voltage of 3V. This feature allows powering directly from a 3V alkaline or lithium battery without the voltage drop and leakage current of a regulator. It is important to monitor the battery voltage to ensure system integrity, and you can also use this information for system purposes. The circuit in this Design Idea maintains constant power to a white-LED-display backlight by adjusting the duty cycle of a boost-power converter. However, an ADC normally needs a fixed voltage reference (Figure 1), which would require two input pins for this function. This Design Idea turns the ADC’s architecture inside out, providing the voltage-reference function using no extra pins.

The monitor circuit in Figure 2 integrates an ADC within the microcontroller. The converter uses the battery voltage as a reference voltage. The principle is the opposite of normal: You want to measure a fixed voltage using a variable-voltage reference (the battery). For an 8-bit converter, the result for this example is (1.18V/ VBAT)x256. Note that a high value indicates that the battery voltage is low. Also, you can use the microcomputer pin that connects to the reference for another purpose. This example normally uses Pin 6 as an output to the pulse-indicator LED, LED1. However, by briefly changing the port direction to analog-input mode, you can complete the battery-measurement operation—including settling, sampling, and conversion—in less than 0.1 msec.

The example uses a PIC12F683 microcontroller and a voltage reference of 1.25V for the LM4041. R1 biases the reference. R2 ensures that the microcontroller output can rise to 3V to turn on transistor Q1 without damaging D1. Resistors R3 and R4 ensure that the transistor is extinguished during the battery measurement. R2, R3, and R4 introduce some attenuation, which you must take into account.

Figure 3 shows the monitor with the addition of a constant-power voltagebooster circuit. The PWM (pulsewidth- modulation) output of the microcontroller drives the converter. For constant power from the booster, the required duty cycle linearly relates to the ADC’s converted value.

Battery technologies vary in their discharge characteristics. Alkaline batteries have high capacity but drop their open-circuit voltage as they operate. The opencircuit voltage can provide a good estimate of battery charge. However, alkaline batteries also have internal resistance and exhibit a recovery phase after supplying a heavy load. The resistance increases with low temperature and low battery charge. To determine the battery’s state, you can take measurements before and immediately after a high-current load is active. This approach allows estimation of both internal resistance and battery charge.