Battery Monitor for Constant Boost Converter

Some controllers allow operation below 3V. This feature allows direct power supply from 3V dry batteries or lithium batteries, eliminating the voltage drop and leakage current of the voltage regulator. It is important at this time to monitor the battery voltage to ensure system integrity. The circuit in this design example maintains constant power to a white LED display backlight by adjusting the duty cycle of a boost converter. However, an ADC usually requires a fixed voltage reference (Figure 1), which may require two input pins. This design example reverses the ADC architecture and provides the voltage reference function without additional pins.

The monitoring circuit in Figure 2 integrates an ADC into the microcontroller. The converter uses the battery voltage as the reference voltage. The working principle is opposite to the normal situation. At this time, a fixed voltage is measured with a variable voltage reference (battery voltage). For an 8-bit converter, the result in this example is (1.18V/VBAT) × 256. Note that a large number indicates low battery voltage. Additionally, the microcontroller pins connected to the reference can be used for other purposes. In this example, pin 6 is generally used as the output for the pulse indicator LED1. However, by simply changing the port direction to analog input mode, battery measurement operations, including stabilization, sampling and conversion, can be completed in less than 0.1ms.

A PIC12F683 microcontroller is used to provide the 1.25V voltage reference for the LM4041. R1 is used as a reference bias. R2 ensures that the output of the microcontroller can rise to 3V. Turning on transistor Q1 without damaging D1. Resistors R3 and R4 ensure that the transistor is inhibited during battery measurements. R2, R3 and R4 will introduce some attenuation and must be taken into account.

Figure 3 shows the supervisor adding a constant power boost circuit. The PWM (Pulse Width Modulation) output of the microcontroller drives the converter. For constant power from the booster, the required duty cycle is linearly related to the conversion value of the ADC. Various battery technologies have different discharge characteristics. Alkaline batteries have large capacities, but their open circuit voltage drops during operation. An open circuit can provide a good estimate of battery charge. However, alkaline batteries also have internal resistance. There will be a recovery period after supplying a large load current. Under low temperature and low battery conditions, the resistance will increase. To determine the state of the battery, measurements can be taken immediately before or immediately after driving a high-current load. This method can simultaneously estimate the internal resistance and battery capacity.

Note: The device used in this article is an intelligent PIC: microcontroller (PIC12F603), which is a microcontroller. Therefore, the pulse width modulation signal (PWM) required in this article needs to be programmed with parameters using assembly language or C language, and then the program is programmed into the chip to realize the above circuit functions. The article only provides one idea.