Nickel Hydride Batteries

　　Analysis of design solutions for two typical battery-powered circuits

　　With the advent of the information age, handheld electronic products emerge in endlessly (such as PDAs, digital cameras, mobile phones, etc.). These products are mainly powered by batteries. How to design power management circuits in such products to ensure the practicality and economy of the products has become a product key issues in design.

　　Hard switching circuit design example

　　The hard switching circuit converts the series voltage of two AA batteries into a voltage of 3.3V through the DC/DC converter MAX756. The circuit diagram is shown in Figure 1. If the Nickel Hydride Batteriesis powered directly without a boost circuit, the voltage generated at the Nickel Hydride Batteriesterminal will drop from high to low. The series voltage of 2 new batteries is above 3V. As the energy is exhausted, it will drop below 2V, causing the machine to fail to work properly. The JM2 button is the on/off button. When JM2 is pressed, malfunction may occur due to the jitter of the button. The charge and discharge circuit composed of R20, C13, R21, R22, R23, and V9 is used to appropriately select the values of R20, C13, and R21 to make the charging and discharging time of the charge and discharge circuit greater than the key jitter time, thereby effectively Eliminate key jitter. After the key pulse output from the V9 collector is debounced, it is further filtered and shaped through three inverters with Schmitt triggers in U25 (74HC14) to generate a single pulse with a complete waveform. This pulse triggers the flip of U24A (74HC74D flip-flop).

　　In Figure 1:

　　① If the 5-pin Q terminal of U24A outputs a high level, the 6-pin Q terminal outputs a low level, and the low level is input to the 1-pin inhibit terminal of the MAX756 (low level is active). At this time, the MAX756 is in the off state, but due to the existence of the pulse rectifier V5 in the DC/DC conversion circuit, the Nickel Hydride Batteriesvoltage still reaches the output terminal 6 of the DC/DC through V5. Therefore, a transistor V11 must be added to the circuit as a switching element. When the 6-pin Q terminal of U24A outputs a low level to disable the MAX756, the 5-pin Q terminal of U24A outputs a high level to put the transistor V11 in a cut-off state, thereby completely shutting down the path from the Nickel Hydride Batteriesto the main circuit power supply VCC. status, the machine is in the shutdown state, and the current of the whole machine is the minimum when it is shut down, and it is measured not to exceed 5uA.

　　② When the key pulse triggers U24A (74HC74D flip-flop) to flip, the 5-pin Q terminal of U24A outputs low level, and the 6-pin Q terminal outputs high level, the MAX756 is in the working state, because the output voltage control terminal 2 pin is high level, So the output voltage is +3.3V. At the same time, the pin 5 Q terminal of U24A outputs a low level to cause the transistor V11 to be in a conductive state, so that the MAX756 output can provide working power for the main circuit and the machine is powered on.

　　In the power-on state, the output SWPW of the microcontroller remains low. When the microcontroller changes the SWPW output to high level, the inverting circuit formed by V10 outputs low level, so that U24A sets the 1 end to be valid, the 5-pin Q terminal of U24A outputs high level, and the 6-pin Q terminal outputs low level. The machine will be shut down, so SWPW can be used as an automatic shutdown signal. Since the output of the 1/O port is high level when the microcontroller is powered on and reset, the high level of SWPW during reset will cause a reset accidental shutdown. In order to prevent this phenomenon from happening, a charging loop composed of R25 and C14 is added to the SWPW output circuit. The values of R25 and C14 are appropriately selected. After reset, the charging loop of R25 and C14 is not charged to the threshold of V10 conduction. Set SWPW to low level before the level is 0.7V to avoid reset and accidental shutdown.

　　The MAX756's 5-pin LBI is the Nickel Hydride Batterieslow-voltage detection pin. If the voltage on this pin drops below the internal reference voltage 1.25V, the MAX756's 4-pin LBO (open-drain output) will output a low level. Can be used as Nickel Hydride Batterieslow voltage alarm signal. There are two basis for setting the alarm voltage point.

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