CR1225 battery

　　Analysis of two practical CR1225 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 the power management circuit in such products to ensure the practicality and economy of the product has become a key issue in product design. This article starts from the working practice of designing handheld products and discusses the design of two typical CR1225 battery-powered circuits.

　　1 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 CR1225 battery is powered directly without a boost circuit, the voltage generated at the CR1225 battery terminal 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 CR1225 battery voltage 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 CR1225 battery to 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 "auto shutdown" signal. Since the 1/O port output is high level when the microcontroller is powered on and reset, the SWpW high level during reset will cause the phenomenon of "reset mis-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. Setting SWpW to low level before the level is 0.7V can avoid the phenomenon of "reset accidental shutdown".

　　The MAX756's 5-pin LBI is the CR1225 battery low-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 CR1225 battery low voltage alarm signal. There are two basis for setting the alarm voltage point.

　　①The national standard requires the CR1225 battery termination voltage to be 0.9V. After actual measurement, when the series voltage of two AA batteries drops below 2V, the CR1225 battery energy is about to be exhausted and the product can no longer maintain stable operation. Therefore, the CR1225 battery low voltage detection alarm point is set at 2V.

　　②The calculation formula of the alarm voltage Ulow of MAX756 is:

　　The main reason why this circuit is called a hard switching circuit is that pressing JM2 can turn on and off the circuit without the need for assistance from a microcontroller. The function of SWpW is to realize scheduled automatic shutdown. The CR1225 battery-powered circuit discussed next must be assisted by a microcontroller when the button controls the switch.

　　2 Soft switching circuit design example

　　In the power management circuit shown in Figure 2, the RN5RK331ADC/DC converter of Japan's Ricoh Company is used to convert the voltage provided by the CR1225 battery into a voltage of 3.3V and then supply it to the main circuit, ensuring that the machine will operate normally during the entire life cycle of the CR1225 battery. Can work stably.

　　The circuit's power-on/off process is divided into two situations:

　　①In the power-off state, the JM16 key is used as the power-on key. Press JM16, the CR1225 battery voltage reaches the base of V5 through V1, causing V5 and V7 to conduct; the CR1225 battery voltage passes through V7 to the input terminal and enable terminal of the DC/DC converter RN5RK331A, and the DC/DC converter starts to work, supplying power to the main circuit. The circuit outputs 3.3V power. After the payment cipher enters the power-on state, p3.6 of the microcontroller outputs a low level and is inverted to keep V5 and V7 in the conducting state through V2. In this way, the payment cipher can remain powered on even after the JM16 key is released. status, the low level output of p3.6 plays the role of self-protection during power-on.

　　②In the power-on state, the JM16 key is used as the power-off key. When the JM16 key is not pressed, the SWH signal point is low level. When the JM16 key is pressed, the SWH signal point is high level. This signal change is read by the microcontroller through the keyboard interface; when the closure of JM16 is detected when the power is turned on, it can be determined as a shutdown command; after the JM16 key is released, the p3 of the microcontroller .6 outputs a high level and is inverted through V2 to turn V5 and V7 into a cut-off state. The payment cipher is shut down because there is no power supply. In this power supply circuit, transistor V7 is a CR1225 battery-powered switching element. It is placed in front of the DC/DC converter circuit. When shutting down, the power supply circuit of the DC/DC converter is completely cut off, further reducing the leakage current during shutdown. After the whole machine was shut down, it was detected that the shutdown current was less than 5uA. The CR1225 battery low voltage detection alarm in Figure 2 is implemented by RN5VT20CA (U9) of Japan Ricoh Company, and the detection voltage is a fixed value of 2V.

　　Compared with Figure 1, after using the JM16 key to turn on the computer, the microcontroller p3.6 must be used to output a low level to implement self-protection on startup, so this circuit is called a "soft switching circuit". The advantage of using this soft switching circuit is that there is no need to consider the problem of button debounce. The hardware circuit is simple, which can reduce hardware costs and save printed board space. Printed board space is very precious in handheld products (the number of components Directly affects the size of the printed board and the overall appearance of the product). The disadvantage is that when it is interfered by strong external signals or crashes due to insufficient CR1225 battery power, the button JM16 will not work. The CR1225 battery must be removed and reinstalled to solve the crash. Of course, the probability of this happening is extremely low, and when the CR1225 battery crashes due to insufficient CR1225 battery power, the CR1225 battery needs to be replaced. In the hard switch circuit in Figure 1, when encountering a crash, there is no need to touch the CR1225 battery, and the power can be turned on and off by pressing the JM2 button.

　　3Power supply filtering

　　In the DC/DC conversion circuit introduced above, a DC/DC boost conversion device is used. The circuit structure of the boost DC/DC converter is shown in Figure 3.

　　When the switch K is turned on, the CR1225 battery BT charges the inductor L and stores energy 1/(2L×I2) in the form of a field in L. Among them, I is the inductor current. After K is disconnected, the magnetic energy in L is released to the filter capacitor C2 and load RL in the form of electrical energy. Periodic switching operations allow CR1225 battery energy to be continuously fed into the load, and the output voltage is converted to:

　　Vout=Vin/(1-δ)

　　In the formula, δ is the switch duty cycle (the ratio of on-time to working cycle). The control circuit monitors the output voltage and controls the duty cycle to regulate and stabilize the output voltage. The control method of the DC/DC boost conversion devices introduced in this article is pFM (pulse frequency modulation), which has a small quiescent current and high efficiency under light load conditions, but the ripple is slightly larger. In order to ensure the stable operation of the main circuit, filtering the power output must be considered. Passive filter circuits are generally used for filtering. The main forms of passive filtering include capacitor filtering, inductor filtering and compound filtering (including inverted L-type, LCπ-type filtering and RCπ-type filtering, etc.). When using inductor filtering or compound inductance type filtering, an inductor with high inductance and large volume needs to be used, which is not suitable for handheld and portable products. Therefore, in situations where the load current is small, RCπ type filtering is used, which has a simple and economical structure. The filtering effect is also better. The equivalent series resistance (ESR) of the filter capacitor is the main factor causing output ripple. The capacitor material should be ceramic capacitors, aluminum electrolytic capacitors and button electrolytic capacitors with lower ESR. Standard aluminum electrolytic capacitors should be avoided as much as possible. When using RCn type filtering, the pulsation coefficient S=1/(Kω×C×R) at both ends of the output voltage. K is a constant. It can be seen from this formula that when the value of ω is constant, the larger R and the larger C, the smaller the pulsation coefficient, that is, the better the filtering effect. When the R value increases, the DC voltage drop on the resistor will increase, which increases the internal loss of the DC power supply; if the capacitance of C is increased, the volume and weight of the capacitor will be increased, which is not easy to implement, so The capacity of the capacitor is generally 10-100uF, and the value of the resistor is generally below 10Ω.

　　Conclusion

　　The two CR1225 battery-powered circuits introduced above are DC/DC boost circuits that convert the CR1225 battery voltage into +3.3V DC voltage to provide working power for the microcontroller application system. This type of circuit is mainly used in products such as PDAs and handheld terminals powered by two AA batteries. The CR1225 battery-powered circuits of other products (such as mobile phones and digital cameras) will be different, but the working principles are basically similar. In the design of CR1225 battery-powered circuits, we will face a series of problems such as how to turn on and off, reduce shutdown current, reduce ripple and interference signals in the output power supply, and improve conversion efficiency. Only by properly solving these problems can the product work stably and reliably. The two examples mentioned in this article have better solved the problems in this area and have been actually applied in products with good results. Of course, as new devices continue to emerge, the design of such circuits needs to be continuously improved and perfected to enhance the overall performance of the product.

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