Two typical battery-powered circuit design solutions

With the advent of the information age, handheld electronic products emerge in an endless stream (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 key issue in product design.

　　Hard Switching Circuit Design Example

　　The hard switch circuit converts the series voltage of two No. 7 batteries into a voltage of 3.3 V through the DC/DC converter MAX756. The circuit diagram is shown in Figure 1. If the power is directly supplied by the battery without the boost circuit, the voltage generated by the battery terminal will drop from high to low. The series voltage of two new batteries is above 3 V. As the energy is exhausted, it will drop below 2 V, causing the machine to fail to work properly. The JM2 button is the on/off button. When pressing JM2, it will cause malfunction due to the jitter of the button. The charge and discharge circuit composed of R20, C13, R21, R22, R23, and V9 is used to select the values ​​of R20, C13, and R21 appropriately, so that the charging time and discharge time of the charge and discharge circuit are greater than the key jitter time, thereby effectively eliminating the key jitter. After the key pulse output by the collector of V9 is de-jittered, it is further filtered and shaped by three inverters with Schmitt triggers of U25 (74HC14) to generate a single pulse with a complete waveform. This pulse triggers the flip of U24A (74HC74 D flip-flop).

　　In Figure 1:

　　① If the Q terminal at pin 5 of U24A outputs a high level, the Q terminal at pin 6 outputs a low level, which is input to the disable terminal at pin 1 of MAX756 (valid at low level). At this time, MAX756 is in the off state, but due to the presence of the pulse rectifier tube V5 in the DC/DC conversion circuit, the battery voltage still reaches the output terminal pin 6 of the DC/DC through V5. Therefore, a transistor V11 must be added to the circuit as a switching element. When the Q terminal at pin 6 of U24A outputs a low level to disable MAX756, the Q terminal at pin 5 of U24A outputs a high level to cut off transistor V11, thereby completely shutting off the path from the battery to the power supply VCC of the main circuit, and the machine is in the off state. When the machine is turned off, the current of the entire machine is at a minimum, and it is measured to be no more than 5uA.

　　②When the key pulse triggers U24A (74HC74 D flip-flop) to flip, the Q terminal of U24A's 5-pin outputs a low level, and the Q terminal of 6-pin outputs a high level, MAX756 is in working state, and because the output voltage control terminal 2 is at a high level, it outputs a voltage of +3.3 V. At the same time, the Q terminal of U24A's 5-pin outputs a low level, causing transistor V11 to be in the on state, so that the MAX756 output can provide working power for the main circuit, and the machine is in the on state.

　　In the power-on state, the output SWPW of the microcontroller remains at a low level. When the microcontroller changes the SWPW output to a high level, the inverting circuit formed by V10 outputs a low level, making the U24A set to 1 terminal effective, the 5-pin Q terminal of U24A outputs a high level, and the 6-pin Q terminal outputs a low level, and the machine will be shut down, so SWPW can be used as an "automatic shutdown" signal. Since the 1/O port output is a high level when the microcontroller is powered on and reset, the high level of SWPW during reset will cause the "reset error shutdown" phenomenon. To prevent this phenomenon from happening, a charging circuit composed of R25 and C14 is added to the SWPW output circuit. The values ​​of R25 and C14 are appropriately selected. After reset, before the R25 and C14 charging circuit is charged to the threshold level of 0.7 V for V10 conduction, SWPW is set to a low level to avoid the "reset error shutdown" phenomenon.

　　The 5-pin LBI of MAX756 is the battery low voltage detection pin. If the voltage on this pin drops below the internal reference voltage of 1.25 V, the 4-pin LBO (open-drain output) of MAX756 will output a low level, which can be used as a battery low voltage alarm signal. There are two bases for setting the alarm voltage point.

　　① The national standard requires that the battery termination voltage is 0.9 V. After actual measurement, when the series voltage of two No. 7 batteries drops below 2V, the battery energy is about to be exhausted and can no longer maintain the continuous and stable operation of the product. Therefore, the battery low voltage detection alarm point is set at 2 V.

　　The circuit is called a hard switch circuit because it can be turned on and off by pressing JM2 without the assistance of a single-chip microcomputer. The function of SWPW is to achieve automatic shutdown at a fixed time. The battery-powered circuit described next must be assisted by a single-chip microcomputer when the button is used to control the power on and off.

　　Soft switching circuit design example

In the power management circuit 　　shown in Figure 2 , the RN5RK331A DC/DC converter from Ricoh of Japan is used to convert the voltage provided by the battery into a 3.3 V voltage before supplying it to the main circuit, ensuring that the machine can operate stably throughout the entire life cycle of the battery.

　　Figure 2 Soft switching circuit

　　The on/off process of this circuit is divided into two cases:

　　① In the shutdown state, the JM16 key is used as the power-on key. When JM16 is pressed, the battery voltage reaches the base of V5 through V1, causing V5 and V7 to turn on; the battery voltage passes through V7 to the input and enable terminals of the DC/DC converter RN5RK331A, and the DC/DC converter starts working and outputs 3.3 V power to the main circuit. After the payment cipher enters the power-on state, the P3.6 of the microcontroller outputs a low level and inverts it through V2 to keep V5 and V7 turned on. In this way, even if the JM16 key is released, the payment cipher can maintain the power-on state, and the P3.6 outputs a low level to play a role in power-on self-protection.

　　② In 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. When the JM16 key is pressed, the SWH signal point is high, and this signal change is read by the microcontroller through the keyboard interface; when the JM16 key is detected to be closed during power-on, it can be determined as a power-off command; when the JM16 key is released, the P3.6 of the microcontroller outputs a high level and after inversion, V5 and V7 are turned off through V2, and the payment cipher shuts down because there is no power supply. In this power supply circuit, transistor V7 is a battery-powered switching element. It is set in front of the DC/DC conversion circuit. When shutting down, the power supply circuit of the DC/DC converter is completely cut off, further reducing the leakage current when shutting down. After the whole machine is shut down, it is tested that the shutdown current is less than 5uA. The battery low voltage detection alarm in Figure 2 is implemented by RN5VT20CA (U9) of Ricoh Company of Japan, and the detection voltage is a fixed value of 2V.

　　Compared with Figure 1, after turning on the machine with the JM16 key, the single-chip P3.6 must also output a low level to achieve self-protection when turned on, so this circuit is called a "soft switch circuit". The advantage of using this soft switch circuit is that there is no need to consider the key de-jitter problem, the hardware circuit is simple, the hardware cost can be reduced, and the printed circuit board area can be saved. The printed circuit board area is very valuable in handheld products (the number of components directly affects the size of the printed circuit board and the overall appearance of the product). The disadvantage is that when it is interfered by a strong external signal or the machine freezes due to insufficient battery power, the key JM16 will not work, and the battery must be removed and re-installed to solve the freeze phenomenon. Of course, the probability of this happening is extremely low, and when the machine freezes due to insufficient battery power, the battery needs to be replaced. In the hard switch circuit of Figure 1, when the machine freezes, there is no need to touch the battery, and the machine can be turned on and off by pressing the key JM2.

　　Power supply filtering

　　In the DC/DC conversion circuit introduced above, a DC/DC boost converter device is used. The circuit structure of the boost DC/DC converter is shown in FIG3 .

　　Figure 3

　　When switch K is turned on, the battery BT charges the inductor L, and stores energy 1/(2L×I2) in the form of a field in L. Where I is the inductor current. After K is disconnected, the magnetic energy in L is released to the filter capacitor C2 and the load RL in the form of electrical energy. The periodic switching operation allows the battery energy to be continuously delivered to 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 the on-time to the working cycle). The control circuit monitors the output voltage and controls the duty cycle to achieve the purpose of regulating and stabilizing the output voltage. The control mode of the DC/DC boost converter 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, it is necessary to consider filtering the power supply output. Generally, passive filtering circuits are used for filtering. The main forms of passive filtering are 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 inductor filtering, inductors with high inductance and large volume are required, which are not suitable for handheld and portable products. Therefore, in occasions with small load current, RCπ type filtering is used, which has a simple structure, is economical, and has a better filtering effect. The equivalent series resistance (ESR) of the filter capacitor is the main factor causing the output ripple. The material of the capacitor should be selected from ceramic capacitors, aluminum electrolytic capacitors and button electrolytic capacitors with low ESR, and standard aluminum electrolytic capacitors should be avoided as much as possible. When using RCn type filtering, the ripple coefficient at both ends of the output voltage is S=1/(Kω×C×R). K is a constant. From the formula, it can be seen that when the ω value is constant, the larger the R and the larger the C, the smaller the ripple coefficient, that is, the better the filtering effect. When the R value increases, the DC voltage drop on the resistor will increase, thus increasing the internal loss of the DC power supply; if the capacitance of C is increased, the volume and weight of the capacitor will increase, which is also not easy to achieve. Therefore, the capacitance of the capacitor is generally 10-100 uF, and the value of the resistor is generally below 10Ω.

　　Conclusion

　　The two battery-powered circuits introduced above are DC/DC boost circuits that convert battery voltage into +3.3 V DC voltage to provide working power for the microcontroller application system. This type of circuit is mainly used in PDAs, handheld terminals and other products powered by two No. 7 batteries. The battery-powered circuits of other products (such as mobile phones and digital cameras) will be different, but the working principles are basically similar. The two examples described in this article have better solved a series of problems faced in the design of battery-powered circuits, such as how to realize power 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 we ensure that the product works stably and reliably.