Design and implementation of power management circuits in embedded systems

Aiming at the battery power management problem of most embedded systems, a unit circuit module for power management of embedded systems, especially those used in handheld and portable devices, is designed. The power management circuit is based on MAX8903, and has the advantages of wide input range, compact size, simple peripheral circuit, high working efficiency, etc. It can be used to manage battery charging, power selection, power detection, etc. in embedded systems, and well meets the functional requirements of power management units.

1 System Introduction

The rapid development of electronic circuit integration technology has made computer systems smaller and more powerful. At the same time, the development of mobile communication technology has made these computer systems more portable, and many portable computers have begun to use batteries for power supply. High-performance computing is usually accompanied by high power consumption, and the serious lag in battery technology and the increase in people's environmental awareness have made the problem between performance and power consumption more prominent. The emergence of power management technology has eased the contradiction between the two and reduced the overall power consumption of the system through effective power distribution. Power management technology is very common in desktop computers and servers. However, in the embedded field, since the development of embedded systems is usually aimed at special applications, the development of power management technology is relatively slow. This paper takes a complete embedded system handheld terminal device as an example to design the power management circuit of the system. With ARM as the control center, it contains 256 MBDDR memory and 512 MB NandFlash memory, and provides asynchronous serial port, USB, WiFi, AC97, display and other circuit units. The charging interface includes two interfaces: USB and AC adapter. The output voltage range of the AC adapter is between 5 and 12 V, and it provides an output current greater than 1 A.

The power supply part mainly includes: battery detection circuit, battery charging circuit, power intelligent selector, DC-DC conversion, power control circuit, etc.

2 Power Management Circuit Analysis

2.1 Introduction to charging management chip

The charging management chip uses MAXIM's MAX8903A, and its basic features are as follows:

(1) High-efficiency DC-DC input range of 4.15 V to 16 V, no need to design a heat sink, which is conducive to the design of small-sized equipment;

(2) Common or separate USB and adapter inputs with current limits up to 2 A (adjustable);

(3) 4 MHz switching frequency allows the use of tiny external components;

(4) Immediate turn-on: Keep working when there is no battery or the battery is over-discharged;

(5) 50 mΩ integrated load switch;

(6) Input OVP (overvoltage protection) up to 16 V;

(7) Thermistor monitoring and thermal adjustment function to prevent overheating;

(8) Charging timer;

(9) 4 mm × 4 mm, 28-pin TQFN package.

2.2 Power Management Circuit Analysis

The system is connected to a dual-input external power mode (AC adapter and USB). When the AC adapter is connected, the chip provides system operating power and battery charging power separately or simultaneously through the internal efficient DC-DC step-down converter. When the USB external power supply is connected, the charging current limit is less than 500 mA. When the system load power supply is greater than the USB power supply capacity, the insufficient part is supplemented by the battery power. The intelligent power selector automatically switches between the external power supply and the battery to ensure uninterrupted power supply to the system. External power detection and charging detection are connected to the GPIO port of the CPU for system monitoring power status.

The external power supply is mainly an AC adapter. USB connection is not recommended because the USB power supply capacity is limited. When the system is in working state, it takes a considerable time to complete charging.

The block diagram of the power management circuit is shown in Figure 1.

Figure 1 Power management circuit block diagram

The schematic diagram of the system power management circuit is shown in Figure 2.

Figure 2 Schematic diagram of the power management circuit

(1) Charging current control

The charging current is controlled by R8P and R9P. The maximum value of the charging current is 1200/R8P. At the same time, the charging current is less than 6000/R9P, where 6000/R9P is the DC power supply current limit setting. As shown in Figure 2, when R8P=1.5 kΩ and R9P=3 kΩ, the DC power supply current limit is 6000/3000=2 A, and the charging current limit is 1200/1500=0.8 A. If R8P=1.2 kΩ and R9P=5 kΩ, the DC power supply current limit is 6000/5000 =1.2 A, and the charging current limit is 1200/1200=1 A.

This system uses R8P=1.5 kΩ and R9P=3 kΩ.

(2) System voltage switching

When DCIN and USB are connected to the system power input at the same time, DCIN input takes priority and USB input is automatically turned off. DCIN supplies battery charging and MBAT (system power supply) at the same time, and the battery can also play a role in reducing MBAT voltage fluctuations.

After the battery is fully charged, the charging circuit is partially turned off, DCIN supplies power to the MBAT system, and the MBAT voltage is stabilized at 4.4 V.

(3) Charging Instructions

The MAX8903 pin DOK is a DC power connection indication output, which is valid at low level. The indicator light D2P is used to indicate the DC power connection status. At the same time, the signal is connected to the GPIO pin of the CPU for software to detect this status.

The MAX8903 pin CHG is the charging indication output, which is valid at low level. The indicator light D3P is used to indicate the charging status. At the same time, the signal is connected to the GPIO pin of the CPU for software to detect the charging status.

The MAX8903 pin FLT is a fault indication output, which is valid at low level. The indicator light D1P is used to indicate fault status, such as charging timeout.

(4) Battery temperature protection

A 10 kΩ negative temperature coefficient thermistor is connected from the THM pin of the MAX8903 to GND to detect the temperature change of the battery during the charging process. When the battery temperature exceeds the set limit temperature, charging the battery is temporarily stopped until the battery temperature drops to a safe temperature range.

(5) DC-DC buck converter inductor selection

The DC-DC buck converter uses a control architecture with a switching frequency of 4 MHz and achieves buck conversion by adjusting the duty cycle. The recommended inductor selection is shown in Table 1.

Table 1 Recommended inductor values ​​for DC-DC step-down converters

The charging current of this system is less than 1 A, the input voltage is about 12V, and the inductor is selected to be 2.2μH.

(6) PCB layout

The PCB layout (partial) is shown in Figure 3.

Figure 3 PCB layout (partial)

The PCB layout of the system circuit is a ten-layer board design, and only the top PCB wiring is shown in the figure. PCB layout principles: short and wide wiring is used for high current parts; multiple vias are used to connect the exposed pad to the heat dissipation ground to facilitate heat dissipation; the current setting resistor is directly grounded to reduce current deviation; and the influence of power current on the voltage stabilization part is reduced.

3 Performance Test Data

The main indicators of the power management circuit are: charging efficiency, output operating voltage, charging current, etc. The circuit test connection is shown in Figure 4.

Figure 4 Power management circuit test connection diagram

3.1 External power supply voltage is fixed

When the external power supply voltage is fixed, the data relationship test data of the charging current and the battery voltage is shown in Table 3. Figure 5 is a schematic diagram of the test data relationship.

Table 3 Test data when the external power supply voltage is fixed

Figure 5 Relationship between charging current and battery voltage when the external power supply voltage is fixed

3.2 External power supply voltage change

The change of the external power supply voltage corresponds to a fixed operating current. The test data of the input current and the power conversion efficiency are shown in Table 4. FIG6 is a schematic diagram of the test data relationship.

Table 4 Test data when external power supply voltage changes

Figure 6 Input current and voltage when external power supply changes

The above test data reflects the external power supply requirements for normal operation of the system.

4 Conclusion

In embedded systems, the power management unit is an essential component of the system. In this system, the power management circuit unit with MAX8903 as the core has the advantages of wide input range, compact size, simple peripheral circuit, high working efficiency, etc., which well realizes the functional requirements of the power management unit.