Design and implementation of power management circ

Design and implementation of power management circuit 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 takes max8903 as the core, and has the advantages of wide input range, compact volume, simple peripheral circuit, high efficiency. Of course, there is one point that is quite important. It can be used to manage battery charging, power selection, power detection, etc. in embedded systems, which well meets the functional requirements of the power management unit

1 system introduction

the rapid development of electronic circuit integration technology has made computer systems smaller and better. At the same time, the development of mobile communication technology has made these computer systems more portable. Many portable computers begin to use battery power supply. High performance computing is usually accompanied by high power consumption, and the serious lag of battery technology and the increase of people's awareness of environmental protection make the problem between performance and power consumption more prominent. The emergence of power management technology eases the contradiction between the two, and reduces 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, because the development of embedded systems is usually for special applications, the development of power management technology is relatively slow. Taking a complete handheld terminal device of embedded system as an example, this paper designs the power management circuit of the system, takes arm as the control center, contains 256 MB DDR memory and 512 MB nandflash memory, and provides circuit units such as asynchronous serial port, USB, WiFi, AC97, display, etc. The charging interface includes USB and AC adapter. The output voltage of the AC adapter ranges from 5 to 12 V, providing an output current greater than 1 A

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

2 analysis of power management circuit

2.1 introduction of charging management chip

the max8903a of maxim is selected as the charging management chip, and its basic characteristics are as follows:

(1) the high-efficiency DC-DC input range of 4.15 V-16 V does not need to design a radiator, which is conducive to the design of small equipment

(2) public or separate USB and adapter input, with a current upper limit of up to 2 a (adjustable)

(its basic properties, limitations and failure mode must be understood 3) the 4 MHz switching frequency allows the use of small external components

(4) immediate conduction: keep working when there is no battery or the battery is over discharged

(5) 50 m Ω integrated load switch

(6) OVP (overvoltage protection) for inputs up to 16 V

(7) thermistor monitoring, 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 into a dual input external power mode (AC adapter and USB). When connecting the AC adapter, the chip provides the system working power supply and battery charging power supply separately or at the same time 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. Intelligent Power Selector realizes automatic switching between external power supply and battery to ensure uninterrupted power supply of the system. The GPIO port connected to the CPU for external power detection and charging detection is used for system monitoring power status

the external power supply is mainly AC adapter, and USB connection is not recommended, because USB power supply capacity is limited, and it takes quite a long time to complete charging under the working state of the system

(1) charging current control

the charging current is controlled by r8p and r9p. The maximum charging current is 1200/r8p, and the charging current is less than 6000/r9p, of which 6000/r9p DC power current limit setting. As shown in Figure 2, when r8p=1.5 K Ω and r9p=3 K Ω, the current limit of DC power supply is 6000/3000=2 a, and the charging current limit is 1200/1500=0.8 a. If r8p=1.2 K Ω, r9p=5 K Ω, the current limit of DC power supply is 6000/5000 =1.2 a, and the charging current limit is 1200/1200=1 a

r8p=1.5 K Ω and r9p=3 K Ω are selected for this system

(2) system voltage switching

when dcin and USB are connected to the system power at the same time for experiment, when the source input, dcin input takes priority, and USB input turns off automatically. Dcin supplies both battery charging and MBAT (system power supply), and the battery can also reduce the voltage fluctuation of MBAT

after the battery is charged, the charging circuit is partially closed, dcin supplies power to the MBAT system, and the MBAT voltage is stabilized at 4.4 v

(3) charging indication

max8903 pin DOK is the DC power connection indication output, and the low level is effective. The indicator light d2p is used to indicate the DC power connection status, and the signal is connected to the GPIO pin of CPU for software to detect this status

max8903 pin CHG is the charging indication output, and the low level is valid. The indicator light d3p is used to indicate the charging state, and the signal is connected to the GPIO pin of the CPU for software detection of the charging state

max8903 pin FLT is the fault indication output, and the low level is valid. The indicator light d1p is used to indicate the fault status, such as charging timeout, etc

(4) battery temperature protection

max8903 pin THM to GND is connected with a negative temperature coefficient thermistor of 10 K Ω, which is used to detect the temperature change of the battery during charging. When the battery temperature exceeds the set limit temperature, temporarily stop charging the battery until the battery temperature drops to the safe temperature range

(5) inductance selection of DC-DC step-down converter

dc-dc step-down converter adopts a control architecture with a switching frequency of 4 MHz, and realizes step-down conversion by adjusting the duty cycle. The recommended inductance selection is shown in Table 1

Table 1 recommended inductance of DC-DC step-down converter

the charging current of this system is less than 1 A, and the input voltage is about 12V. Select inductance 2.2 μ H。

(6) PCB layout

the PCB layout of the system circuit is a ten layer board design, and only the top-level PCB wiring is shown in the figure. PCB layout principle: the high current part adopts short and wide wiring connection; The bare pad is connected with the heat dissipation ground through multiple vias to facilitate heat dissipation; The current setting resistance is directly grounded to reduce the current deviation; Reduce the influence of power current on the voltage stabilizing part, etc

3 performance test data

3.1 when the external power supply voltage is fixed

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

3.2 change of external power supply voltage

the change of external power supply voltage corresponds to the fixed working current, and the test data of its input current and power conversion efficiency are shown in Table 4. Figure 6 shows the relationship between test data

the above test data reflect the external power supply requirements required for the normal operation of the system

4 conclusion

in embedded system, power management unit is an important part of the system. In this system, the power management circuit unit with max8903 as the core has the advantages of wide input range, compact volume, simple peripheral circuit and high work efficiency, which well realizes the functional requirements of the power management unit