Aiming at the battery power management problem of most embedded systems, a unit circuit module for power management of embedded systems, especially embedded systems for handheld and portable devices, is designed. The power management circuit is based on the MAX8903 and has the advantages of wide input range, compact size, simple peripheral circuit and high working efficiency. It can be used to manage battery charging, power supply selection, power detection, etc. in embedded systems. The functional requirements of the power management unit.
1 System Introduction
The rapid development of electronic circuit integration technology has led to the shrinking of computer systems and the continuous improvement of performance. 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 battery power. High-performance computing is often accompanied by high power consumption, and the serious lag in battery technology and increased environmental awareness have made the problem between performance and power more prominent. The emergence of power management technology has eased the contradiction between the two, reducing the overall power consumption of the system through effective power distribution. Power management technology is very common on desktop computers and servers. However, in the embedded field, power management technology is relatively slow due to the development of embedded systems for specific applications. This paper takes a complete embedded system handheld terminal device as an example, and designs the power management circuit of the system. ARM is the control center. It contains 256 MB DDR memory and 512 MB NandFlash memory. It provides asynchronous serial port, USB, WiFi, AC97. , display and other circuit units. The charging interface includes two interfaces, USB and AC adapter. The AC adapter output voltage ranges from 5 to 12 V, providing an output current greater than 1 A.
The power supply part mainly includes: a battery detection circuit, a battery charging circuit, a power smart selector, a DC-DC conversion, a power control circuit, and the like.
2 power management circuit analysis
2.1 Introduction to Charge Management Chip
The charging management chip uses the MAX8903A of MAXIM. The basic features are as follows:
(1) 4.15 V to 16 V high-efficiency DC-DC input range, no need to design a heat sink, which is conducive to the design of small 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 conduction: keep working when there is no battery or battery over discharge;
(5) 50 mÎ© integrated load switch;
(6) Input OVP up to 16 V (overvoltage protection);
(7) Thermistor monitoring, thermal adjustment function to prevent overheating;
(8) charging timer;
(9) 4 mm & TImes; 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 connected to an AC adapter, the chip provides system operating power and battery charging power, either individually or simultaneously, through an internal high-efficiency DC-DC buck converter. When the USB external power supply is connected, the charging current limit is less than 500 mA. When the system load power is greater than the USB power supply capability, the insufficient portion 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 charge detection The GPIO port connected to the CPU is used to monitor the power status of the system.
The external power supply is mainly based on the AC adapter. It is not recommended to use the USB connection. The reason is that the USB power supply capacity is limited. It takes a long time to complete the charging in the system working state.
The power management circuit block diagram is shown in Figure 1.
Figure 1 power management circuit block diagram
The circuit schematic of the system power management part is shown in Figure 2.
Figure 2 Power management part circuit schematic
(1) Charge 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 supply is set. As shown in Figure 2, when R8P = 1.5 kÎ© and R9P = 3 kÎ©, the DC power supply current limit is 6000/3000 = 2 A, and the charge current limit is 1200/1500 = 0.8 A. If R8P = 1.2 kÎ©, R9P = 5 kÎ©, the DC power supply current limit is 6000/5000 = 1.2 A, and the charge current limit is 1200/1200 = 1 A.
This system uses R8P=1.5 kÎ© and R9P=3 kÎ©.
(2) System voltage switching
When DCIN and USB are connected to the system power input at the same time, the DCIN input takes precedence and the USB input automatically turns off. DCIN also supplies battery charging and MBAT (system power supply), and the battery can also reduce the fluctuation of MBAT voltage.
After the battery is fully charged, the charging circuit is partially turned off, DCIN is supplied to the MBAT system, and the MBAT voltage is stabilized at 4.4 V.
(3) Charging indication
The MAX8903 pin DOK is a DC power connection indication output, active low, the indicator D2P is used to indicate the DC power connection status, and the signal is connected to the GPIO pin of the CPU for software detection of this state.
The MAX8903 pin CHG is the charging indication output, active low, the indicator D3P is used to indicate the charging status, and the signal is connected to the GPIO pin of the CPU for software to detect the charging status.
The MAX8903 pin FLT is the fault indication output, active low, and the indicator D1P is used to indicate the fault status, such as charging timeout.
(4) Battery temperature protection
The MAX8903 pin THM to GND is connected to a 10 kÎ© negative temperature coefficient thermistor 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 safety. temperature range.
(5) DC-DC buck converter inductance selection
The DC-DC buck converter uses a control architecture with a switching frequency of 4 MHz to achieve buck conversion by adjusting the duty cycle. The recommended inductor selection is shown in Table 1.
Table 1 DC-DC buck converter inductance recommended value
The charging current of this system is less than 1 A, the input voltage is about 12V, and the inductance is 2.2Î¼H.
(6) PCB layout
The PCB layout (partial) is shown in Figure 3.
Figure 3 PCB layout (partial)
The system circuit PCB layout is a ten-layer board design, and only the top-level PCB layout is shown in the figure. PCB layout principle: the large current part adopts short and wide wiring connection; the exposed pad uses multiple via holes to connect the heat dissipation ground to facilitate heat dissipation; the current setting resistor is directly grounded to reduce current deviation; and the power current is reduced to the voltage regulation part. Impact and so on.
3 performance test data
Main indicators of power management circuit: charging efficiency, output working 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 between the charging current and the battery voltage is shown in Table 3. Figure 5 is a schematic diagram of the relationship of test data.
Table 3 Test data when the external power supply voltage is fixed
Figure 5 When the external power supply voltage is fixed. Relationship between charging current and battery voltage
3.2 External power supply voltage change
The change of the external power supply voltage corresponds to the fixed operating current, and the test data of the input current and power conversion efficiency are shown in Table 4. Figure 6 is a schematic diagram of the relationship of test data.
Table 4 Test data when the external power supply voltage changes
Figure 6 Input current and current when the external power supply voltage changes
The above test data reflects the external power requirements required for the normal operation of the system.
In embedded systems, the power management unit is an essential 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 size, simple peripheral circuit and high working efficiency, which fully realizes the functional requirements of the power management unit.
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