Design of Embedded Single Board Computer Power Supply
Embedded operating speed is high, the system is more complex, often integrated ultra-large-scale FPGA devices, DSP devices, DDR memory and a variety of interface circuits. This puts higher demands on the output voltage, power, voltage accuracy, power-up sequence, and power integrity of the power supply. Here is a CPCI-based embedded single-board computer power supply design. The design is mainly used in aviation equipment and military vehicle equipment.
Embedded operating speed is high, the system is more complex, often integrated ultra-large-scale FPGA devices, DSP devices, DDR memory and a variety of interface circuits. This puts higher demands on the output voltage, power, voltage accuracy, power-up sequence, and power integrity of the power supply. Here is a CPCI-based embedded single board computer power supply design. The design is mainly used in aviation equipment and military vehicle equipment.
System power demand analysis and device modeling
The system consists of the CPU and its associated DDR memory, PCI interface, clock, power, EBC bus and external interface circuit. CPU using AMCC's PowerPC 440EPx.
2.1 System Power Requirements
The system power supply is more complex, with up to eight different supply voltage values, with 5 V and 3.3 V provided by the CPCI chassis. 5 V supplies the DC / DC device with buck to generate other supply voltages, and supplies power to the 1553 bus's transformer. 3.3 V is the main system power supply, including the USB PHY, clock devices, FPGA and CPU and PCI bridge devices (PLX6466) I / O parts. Other supply voltages are derived from a 5V or 3.3 V supply voltage step-down.
The system uses DDR2 as the memory, the use of four Micron's MT47H64M16, capacity of 512 MB. Each chip DDR2 device core, interface and DLL power supply voltage is 1.8 V, the maximum current of 440 mA. Also pay special attention to DDR2 VREF and address and control signal port voltage VTT, the voltage value is 0.9V. Among them, VREF on the tolerance requirements are very strict (less than 2%), but its less demanding on the current. VTT is not only strict tolerance requirements, but also requires its ability to instantaneous output or absorption of a large current. At the same time, VREF 岍 to change with the changes in VDD, VTT also to track VREF changes. LDO usually difficult to complete such work, must use a dedicated DDR termination power supply devices.
The system uses Spartan3 FPGA device XC3S200 to achieve 1553 transceiver and some interface circuit design. The device uses three voltage core voltages VCCINT (1.2 V), auxiliary voltage VCCAUX (2.5 V) and interface voltage VCCO (3.3 V). FPGA internal power-on reset circuit, only when the three power signals have reached their respective threshold voltage, the release of the reset signal. Therefore, there is no requirement for the power-up sequence of the three power signals. However, if VCCINT is powered up prior to VCCAUX, an additional current of several hundred milliamperes will be added at power-up. Estimation of FPGA device power consumption can be based on spreadsheet tool XPower Estimator (XPE) or in the ISE directly call XPower. The system uses XPower software to estimate the design power requirements: VCCINT is 50 mA, VCCAUX is 10 mA. The system uses two 88E1111s as gigabit Ethernet PHY devices, with 2.5 V for the die voltage (410 mA) and 1.0 V for the core voltage (250 mA). In addition to the above integrated circuits, the system also has such as serial interface, USB interface, clock and other circuits, but the power consumption is low. From the analysis we can see: 1.5 V and 1.8 V need to use high-power power devices, DDR2 power supply requires a dedicated power supply devices, other voltage power requirements smaller.
2.2 power supply device selection
Power devices are mainly divided into linear regulators and DC / DC converter two types. LDO is a linear regulator is mainly used in the input and output voltage difference of small occasions, which is characterized by: low cost, low noise, quiescent current is small, less external components, but the conversion efficiency is not very high, and the output current Generally not very big. DC / DC converter conversion efficiency, high output current, low quiescent current. However, due to the use of PWM control, the switching noise is relatively large, the cost is relatively high. And the external circuit is more complex, generally need an external switch, inductors and capacitors. Many new DC / DC switches integrate the device inside the device, so only an external inductor and filter capacitor.
According to the characteristics of power devices, and the analysis of system power requirements, these two types of power devices in the system are used. But to simplify the design, to facilitate mass production and materials management, the system uses only three different types of power devices, namely: LT3501, LDO devices TPS51100 and TPS74801. Among them, 1.5 V and 1.8 V power supply circuit with larger power consumption is realized by LT3501. The power supply and reference power supply of DDR2 are provided by TPS51100. The other power supply of the system is provided by TPS74801.
3.1 Parameter Configuration
3.1.1 Output voltage
The choice of output voltage is simple, by connecting the VOUT and VFR between the two resistive voltage to be
3.1.2 switching frequency
The switching frequency of the LT3501 is determined by the resistance connected to the RT / SYNC pin
3.1.3 Inductance value
For switching power supply, the value of the inductance is very important.
3.1.4 Input Capacitance and Output Capacitance
Since the input to the switching power supply supplies current to the output in pulsed form, the rise and fall times are very fast.
Therefore. The input capacitor is used to filter the voltage ripple to reduce EMI. Bypass the input signal with a 4.7μF or larger X7R or X5R capacitor, or use a tantalum capacitor in parallel with a smaller-capacity ceramic capacitor. The ceramic capacitor should be placed as close as possible to the input pin of the device.
The output capacitor filters the current flowing through the inductor to produce a very small ripple output voltage. At the same time, its energy storage function can also meet the instantaneous load, and stability LT3501 control loop. The control loop of the LT3501 uses a current mode that does not require RESR (series equivalent resistance) for the output capacitor.
Therefore, a ceramic capacitor can be used as the output capacitor.
3.2 PCB layout
For switching power supplies, the PCB layout is very important. When the switching power supply, the circuit part of the branch there is a large step current. This current flows mainly between the switch tube inside the device, the loop diode outside and the input capacitance. The loop formed by these elements should be as small as possible. In the layout, these devices and the inductor and the output capacitor should be laid out on the same layer of the board, and the wiring is done in the same layer as much as possible. Underneath these elements, there is a continuous localized ground. The local and system connection using a single point of connection, the connection point is best selected in the output capacitor ground. In addition, SW and BST signal wiring as short as possible. LT3501 devices have bare leadframe at the bottom, the structure of a good heat. In the design of PCB. Can be placed in the corresponding position of the device at the bottom of a piece of copper, and through a number of vias and the inner layer of large area copper connection.
Based on the analysis of the power consumption of the system, three kinds of power devices are designed to design the power supply circuit of the embedded system. And use MAX705 power monitoring devices to improve system reliability. The system has been successfully verified in a number of practical applications, and performed well.