Design of power supply module based on high performance digital signal processor

1 Introduction

With the continuous development of chip manufacturing technology in recent years and the market demand for high-performance digital signal processors, new digital signal processor (DSP) products with stronger functions, faster speeds and lower power consumption have been continuously launched, which has brought great convenience to circuit design. But at the same time, the use of these high-performance devices has put forward higher requirements for the design of power supply modules. The design of power supply modules that are efficient, stable and meet the power-on sequence is of great significance and will directly affect the stability of the entire system and even the realization of the entire system.

At present, there are three main ways to power chips such as DSP and FPGA: using linear power chips, using switching power chips, and using power modules. An overall comparison of these three methods is shown in Table 1.

The basic principle of a linear power supply is to adjust its internal resistance according to the change of the load resistance, so as to ensure that the voltage at the output end is within the required range. Due to the use of the linear regulation principle, the transient characteristics are good and there is essentially no output ripple. However, as the input-output voltage difference increases or the output current increases, the chip heat will increase proportionally, so the linear power supply requires better heat dissipation control. The input current of a linear power supply is close to the output current, and its efficiency (output power/input power) is close to the output/input voltage ratio. Therefore, the voltage difference is a very important performance, because a lower voltage difference means higher efficiency. The low voltage difference characteristic of the LDO linear power supply is conducive to improving the overall efficiency of the circuit. Linear power supplies provide a small and inexpensive design solution for application systems with small current input.

The power module is a switching regulator in principle, with very high efficiency. Compared with ordinary switching regulators, it has a higher degree of integration, and only requires an input capacitor and an output capacitor to work. It is simple in design and suitable for applications that require a very short development cycle.

2 Chip selection and function introduction

Since the signal processing part of ADSPTS101 is only a sub-part of the whole system, combined with the power supply requirements of other parts, the FPGA chip uses ATERA's EPlCl2F324, IO voltage 3.3 V, core voltage 1.5 V, ADSPTS101's IO supply voltage 3.3V, core voltage 1.2V. Among them, EPlCl2F324 does not have too strict requirements on the power-on sequence, and the power supply design is relatively simple. The AS2830-1.5 power supply chip can meet the requirements. However, ADSPTS101 has stricter requirements on the power-on sequence. When the power-on sequence does not meet the requirements, even after reset initialization after power-on, the initial state may still be incorrect. Therefore, the focus of the system power supply design is to meet the power-on requirements of ADSPTS101. Of course, the use of power modules, such as PT6944 chips, can meet the design requirements, but based on the comparative advantages of switching power supplies and power modules, this system uses switching power supplies for design. The power chips used are TI's TPS54616 and TPS54312.

TPS54616 is a power supply chip launched by TI for DSP, FPGA, ASIC and other multi-chip systems. It is a synchronous step-down DC/DC regulator with low voltage input and high current output. It contains 30MQ, 12 A peak current MOSFET switch tube, and can output a maximum current of 6A. The output voltage is fixed at 3.3V, and the error rate is 1%. The switching frequency can be fixed at 350 kHz or 550 kHz, or it can be adjusted between 280 kHz and 700 kHz. In addition, it also has a current limiting circuit, a low voltage lockout circuit and an overheating shutdown circuit.

TPS54312 is also a low voltage input, high current output synchronous step-down DC/DC regulator launched by TI. The difference is that TPS54312 has a high efficiency output of 3 A continuous current, the integrated MOS-FET switch tube is 60MQ, and its fixed voltage output is 1.2V.

In addition, both TPS54616 and TPS54312 adopt an integrated design, which reduces the number of components and volume. Therefore, they can be widely used in decentralized power supply systems with low voltage input and high current output.

The functional pin definitions of TPS54616 and TPS54312 are similar, and their pin packages are shown in Figure 1.

Taking TPS54616 as an example, the functions of each pin are briefly described. The corresponding pins of TPS54312 with the same name have similar functions.

AGND: analog ground; BOOT: startup input, a 0.02~0.1μF capacitor should be connected to the PH pin; NC: not connected; PGND: power ground, connected to AGND at a single point when used; PH: voltage output terminal; PWRGD: when VSENSE>90% of the reference voltage, the output is high impedance, otherwise the output is low level. This can be used to control the I/O port voltage and core voltage to design a power-on sequence that meets the requirements; RT: frequency setting resistor input, different resistance values ​​can be selected to set different power switching frequencies; SS/ENA: slow start or input and output enable control; FSEL: frequency selection; VBIAS: internal bias adjustment, a 0.1~1μF ceramic capacitor should be connected to AGND; VIN: external voltage input; VSENSE: error amplifier feedback input, which can be directly connected to the output voltage terminal.

3 Circuit Design

Build the schematic diagram in Protel, as shown in Figure 2.

The design mainly considers issues such as input filtering, feedback loop, frequency operation, output filtering, and delayed start.

3.1 Input and output filtering

The input voltage of both power chips is 5 V. To effectively eliminate the high-frequency components in the input power, a 10μF bypass capacitor is connected to the input. At the same time, to reduce the input ripple voltage, a 100μF and 180μF filter capacitor are connected to each. After such combined filtering, a relatively clean input power can be obtained.

At the output end, in order to obtain a better quality output waveform, the output filter network consists of a 4.7μH inductor and a 470μF and 1 000pF capacitor.

3.2 Feedback loop

TPS54312 has direct feedback, and the voltage after filtering output is directly connected to VSENSE. TPS54616 adds a feedback resistor, and the functions are actually the same, both are direct feedback.

3.3 Switching frequency design

If the RT pin is left unconnected and FSEL is grounded or connected to VIN, the switching frequency is 350 kHz or 550 kHz. If an external resistor is used to select the switching frequency, the formula for calculating the resistance value is: R = 500 kHz / selected switching frequency × 100 kΩ. The switching frequency selected in the design is 700 MHz, and the calculated resistance value of the connected resistor is 71.5 kΩ.

3.4 Delayed start

Both chips have slow start and output input enable control functions. Different slow start times can be obtained by connecting capacitors of different capacitances to the SS/EN pins. Although there are special calculation formulas for calculation, the design here can use TI's Swift Designer software for special power supply design, which can provide great convenience for design. Swift Designer provides a series of power supply chip support designs, including support for TPS54312 and TPS54616.

Set the parameters in Swift Designer, and then press "GO". The software will automatically select the power chip and build the peripheral circuit according to the required parameters. Set the parameters as follows: output voltage 1.2V, output current 3A, input minimum voltage 4.8V, maximum 5.2V, slow start time 3 ms, switching frequency 700 kHz. The software can automatically generate a circuit diagram. The power chip automatically selected by the software is TPS54312, and the peripheral circuit has been connected.

Modify the parameters in the same way: output voltage 3.3V, output current 6A, input minimum voltage 4.8V, maximum 5.2V, slow start time 6 ms, switching frequency 700 kHz. Similarly, the software automatically generates a 5V to 3.3V circuit diagram (omitted).

With the help of Swift Designer software, the design becomes flexible and simple. To obtain the correct power-on sequence, some adjustments should be made in the design. Connect the PWRGD pin of TPS54312 to the SS/ENA pin of TPS54616, as shown in the schematic diagram in Figure 2, and connect it to the pull-up state at the same time. In this way, only when the output voltage of TPS54312 is greater than 1.2 V*90%, the output of the PWRGD pin is low, thereby enabling TPS54616 to generate a 3.3 V voltage output, thereby obtaining the correct power-on sequence requirements. When the output voltage of TPS54312 does not meet the requirements, TPS54616 is pulled up and cannot generate a 3.3 V output. In this way, through the setting of the slow start time and the control of the enable pin, the correct power-on delay and power-on sequence can be fully ensured. At the same time, we can flexibly adjust the power-on delay and power-on sequence according to different chips to meet the power-on requirements.

4 Simulation Analysis

Swift Designer software also provides preliminary simulation analysis, which can intuitively provide analysis tables, cycle response diagrams, input voltage jitter impact diagrams, efficiency diagrams and PCB wiring diagrams. The following is a series of related simulation analysis.

From the simulation, it can be seen that the power conversion used in the design has a high conversion efficiency. At the same time, the impact caused by input jitter is also within the acceptable range of the system. After the peripheral capacitor filtering, the output voltage ripple effect will be improved. Since the software does not give an intuitive simulation of the power-on sequence, the system power-on sequence is better guaranteed by setting the slow start time of the two power chips and controlling the enable terminal.

5 Conclusion

The design of the power supply module is of great significance to the realization of the entire system and the smooth operation of the system, especially for some high-performance devices with special power supply requirements. In the design of the power supply module, it is necessary to comprehensively consider the system requirements, design flexibility, implementation difficulty, cost, efficiency, packaging and other corresponding factors, so as to make comprehensive and compromise considerations to seek the best design solution. After actual application on the radar signal processing board, the design meets various voltage, current and power consumption requirements. At the same time, due to the adoption of a better power-on sequence design, it is ensured that the core of ADSPTSl01 is powered on before IO, so that the stability and reliability of the entire system are better guaranteed.