Design of FPGA high-efficiency power supply based on TPS6211X

1 Introduction

With the development of ARM microprocessor technology and field programmable logic array technology (FPGA) and their application in handheld devices, the requirements for low power consumption and miniaturization of handheld devices are getting higher and higher. As an inevitable trend, energy saving will be an inevitable feature of future handheld electronic devices, but high efficiency and energy saving are a contradiction. Although the capacity of lithium batteries as the energy source of handheld devices is getting larger and larger, with the improvement of functions, the total power consumption of handheld devices is also increasing, and the energy volume ratio of lithium battery capacity is limited, which requires continuous improvement of the conversion efficiency of power devices and improvement of power management solutions.

In handheld devices, in order to obtain higher total power, the battery voltage of many systems is above 7 V, such as 7.2 V, 9 V, 12 V, etc. Based on this, Texas Instruments (TI) launched the 1.5 A (typical) DC/DC step-down converter series TPS6211X that supports an input voltage range of 3.1 to 17 V. The power conversion efficiency of this small integrated circuit (IC) can reach up to 95%, which can significantly extend the battery life of handheld test instruments and other high-end handheld devices powered by 2 to 3 lithium batteries.

2 Introduction to TPS6211X Devices

The TPS6211X series of devices includes three DC/DC power supply devices: TPS6211, TPS62111, and TPS62112. TPS62111 and TPs62112 have fixed outputs of 3.3 V and 5 V respectively, and TPS62110 has an adjustable output with an output voltage range of 1.2 to 16 V. Its characteristics can meet the power requirements of most handheld devices.

2.1 Pin Function Description

The standard package of TPS6211X is QFN-16, with a size of 4 mm × 4 mm. Figure 1 shows the pin arrangement. The pin function description is shown in Table 1.

2.2 Performance characteristics

This series of devices is a synchronous buck converter with low ripple voltage noise. The switching frequency is typically 1 MHz. It has built-in FETs and does not require additional MOSFETs and voltage regulators, which reduces the design difficulty and allows designers to use smaller external components without worrying about the impact on component performance or conversion efficiency. It can save board space and shorten system design time. In addition, the TPS6211X can be synchronized with an external frequency signal of 0.8 to 1.4 MHz, making the design flexible.

Figures 2a and b are the current-voltage conversion efficiency curves of the TPS62111 and TPs62112 under various input voltages, working in PFM (pulse frequency modulation) mode and PWM (pulse amplitude modulation) mode respectively. As can be seen from Figure 2, in PFM, that is, when working under light load, the conversion efficiency is quite high, reaching 90%; and when the input voltage, internal parameters and external load change, it works in PWM mode, and the conversion efficiency can reach more than 85%, which is a very useful feature for directly converting from voltage values ​​above 7 V to the required voltage value. Figure 2c shows the maximum value of the output current under different input voltages. When the input voltage is higher than 5 V, the maximum output current of the TPS6211X series can reach 1.8A, which is very suitable for driving FPGA devices that require high voltage accuracy and large current amplitude.

3 Application and Implementation

Unlike LDO (linear voltage regulator) type power supply devices, TPS6211X series devices are DC/DC (switching) type voltage converters. In the development of handheld test instruments, ARM and FPGA are the two main devices. ARM, as the main control device, carries the operating system and is responsible for the scheduling and power management of global tasks. FPGA, as the main computing unit, is the main module that consumes power. Therefore, good power management of FPGA modules can significantly improve system performance.

3.1 Introduction to FPGA devices and analysis of their power configuration

In the developed handheld tester, Cyclone III EP3C40_780 FPGA devices are used. Cyclone III FPGA is the third generation product of Altera Cyclone series. The CycloneIII FPGA series has unprecedentedly achieved low power consumption, low cost and high performance at the same time, further expanding the application of FPGA in cost-sensitive fields.

The power-on timing of the core voltage and I/O voltage of the FPGA device should follow a certain order, that is, the core power-on time should not be later than the I/O power-on time. Although the Cy-cloneIII FPGA series devices have no special requirements for this, it is beneficial to follow certain power-on conditions. For example, Altera's Cyclone III must allow the VC-CINT power supply to power on within 3 to 9 ms (fast power-on mode) or 50 to 200 ms (standard power-on mode), and the typical startup time of the TPS6211X series devices is 1 ms (1 MHz), which fully meets this requirement.

After all the code is compiled, the Quartus II development platform is used to calculate the usage of its internal resources and estimate the power consumption: 1.2 V/3 A (core voltage and digital voltage required by PLL), 2.5 V/1 A (analog voltage required by PLL), 3.3V/1 A (I/O voltage).

3.2 Solution Selection

In the handheld tester, a 7.2 V lithium battery is selected for power supply. This property is the main reason for choosing the TPS6211X series power supply device. There are two power management solutions to choose from, as shown in Table 2.

3.3 Circuit Implementation

According to the selected solution 2 and the data sheet of TPS6211x, the circuit connection is shown in Figure 3 and Figure 4. The inductors, capacitors, and resistors in the two circuits are all packaged in SMD type, which can minimize the PCB area. The smaller the DC resistance of the inductor, the better (tens of milliohms), and the saturation current must be higher than the maximum output current. Therefore, a power supply type inductor is selected to reduce heat loss; the input capacitor can use a non-polar ceramic capacitor, and the output capacitor should use a polar tantalum capacitor. The withstand voltage of the capacitor is preferably above 16 V, which can ensure the unidirectionality and good ripple characteristics of the output current. The device pin FB in Figure 3 is the output voltage feedback terminal to adjust the output voltage accuracy. The resistance accuracy of R3 and R4 in its external resistor divider network should be above 5% to obtain an output voltage with appropriate accuracy. The relationship between R3 and R4 is as follows:



Where: VFB is 1.153 V. As long as R4 and the required output voltage Vo are given, R3 can be determined. In addition, for better device operation, the sum of R3 and R4 should be between 400 kΩ and 1 MΩ. The circuit for obtaining 2.5 V from TPS62110 is similar to Figure 3, and the resistance value of the feedback resistor network can be obtained according to formula (1). The circuit for obtaining 5 V from TPS62112 is similar to Figure 4 and will not be repeated.

It should be noted that the output current of the power supply device in the system is large, so its heat dissipation end PowerPAD should have good contact with the copper plate of the PCB. When a certain voltage in the system requires a large driving current, the parallel method can be used to meet the requirements. In the developed handheld tester, the FPGA requires a power supply of 1.2 V/3 A, 2.5V/1 A, and 3.3 V/1 A, so two TPS62110s can be connected in parallel. The maximum driving current of 3.6 A is obtained, which has been proven to be completely feasible in practice. This approach can not only meet the needs of FPGA devices, but also provide some support for other modules of the system.

4 Conclusion

Although the capacity of lithium batteries as the energy source of handheld devices is getting larger and larger, with the improvement of functions, the total power consumption of handheld devices is also increasing, and the energy-to-volume ratio of lithium battery capacity is limited, which requires continuous improvement of the conversion efficiency of power supply devices and improvement of power management solutions. The TPS6211X power conversion device here is stable in the system and can provide good voltage support for FPGA devices, achieving the expected goal.