Two power supply designs in handheld terminal tester

Introduction
In multifunctional intelligent instruments, different power supply methods are sometimes required at different work sites. At present, in the research of power supply, redundant power supply and multiple power supply methods are the current research hotspots. Redundancy technology is relatively mature and widely used. Now many instruments are designed based on redundant power supply. Multiple power supply methods are also widely used, but there are not many that integrate multiple power supply methods into the same electronic product. This paper implements the design of two power supply methods and applies them to multifunctional handheld test terminals, which effectively reduces the cost of products and brings convenience to equipment maintenance.

1 Design of two power supply modes
In the power supply system of electronic products, the most common one is based on lithium battery power supply, and the more novel one is bus power supply system. This design integrates bus power supply and lithium battery power supply, and performs special processing on the two power supply modes to avoid the influence of one power supply on the other power supply.
1.1 Bus power
supply circuit The bus power supply system provides voltage to the devices hanging on the bus through the bus. This design can provide 5 V, 3.3 V and 1.8 V voltages to the devices. Because RJ45 outputs standard +24 V, in order to obtain 5 V, 3.3 V and 1.8 V voltages, level conversion must be performed. This design uses LM2576-5, AS1117-1.8 and AS1117-3.3 power conversion chips to obtain the required voltage. The bus power supply circuit is shown in Figure 1.

The 24 V voltage provided by RJ45 is input to the VIN terminal of the voltage conversion chip LM2576-5HV through a resistor fuse, and a 5 V voltage VCCl is output from the FOB. After decoupling and filtering, VCCl is input to the IN terminal of the voltage conversion chip ASlll7-1.8V and ASlll7-3.3V, and a voltage of 1.8 V and a voltage of 3.3 V DVCC are obtained. At the same time, bus power supply can also provide charging voltage for lithium batteries. The charging voltage of the lithium battery charging control chip is 3.5~7 V, so VCCl can be used to charge it. In the specific implementation, a charging switch is designed between VCCl and the DC input of MAXlll5. When charging by bus, turn the switch to the open position; when charging with a charging adapter, turn the switch to the closed position. [page]

1.2 Design of lithium battery powered circuit
In the lithium battery powered system, the battery output voltage passes through the TPS60110 and TPS60100 power chips, and after level conversion, the required 5 V, 3.3 V and 1.8 V voltages are obtained. In the charging circuit, MAXl555 is used as the control chip. MAXl555 charges a single lithium ion (Li+) battery through the charging interface and AC adapter power supply. It does not require an external FET or diode and can accept an input voltage of up to 7 V. The on-chip temperature limit simplifies the PCB layout, and by optimizing the charging rate, it can be unconstrained by heat dissipation issues when the battery condition and input voltage are in the worst case. When the MAXl555 temperature limit is reached, the charger does not shut down, but gradually reduces the charging current. The battery charging circuit is shown in Figure 2.

The charging voltage is input to VCC_PLUG, and the lithium battery starts to charge. The indicator D1 turns on, indicating that the charging is complete.
In order to obtain three voltage specifications (5 V, 3.3 V and 1.8 V), the output voltage of the lithium battery needs to be converted. Here, two integrated DC-DC charge pump chips, TPS60110 and TPS60100, are selected. TPS60110 can output a voltage of 5 V ± 0.2 V, and TPS60100 can output a voltage of 3.3 V ± 0.132 V. The two chips have the following characteristics:
① The maximum output current can be 300 mA;
② It has a wide input voltage range;
③ It has energy storage function when low power consumption output;
④ It can well suppress electromagnetic interference.
The peripheral circuits of the two power conversion chips are relatively simple, and only input capacitors, output capacitors and inductors need to be configured outside. The specific circuit is shown in Figure 3.

Since the system also requires a 1.8 V voltage, a AS1117-1.8V is used to achieve 1.8 V voltage conversion.

2 Application of two power supply modes
The handheld test terminal based on industrial Ethernet and industrial wireless communication is a multifunctional test equipment. The handheld test terminal uses the AT91 series ARM microcontroller chip, which includes important components such as microprocessor (AT91R40008), memory, communication module, bus-powered communication interface, display terminal, wireless communication module, handheld keyboard, etc. It integrates two protocols: EPA (Ethernet for Plant Automation) and IEEE 802.15.4 (low data rate WPAN standard). It can realize network field testing, EPA protocol analysis and equipment calibration for wired and wireless sites. Since it is used in two industrial sites, two different power supply modes are adopted, namely, power supply through bus power supply in industrial Ethernet and power supply by lithium battery in industrial wireless transmission mode. The lithium battery can be charged while the bus power supply is working normally, or it can be charged by an external power adapter. Its hardware block diagram is shown in Figure 4.

3 Voltage characteristics of two power supply modes in handheld terminals
In the application of handheld test terminals, the voltage characteristics of the two power supply modes were measured many times. The voltage conditions of the handheld test terminal when working stably are listed in Table 1.

Conclusion
This article introduces the hardware implementation process of the two power supply methods in detail, focusing on their application in handheld test terminals. The design of the two power supply methods can provide 5 V, 3.3 V and 1.8 V voltages, which meets the requirements of two power supply methods required for an intelligent instrument to be used in multiple industrial sites. In the design of multi-functional intelligent instruments, it has broad application and market prospects.