Design strategies for two power supply circuits for test terminals

　　Two power supply designs

　　In the power supply system of electronic products, the most common one is based on lithium battery power supply, and the newer 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.

　　Bus power supply circuit

　　The bus power supply system provides voltage to the devices connected to the bus through the bus. This design can provide 5 V, 3.3 V and 1.8 V voltages to the devices. Because the RJ45 outputs a 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 (＄0.9272) , 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 LM2576-5 (＄0.9272) HV 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 terminals of the voltage conversion chips 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 by a charging adapter, turn the switch to the closed position.

　　Lithium battery power supply circuit design

　　In the lithium battery powered system, the battery output voltage passes through the TPS60110 (＄0.9000) and TPS60l00 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, the lithium battery starts to be charged, and 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 (＄0.9000), are selected . TPS60110 (＄0.9000) can output a voltage of 5 V±0.2 V, and TPS60100 (＄0.9000) can output a voltage of 3.3 V±0.132 V. The two chips have the following characteristics:

　　①The maximum output current is 300 mA;

　　② Has a wide input voltage range;

　　③It has energy storage function when outputting at low power consumption;

　　④Can effectively suppress electromagnetic interference.

　　The peripheral circuits of the two power conversion chips are relatively simple, requiring only external configuration of input capacitors, output capacitors and inductors. The specific circuit is shown in Figure 3.

　　Since the system also requires a 1.8 V voltage, a 1.8 V voltage conversion is achieved through an AS1117-1.8V.

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