TWS solution based on MAXIM PLC technology

In recent years, the overall market of TWS wireless headsets has continued to grow rapidly, and the market has become increasingly popular. At the same time, the continuous improvement and development of smart terminals, Bluetooth technology, and chip technology have further accelerated the popularization of TWS wireless headsets. Each manufacturer has launched its own solution. This article introduces Maxim's unique solution.

System Architecture

Figure 1 shows the block diagram of this design, which includes two parts: the charging box and the earphones.

The charging box is powered by a 3.7V@125mAH lithium battery, which is charged via USB. The battery is managed by Maxim's high-performance fuel gauge chip MAX77818, and the system is powered by the integrated power management chip MAX17270 using Maxim's SIMO technology. The main control system uses Maxim's low-power Cortex M4 processor MAX32660.

The earphones are powered by a 3.7V small lithium battery. The charging box's PLC MAX20340 charges and communicates with the earphones. The low-power fuel gauge chip MAX17260 mainly manages and measures the battery. The integrated power management chip MAX77651 using Maxim SIMO technology powers the system. The main control system uses the same processor MAX32660. It is also equipped with an audio codec MAX98090 to facilitate voice processing and testing. This codec is only used to illustrate the effect and verify the feasibility of the solution.

The size of the entire circuit board (including all components) is charging box: 46mm*46mm, earphones: 58mm*46mm. Because all devices have small package sizes, the actual product size can be very small to meet the actual earphone size requirements.

Figure 1 Design system block diagram

Figure 2 is the actual circuit of this design, and all our tests are based on it.

Figure 2 MAXREFDES1263

Data communication and energy transmission

In TWS earphones, data transmission and energy transfer between the charging box and the earphones are critical. Currently, true wireless earphones on the market often use 3 (or more) pins to connect to their charging boxes for data and power transmission. Additional pins require more space and also cause reliability risks. In addition, a fixed voltage is often used during the charging of the earphone battery, which can cause harmful heating. This design uses Maxim's proprietary MAX20340 to achieve power and data transmission between the earphones and their charging box, superimposing the data signal on the power supply, so that the interface uses only two pins, effectively reducing the number of failure points and improving reliability. MAX20340 is a universal bidirectional DC power line communication (PLC) management IC with a maximum bit rate of 166.7kbps and supports a maximum charging current of 1.2A. One of its notable features is that it can simultaneously realize charging and data transmission. MAX20340 has a slave detection circuit that generates an interrupt to the system when the PLC host detects the presence of a PLC slave on the power line. This function allows the system to remain in a low-power state until a slave device is connected. This feature is very suitable for charging box applications.

Figure 3 MAX20340 PLC data communication waveform

Many features of the MAX20340, such as master/slave mode, I²C address, dual/single PLC slave mode, and PLC slave address, are pin-configurable, simplifying design. The device is available in a 9-bump, 0.4mm pitch, 1.358mm x 1.358mm wafer-level package (WLP).

Figure 4 MAX20340 block diagram

Figure 5 MAX20340 configuration flow chart

MAX20340 Workflow

At the beginning, the PLC line is in a clamped state to ensure safety. After power-on, the device is first initialized, mainly to detect the device configuration resistance value (RSEL), so as to confirm the device's working mode (master/slave mode, number of slaves), device I2C address, and slave address. This process takes about 3mS. You can use the wait method or the interrupt method (Rsel_Donei) to confirm whether the initialization is completed. After the initial configuration is completed, we can confirm the configured working mode by reading the register DEV_STATUS.

When the device is in host mode, it is in low-power shutdown state. After enabling (/EN), it enters the slave detection state. If the slave is successfully detected, the host will turn on the switch (Q1/2) on the PLC channel and enter the slave charging mode, so that the PLC end can be charged through VCC, and the charging current can reach 1.2A. At the same time, the host can actively start PLC online communication, enter PLC mode, and exchange data with the slave. During the whole process, if any short-circuit unsafe factors occur, the device will automatically disconnect the switch (Q1/2) and enter the safe mode.

When the device is in slave mode, it is in low-power shutdown state. After enabling (/EN), it enters the host detection state. If the host is successfully detected and the online PLC voltage is greater than the set threshold voltage, the device enters the host find mode, enables data communication, automatically configures the switch (Q1/2) in LDO mode, and waits for the host's data communication command. When the host protocol is received, the data can be replied accordingly. The host can restore the slave to the idle waiting state at any time by command and disconnect the switch (Q1/2). It should be noted that data communication between the master and the slave must be initiated by the host. When there are two slaves, the two slaves are confirmed by specific bit addresses.

High performance fuel gauge

The battery part of the charging box design uses Maxim's latest fuel gauge management chip MAX77818. MAX77818 integrates a high-performance switching charger and a proprietary ModelGauge™ m5 fuel gauge in a WLP package, making it an ideal choice for USB-powered portable devices. The intelligent power path charger supports two inputs, has reverse blocking and USB-OTG, integrates all power switches, operates at a higher switching frequency, has high efficiency, and supports low thermal design using smaller external components. The Maxim ModelGauge m5 algorithm has both the excellent short-term high accuracy and high linearity characteristics of the coulomb meter and the excellent long-term stability of the voltage fuel gauge, while temperature compensation also provides industry-leading measurement accuracy. The device also integrates two high-voltage input LDOs, which can be flexibly programmed through the I2C interface. It should be noted that in order to accurately measure the battery power, it is necessary to model the battery and obtain the characteristic parameters of the battery. Maxim has a dedicated team to help with battery modeling. Of course, in the initial stage, you can also use the tools on Maxim's website to generate simple modeling parameters for test evaluation.

The headphone part uses the MAX17260. This is an ultra-low-power fuel gauge IC with an operating current as low as 5.1μA. It uses the Maxim ModelGauge™ m5 algorithm to monitor a single battery and supports high-side and low-side current detection. It is worth noting that the MAX17260 supports the ModelGauge m5 EZ algorithm, which does not require battery characterization, making it easy to implement fuel measurement and simplifying the host software. The algorithm has a high tolerance and supports a variety of lithium battery applications. The IC automatically compensates for battery aging, temperature, and discharge rate under a wide range of operating conditions without calibration, and provides an accurate state of charge (SOC) in percentage (%) and remaining capacity in milliampere hours (mAh). When the battery reaches a critical area close to empty power, the algorithm activates special error correction to further eliminate errors. The IC accurately estimates the remaining operating time and the time it takes for the battery to be fully charged, and provides three methods for reporting battery life: reduced capacity, increased battery resistance, and increased charging times.

Figure 6 MAX77818 block diagram

Figure 7 MAX17260 block diagram

Processor Unit

MAX32660 is an ultra-low power ARM M4 core MCU with a floating point unit (FPU). It comes with 256KB flash memory, 96KB RAM, 16KB instruction cache, 14 general-purpose I/O pins, and uses simple SWD programming. Its internal oscillator operates at a frequency of up to 96MHz and supports SPI, UART and I2C communications. It integrates an optimized power management unit with a standby current of only 2uA, which can maximize the battery life. The ultra-low standby power consumption, flexible power configuration, and ultra-small size (1.6mm x 1.6mm) of MAX32660 make it very suitable for wearable design applications, including wireless headphones, sports watches, bracelets, fitness equipment, handheld medical devices, and Internet of Things (IoT). In this design, MAX32660 is used to handle various software tasks. The software flow chart is shown in Figure 8. The flow chart consists of two parts: the charging box end and the headphone end.

The software flow of the charging box is as follows: First, after power-on, the GPIO port and various peripherals are initialized, including I2C, SPI, Timer1, and the interrupt vector is configured. Next, the peripheral devices are configured, including the fuel gauge chip MAX77818, the DC-DC power supply MAX77813, the PLC chip MAX20340, and the OLED display. The MAX77813 is used to power the MAX20340 and is directly configured as a 5V output (VOUT[6:0]=0x78). The MAX20340 is configured in host mode, and the PLC data format, PLC current and voltage, charging voltage threshold, LDO voltage difference, and interrupt enable are set. Because the reference design defaults to the charging box always working in standby mode, when there is an external USB power input, the charging box starts charging until the battery is fully charged. The OLED always displays the battery power. When a slave PLC is connected, the host immediately receives an interrupt signal and performs slave identification judgment. If the correct slave is detected, the PLC output is turned on to charge the slave and the slave is queried periodically. After receiving the correct feedback from the slave, the data sent by the slave (the power level of the earphones and the percentage of charging status) is read and displayed.