TWS Bluetooth earbuds have always been popular, and the matching earbud case – the charging case – is also very important. The charging case can manage and charge the TWS earbuds and itself, display the battery level, and transmit the state of charge (SOC) to the mobile app. The design of the charging case for these Bluetooth earbuds has a low standby current of as little as 0.5µA, which meets the requirements for small storage space and low standby power consumption.
Solution Features:
This solution is based on the BP66FW1240 wireless charging receiver dedicated Flash MCU, combined with the low-power Bluetooth chip BC7161 and power chip HT7133-1, which can manage the charging and discharging of both the charging case and earphones. The built-in Coulomb counter software library can calculate SOC (state of charge) of the battery, and the SOC data can be transmitted to the smartphone app via the BC7161 Bluetooth module.
There are two ways to display the battery level in this solution: LED light display and Bluetooth app display. The LED light display shows the battery level by the number of lights lit. For example, 1 light indicates 0-40% battery level, 2 lights indicate 40-60% battery level, 3 lights indicate 60-80% battery level, and 4 lights indicate 80-100% battery level.
In addition to displaying the battery level, the LED lights can also indicate the charging status of the charging case through a flowing light effect. To use the Bluetooth app display, the corresponding app needs to be pre-installed on the smartphone. When the charging case cover is opened, the Bluetooth connection can be established through the app, and the SOC data broadcast by the BC7161 module can be observed.
Basic features of this solution are as follows:
- Operating voltage: DC3.8V (powered by lithium battery)
- Operating current: standby current 12µA, earphone charging current 340mA
- Temperature conditions: -40°C to 85°C
- Charging case charging current: 140mA
- LDO output voltage: 5V
- BLE operating frequency: 2426MHz
- BLE data rate: 1Mbps
- BLE transmission output power: -2dBm
- Maximum charging current: 600mA
The principle of this scheme:
This scheme is mainly composed of a main control module and a Bluetooth broadcasting module. The main control chip BP66FW1240 has 4K ROM program storage space, 20 bidirectional I/O interfaces, and multiple timer modules for users to use. In addition, it also has an I2C serial interface module for data communication. The wireless charging receiver part is built-in with a high-efficiency synchronous rectification circuit, linear charging function, LDO, and communication modulation function.
1. Key Chips
The main control module has two ways to charge the internal battery: one is to charge directly through the USB-Type-C interface, and the other is to charge through a wireless charging base that complies with the QI protocol. The coil connected to the main control board can convert the changing magnetic field into AC current. The built-in synchronous rectification circuit of the MCU converts the AC current into DC current. After rectification, the OVP voltage does not exceed 7V, and then through the built-in 5V 30mA LDO circuit. Both charging methods can provide power to the linear charging circuit and MCU. The integrated linear charging can manage the battery charging.
The main control IC also controls the CH+ and CH- (headphone charging contacts) through the control of the DC/DC boost circuit, and ETA1061 is the boost converter. The charging and discharging current data is collected by 12-bit A/D, and the current battery capacity is calculated. The U2 Hall element determines the state of the charging case when the cover is opened, and when the cover is opened, the four LED lights display the charging status and SOC status.
The main component of the Bluetooth broadcasting module is the BC7161 module, which is a fully integrated 2.4GHz transmitter composed of a fractional N frequency synthesizer, a programmable power amplifier (PA), and a power management module. It communicates with the main control IC through I2C, and broadcasts the Coulomb value calculated by the main control IC when the cover is opened.
2. Coil Selection
To balance cost and performance, the appropriate Rx coil wire specifications are selected. Large-diameter wires or twin-wire (two parallel wires) have high efficiency but higher prices. The physical Rx coil used in this design is shown in Figure 3, with a magnetic material placed under the coil.
The specific parameters are as follows:
Inductance: 14µH;
Turns: 14 turns, single-core wire;
Size: 28.3mm in length, 16.2mm in width;
Wire diameter: 0.33mm.
Firstly, it is necessary to determine the LC network matching and calculate the relevant parameters. The Rx coil network in the scheme consists of series resonant capacitors C27, C28 and parallel resonant capacitors C25, C26, which can be simplified as shown in Figure 4. These two capacitors form a double resonant circuit, and their size must be correctly selected according to the Wireless Power Consortium (WPC) specification.
According to the WPC electrical specifications, the resonant frequency must be 100kHz, and the capacitance calculation formula of the double resonant circuit C1 is:
C1 = 1 / [(100kHz × 2π)2×LS’]
Where LS’ is the mutual inductance value. By placing the coil on a wireless charging pad that complies with the QI certification and measuring the mutual inductance value, LS’ can be obtained. After measurement, we obtained the mutual inductance value LS’ of about 15.3µH.
After C1 is determined, calculate C2 and LS of the double resonant circuit. At this time, the secondary resonance frequency must be 1.0MHz, and the mutual inductance value LS is about 14µH.
C2 = 1 / [(1MHz × 2π)2 × ( LS – 1/C1)]
The next step is to determine the magnetic isolation material. The coil isolation material is a ferrite sheet, which has two main functions.
(1) To provide a low-impedance path for magnetic flux, reduce the leakage phenomenon, and improve efficiency.
(2) To use fewer turns to achieve higher inductance of the coil, so that it will not produce too high resistance and improve energy transfer efficiency.
3. PCB Wiring Considerations
Below figure shows the PCB layout for the main board (top) and Bluetooth module (bottom).
The upper left shows the front side of the mainboard PCB, and the upper right shows the back side of the mainboard PCB. The lower left shows the front side of the Bluetooth module PCB, and the lower right shows the back side of the Bluetooth module PCB.
4. BOM Analysis
The BOM table of this TWS earphone charging case solution consists of two parts: the main control board and the Bluetooth module, with a total of 70 components.
The BOM table of the mainboard consists of 42 components, including BP66FW1240 wireless charging receiver dedicated flash MCU, two P-channel power MOSFETs, a USB_TYPE_C interface connector, as well as bipolar transistors and resistive and capacitive components.
The BOM table of the Bluetooth module consists of 28 components, including the BC7161 low-power Bluetooth chip (U1), HT7133-1 power chip (U5), ETA1061 boost converter (U2), two N-channel MOSFETs, a 32MHz quartz crystal resonator, and it is worth noting that the matching capacitors should be 12pF NP0 class MLCC components.
If you have a similar earbud charging case project, please don’t hesitate to contact SuperPCBA. With over 15 years of design and manufacturing experiences, our factory will definitely assist you in achieving your project goals quickly and cost-effectively, meeting your expectations.
