TWS Bluetooth Headset Charging Compartment Design Solution

TWS Bluetooth headset has been very popular, with the matching headset mother - the charging compartment is also very important. The charging compartment can manage the TWS headset's charging, display the power level, and transmit the SOC (state of charge of the battery) to the cell phone APP. The solution sleeps at a low current of 0.5µA, which meets the requirements of compact space of the storage box and low standby power consumption and other energy-saving requirements.

Solution features

The solution is based on a BP66FW1240 wireless charging receiver, particular Flash MCU, combined with low-power Bluetooth chip BC7161 and power chip HT7133-1, which can perform charging and discharging management for itself and the headset and comes with a software coulometer to calculate SOC (battery state of charge). At the same time, the SOC is transmitted to the cell phone APP through the BC7161 Bluetooth module.

The power display has an LED light display and Bluetooth APP display. LED light display shows the power by lighting the number of lights, 0~40% power on 1 light, 40~60% power on 2 lights, 60~80% power on 3 lights, 80~100% power on 4 mornings.

LED lights can show the power and through the flowing light effect to indicate the charging status of the charging bin. Bluetooth APP display requires the corresponding Bluetooth APP pre-installed on the cell phone, and the power data broadcast from the BC7161 module can be observed by Bluetooth connection through the APP when the charging bin lid is opened.

The basic characteristics of this program are as follows. 

• Operating voltage: DC3.8V (lithium battery powered) 
• Operating current: standby current 12µA
• Headset charging current: 340mA
• Temperature condition: -40℃~85℃ 
• Storage box charging current: 140mA
• LDO output voltage: 5V
• BLE operating frequency: 2426MHz
• BLE data rate: 1Mbps
• BLE transmit output power: -2dBm
• Maximum charging current: 600mA

Program Principle

 Key Chip

There are two ways to charge the internal battery by the main control module, one is directly through the USB-Type-C interface, and the other is through the wireless charging dock that complies with the QI protocol. The coil connected to the main control board converts the changing magnetic field into AC, and the MCU's built-in synchronous rectification circuit converts AC into DC. The rectified OVP voltage does not exceed 7V, followed by the built-in 5V 30mA LDO circuit. Both charging methods can supply power to the linear charging circuit and the MCU, and integrated linear charging can manage the battery charging.

The main component of the Bluetooth broadcast module is the BC7161 module, a fully integrated 2.4GHz transmitter consisting of a fractional N division synthesizer, programmable power amplifier (PA) and power management module. It communicates with the main control IC via I2C and broadcasts the coulomb value calculated by the main control IC when the lid is opened.

Coil Selection

Weighing cost and performance, select the appropriate Rx coil wire size. Large-diameter or double-stranded wire (two parallel wires) has high efficiency but a higher price. The physical coil of this program is shown in Figure 3, and the magnetic isolation material is placed below the coil with the following specific parameters.

• Inductance: 14µH.
• The number of turns: 14 turns, single-core wire.
• Size: 28.3mm long and 16.2mm wide.
• Wire diameter: 0.33mm.

First, 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 and C28 and parallel resonant capacitors C25 and C26, which can be simplified as shown in Figure 4 below. These two capacitors form a dual resonant circuit whose size and dimensions must be properly selected according to the Wireless Power Consortium (WPC) specification.

According to the WPC electrical specification requirements, the resonant frequency must be 100 kHz, and the dual resonant circuit C1 electrical capacity is calculated by the formula.

C1 = 1 / [(100kHz × 2π)2 × LS']

Where LS' is the mutual inductance value, the mutual inductance value can be obtained by placing the coil on a QI certified wireless charger and then measuring the inductance value. After the measurement, we get the mutual inductance value LS' is about 15.3µH.

After C1 is determined, calculate the dual resonant circuit C2, LS when. At this time, the secondary resonant frequency must be 1.0MHz, and the measurement yields a mutual inductance value LS of about 14µH.

C2 = 1 / [(1MHz × 2π)2 × ( LS - 1/C1)]

The next step is to determine the isolation material. The coil spacer material is a ferrite sheet with two main functions.

(1) To provide a low impedance path for the magnetic flux, reducing the leakage inductance phenomenon to improve efficiency.

(2) the use of fewer turns to achieve higher coil inductance so that it does not produce too high resistance to improve energy transfer efficiency.

PCB Wiring Matters

Where the top left is the front side of the mainboard PCB, and the top right is the reverse side of the mainboard PCB. The lower left is the front side of the Bluetooth module PCB, and the lower right is the reverse side of the PCB.

BOM analysis

This TWS headset charging bin solution BOM table consists of the main control board and the Bluetooth module, with 70 components.

Main control PCB BOM table components 42, including BP66FW1240 wireless charging receiving special Flash MCU, two P-channel power MOS tubes, a USB_TYPE_C interface connector, bipolar transistors and resistive components, etc.

Bluetooth module PCB BOM table components 28, including BC7161 low-power Bluetooth chip (U1), HT7133-1 power chip (U5), ETA1061 boost converter (U2), two N-channel MOS tubes, a 32MHz quartz crystal resonator, it should be noted that the corresponding matching capacitor should be selected 12pF NP0 class MLCC components.