HOLTEK WAS-1971EN TWS Earbuds Wireless Charging Case User Manual

**HOLTEK WAS-1971EN TWS Earbuds Wireless Charging Case User Manual

**

Introduction

TWS Bluetooth earbuds have become extremely popular audio products in recent years. Every TWS Bluetooth earbuds can be equipped with a matching charging case that provides a power supply and storage function.

The Holtek TWS earbuds wireless charging case solution contains a combination of Holtek devices. These are a BP66FW1242 wireless charging receiver dedicated Flash MCU and a 5V boost circuit. This combination of devices can implement internal battery charging management. The BP66FW1242 includes an integrated linear charging circuit, this circuit current can be programmed and the device has a temperature management function. In addition, the device can be used together with a boost circuit which can boost the battery voltage to 5V to charge the earbuds

Figure 1. System Block Diagram

Application Area
Electric toothbrushes, electronic watches, earbuds charging cases, etc.

Solution Features

1. High level of functional integration: includes a full range of functions such as an integrated full-bridge synchronous rectifier, a communication modulator, a linear charger and an LDO. The BP66FW1242 contains an integrated high efficiency synchronous rectifier circuit, a linear charging circuit, an LDO and a communication modulator, which provides the circuits required to implement a wireless charging receiver. It is very suitable for small size wireless charging receiver devices.
2. Short development cycle: fully compatible with Qi specifications, can achieve a software coulombmeter. As the solution is fully compatible with Qi specifications, it has enhanced product versatility. The software coulombmeter function can be realized by a high resolution current sampling. The charging and discharging current data is sampled using a 12-bit A/D converter to calculate the current battery capacity. The coulombmeter parameters can be transmitted to the mobile phone via upgrade the bluetooth function.

Operating Principles

Both wireless charging and charging management form the main subjects of this solution. Their operating principles will be introduced below.

Wireless Charging
The wireless charger uses electromagnetic induction principles to charge using a method similar to a transformer.

There is a coil at both the transmitter and the receiver. The transmitter coil is connected to an transmit controller which generates an AC voltage which forces the coil generate a fluctuating magnetic field. The receiver coil detects this transmitter fluctuating magnetic field and then generates its own AC voltage.

Figure 2. Wireless Charging Principle Diagram

Charging Management

The BP66FW1242 has an integrated linear battery charger for charging management. There are three charge control modes for the linear charger, which are the trickle mode, constant current charge mode and constant voltage charge mode. The required charging voltage and current for these modes is different according to different user designs. These charge modes are described below.

Trickle Mode: this mode is used for completely discharged or batteries which have a battery voltage of less than 3V. This mode is the first charging stage. When the voltage is less than 2.5V, the battery will be pre-charged using a typical current value of 8.3mA. When the voltage is greater than 2.5V, the battery will be pre-charged using a typical current value of 1/12×IBAT_CC.

Constant Current Charge Mode: when the battery voltage is equal to or larger than 3V and less than 4.2V, the charging current will then switch to a typical constant current during this second charging stage. The constant charging current is controlled by the internal hardware of the MCU. The constant charging current can be changed through a program, and is within the range of 100mA to 1000mA.

Constant Voltage Charge Mode: once the battery voltage has reached 4.2V, it will be charged using a constant voltage during this third stage. The charging voltage should be fixed at 4.2V with a tolerance within 1%. The charging current gradually decreases as the constant voltage charging time increases. Typically, the constant voltage charging stage is completed when the charging current reduces to a value of less than 1/12×IBAT_CC. At this time the battery is full and stops charging. After fully charged, the battery voltage will continue to be monitored. When the battery voltage is equal to or less than 4.0V, the battery will be recharged and enter the next charging cycle

Figure 3. Lithium Battery Charging Curve

Functional Description

Solution Features

• Operating voltage : DC3.7V- powered by a lithium battery
• Operating current: standby current – 6µA
• Charging case charging current: 150mA (programmable current)
• LDO output voltage: 5V

Solution Functions
The product hardware is shown in **Figure 4.

Capacity Display**
The TWS earbuds wireless charging case displays the battery capacity via the number of LEDs which are on as the following table shows:

Table 1

The LED can not only display the battery capacity but also shows the charging case charging state using a water flow lighting effect

Solution Design Description

This solution uses the BP66FW1242 as a master MCU, which provides an internal 4KW Program Memory for storage. There are also 18 directional I/O ports and multiple timer modules. The wireless charging receiver includes a high efficiency synchronous rectifier circuit, a linear charging function, an LDO and a communication modulator.

The complete system will be introduced in the following section.

**Hardware Description

Figure 5. Application Circuit**

The solution charges the internal battery in two ways. One is directly charging via a USB-Type-C interface, the other is charging via a wireless charging base compliant with the Qi specification. The wireless charging receiver circuit connected to the master board can convert a changing magnetic field into an AC voltage. The MCU integrated synchronous rectifier circuit can then convert this AC voltage into a DC voltage, which is provided to the LDO and the linear battery charger. Both charging methods can provide the power to the linear battery charger and the MCU. The integrated linear charging circuit is used for battery charging management.

The master MCU also controls two earbuds charging contacts, CHL+ and CHR+, by controlling a DC/DC boost module for implementing 5V discharging output. The U3 Hall element is used to determine the charging case open or closed state. If it is opened, four LEDs will display the charging state and the SOC state.

Coil Selection Description
A balance between cost and performance is required to select the corresponding Rx coil wire specification. Large-diameter wires or double-stranded wires will have a higher efficiency but will have a higher cost. The coil parameters for this solution are shown as follows. The actual coil is shown in the figure below where some magnetic insulation material is placed under the coil.

**Figure 6

Inductance: 14µH.
Coil wire : 14 turns of single-core cable.
Dimensions: 28.3mm in length, 16.2mm in width.
Wire diameter** : 0.33mm.

• LC Network Matching
  In this solution, the Rx coil circuit is composed of resonant capacitors C10/C14 in series with it and C8/C9 in parallel with it. This can be simplified to the circuit diagram shown in Figure
  7. These two capacitors form a double resonant circuit, whose size must be correctly selected according to the WPC (Wireless Power Consortium) specification

Figure 7

According to the WPC electrical specification requirement, the resonant frequency should be 100kHz.

The capacitance C1 calculation formula in a double resonant circuit is as follows: When calculating C1, LS’
??1 = ____1_____
(100kHz × 2π)2 × ????’

LS’ is a mutual inductance whose value can be measured by placing the coil on the wireless
charger compatible with the Qi specification. The mutual inductance LS’ is measured to be
about 15.3µH.
For the C2 capacitance calculation formula in the double resonant circuit: when calculating C2
and LS’, the secondary resonance frequency should be 1.0MHz and the mutual inductance value
LS’ is measured to be about 14µH. This is first required to determine the C1 value before the
calculation is done.

??2 = __1_____
(1MHz × 2π)2× ?S- 1
C1

Magnetic Shielding Materials

The coil magnetic shielding material is a ferrite sheet which has two main functions:

1. Provides a low impedance path for the magnetic flux to reduce leakage inductance and improve efficiency.
2. Requires fewer turns to achieve higher inductance value coils. This results in lower resistance which improves the energy transmission efficiency.

Layout and Hardware Considerations

Figures 8 and 9 show the front and back view of the master board PCB layout.

Figures 10 and 11 show the front and back view of the boost module PCB layout. Considerations:

1. The filter capacitor next to the MCU pin should be placed as close as possible to the pin (A/D sampling pin, power input/output pin).
2. The power loop circuit (LC resonant circuit, Type-C power input and 5V boost circuit) needs enough routing width to carry the large current

PCB BOM Table

Table 2 Master Board BOM Table

Table 3 Boost Module BOM Table

Software Description

Main Program

Figure 12. Software Main Flow Chart

As shown in Figure 12, after power on initialisation, the program will first initialise the user-related data. In the main loop, the program will determine whether a wireless charger has been accessed.

If yes, the program will then prepare to send code and then determine whether a Type-C interface has been connected. If yes, then skip the code sending program and 1ms timing subroutine and execute the 250ms timing subroutine.

Among these routines, the 1ms timing subroutine is mainly used for wireless charging access detection. The 250ms timing subroutine is used for lighting effect processing etc. Refer to the following section for details.

Code Sending Program

**Figure 13

** Signal strength = _Voltage value after rectification____× 256
(_Targetmaximum voltage valueX5X4096) 10                                              Reference voltage

The voltage value after rectification refers to the voltage VCC value. The target maximum voltage value refers to the maximum voltage value after rectification.

When Phase=3, send an identification packet to verify that the wireless device has been accessed correctly.

When Phase=4, send a configuration packet.

When Phase=5, calculate the CE value which is the required power

CE =( __?cc Target value__− 1) 128
____ADC measured valuex ?DC Reference voltage × 5
4096

During charge case charging, the charge voltage is usually expected to be kept at a certain value and the target VCC value is the charge voltage value. The CE value ranges from +127 to -128 = 100% to -100%, which can be simplified as CE=(V(target value)/V(now)-1)×128

When Phase=6, calculate the received power

To calculate the received power which includes the power consumed by the load and Rx, this solution only calculates the lithium battery charge current and the MCU operating power consumption. The simple calculation formula is as follows. It needs to be adjusted according to the actual situation due to errors.

Power =(Charging current×ADC reference voltage + System current consumption)x ADC measured value×ADC reference value x5
4096
___X 128
5W

The charging current refers to the lithium battery charging current sampled on PA7. The current sampling resistor, R13, has a value of 2R2. The current used by the systemis the charging case current consumption sampled on PB4, and the ADC measured value is the VCC voltage sampled on PB1.

1 ms Subroutine
**Figure 14

** As shown in Figure 14, the 1ms timing subroutine is used to detect whether the TX is transmitting energy. If the detected received voltage is normal (greater than 3.3V), it represents that the TX is transmitting energy to the RX, and the RX enters the charging mode. If the voltage received during the communication is abnormal (less than 3.3V), the TX removal detection will be executed. If a time-out occurs and the voltage is not recovered, this indicates that the TX has been removed and the charging is completed.

**250ms Subroutine
Figure 15

**As shown in Figure 15, the 250ms timing subroutine will process the illumination effects for various situations, including the water flow effect during charging and the capacity display when the cover is open etc. The Type-C charging processing is mainly used for Type-C access debounce and will determine whether to charge or not. The sleep state are also processed here

Test Data

Test project – Current setting linearity.
Table 4

Solution Comparison

Table 5

| Holtek Solution| Traditional Solution
---|---|---
Function| An integrated linear charging single chip solution, supports Qi specification wireless charging receiver function| MCU + wireless charging IC, multi-chip solution
Performance| Standby current as low as 6µA| Standby current is more than 20µA
Cost| The MCU includes an integrated high efficiency synchronous rectifier, an LDO, a linear charging and a communication modulator.| Requires an external rectifier bridge circuit, an LDO, linear charging, a communication modulator and other components, high cost

Conclusion

This application note has used the BP66FW1242 as a master MCU, combined with a 5V boost circuit and has introduced the TWS earbuds wireless charging case solution in detail. The BP66FW1242 provides an integrated high efficiency synchronous rectifier circuit, a linear charging circuit, an LDO and a modulator. Its current can be as low as 0.5µA in the Sleep mode which meets the small size and the low standby current requirements of TWS earbuds wireless charging case.

Reference File

Reference file: Qi specification 1.2.4, BP66FW1242 Datasheet.
For more information, consult the Holtek official website: www.holtek.com.

Revision and Modification Information

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References

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