BP45F0044 Wireless Electric Toothbrush Application Note

AN0569EN V1.00 1 / 8 November 23, 2020

BP45F0044 Wireless Electric Toothbrush Application Note

D/N: AN0569EN

Introduction

Wireless rechargeable toothbrushes are divided into two main parts, a wireless charging transmitter

(Tx) and a sonic electric toothbrush (Rx). Communication between these two parts is one-way from

the toothbrush to the charging base. Holtek provides the charging base communication function,

which can enable the charging base to receive toothbrush status, ID, toothbrush presence, etc. The

communication principle is based on AM load modulation. The toothbrush terminal uses a resistor

and a transistor to control the load to change the current. When the charging base receives these

changes in current, it can then implement communication using the integrated OCP, CMP and

demodulation algorithm after RC filtering.

This application note describes the transmitter communication demodulation and how to use the

Holtek Library to implement wireless electric toothbrush communication.

Functional Description

The charging base hardware design is shown in Figure 1. In the initial standby state, the charging

base will Ping transmitting power to the toothbrush at regular intervals. If the toothbrush receives

a Ping action, the charging base will demodulate the returned code. If the ID code is correct, the

charging base will then continuously transmit power to charge the toothbrush. During this period,

the toothbrush will send back its ID code to indicate the toothbrush presence at regular time

intervals. Holtek has integrated the parameters that users will encounter into the program and has

provided related subroutines for the user to call. Users can quickly adjust these parameters and

implement an ID identification function according to this application note.

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Figure 1. Charging Base Application Circuit

Operating Principles

1. Charging base coil:

Inductance: 32.01µH

Type: cylindrical, 8mm in height, 10mm in diameter, with magnetic column

2. Toothbrush coil:

Inductance: 53.59µH

Type: cylindrical, 9mm in height, 15mm in diameter, without magnetic column

3. Connection between the charging base and the toothbrush:

The distance between the charging base and the toothbrush is about 4mm. The coil type is

shown in Figure 2.

Figure 2. Charging Base Coil (Left) and Toothbrush Coil (Right)

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Hardware Description

The toothbrush terminal AM extracted load current circuit is shown in Figure 3.

The toothbrush uses a resistor and NPN transistor to control load changes. As the charging base

terminal will result in a current difference when the load changes, the charging base can implement

demodulation by monitoring this difference.

Figure 3. Toothbrush Extracted Load Current Circuit

The BP45F0044 device provides users with a complete charging base demodulation solution,

including an integrated OCP with programmable amplifier gain, adjustable DAC, comparator and

decoding debounce. The wireless charging transmission and demodulation functions can be

implemented using these internal hardware circuits together with the decoding algorithm provided

by Holtek. The demodulation block diagram of the BP45F0044 device is shown in Figure 4.

The OPA will be configured to operate in the non-inverted mode, where S0 and S3 are on and S1

and S2 are off. The sampling resistor signal R1 is connected to the OCPI pin, amplified up to 50

times using the OPA and then compared with the DAC value. The DAC value can be obtained by

directly calling the Holtek program subroutine "OCP_demo_trim" to complete the calibration. The

CMP output will be demodulated by the algorithm after the filter debounce.

TX decoding DAC

TX decoding debounce

Sampling amplifier gain adjustment Figure 4. Charging Base Decoding Block Diagram

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Software Description

The details for the customised communication packet format is described as follows.

Complete Packet Format

The complete packet format is composed of 5 bytes of data, including Preamble, 3 bytes of data

and checksum. Details are as follows:

Figure 5. Complete Packet Format Diagram

The user-defined data is c_ID_H, c_ID_L and c_Rx_STA, 3 bytes in total.

The complete packet format includes:

Preamble (11~25 bits, all =‘1’)

Toothbrush ID (3 bytes)

Checksum (1 byte)

Toothbrush ID Setting

Before the charging base implements demodulation, the Rx terminal should be prepared with

the corresponding settings. In the toothbrush terminal program provided by Holtek, 2 bytes of

data is used as the toothbrush ID code and 1 byte of data is used as the user-defined data. Users

can find the following parameters in the my_define.h file in the toothbrush program of the

attachment file.

c_ID_H: Toothbrush ID code - high byte

c_ID_L: Toothbrush ID code - low byte

c_Rx_STA: Toothbrush customised parameters

Toothbrush Sending Code Program

In addition to the toothbrush terminal ID setting, the subroutine "madulation_function()" should

be called in the program. By calling this program, the toothbrush can send the above-mentioned

three parameters of c_ID_H, c_ID_L and c_Rx_STA, which have already been called in the

complete toothbrush program provided by Holtek. Users can also modify the aforementioned

ID.

Charging Base Decoding OCP Setting

The sampling resistor should be selected to match the power, this resistance is usually between

0.1Ω~1Ω. A voltage signal will be generated according to the resistance value. After RC

filtering, a more useable voltage difference will be obtained using the OCP operational

amplifier which has a programmable gain. In this example, the sampling resistor R1 is 250mΩ,

the amplifier gain is 50 by default and the filter debounce time selects 64. See Figure 1.

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CH1:Waveform after sampling resistor filtering

CH2:Demodulated waveform

Figure 6. Charging Base Demodulation Waveform

As shown in Figure 6, after the filtering shown in CH1, the signal is composed of a sinusoidal

waveform with a small amplitude and many spikes. After the signal enters the OCP non-

inverted mode, the signal will be firstly amplified by the OPA to increase the sinusoidal

waveform amplitude. If the CMP compares the central position of the sinusoidal waveform, it

can then be correctly demodulated. However, the component errors in the coil or sampling

resistor must be taken into account. To resolve this, Holtek has devised a way to make the DAC

adaptive to ensure that for each demodulation, the DAC can output a central value of the

sinusoidal waveform, which the CMP can compare to allow the MCU to correctly demodulate

the waveform. The signal spikes are filtered by the debounce operation, the demodulated

waveform for this is shown as CH2.

Holtek has simplified the adaptive program to make the adjustment method easier to use. Users

only need to call "OCP_demo_trim" in the program to complete the calibration of the

comparator voltage for the sinusoidal waveform. If there is a deviation in the demodulation

waveform, the parameter "c_OCPDA_offset" can be trimmed.

The OCP_demo_trim operation is as follows:

1. When the PWM function is initiated, it needs to wait for some time. The wait time is about 200µs in this application.

2. CALL OCP_demo_trim.

3. If there is a deviation in the OCP_COUT waveform, this can be resolved by trimming the "c_OCPDA_offset".

Demodulation Program

The packet demodulation results of the charging base are obtained by a series of complex

algorithms. Holtek has shortened the user development time by calling the demodulation

subroutine. After users have debugged according to the aforementioned "Charging base

decoding OCP setting", the subroutine "Demodulation" is called every 50µs and the flag "

user_fg_demodu_ok" is checked to ensure that the ID code is correct. When the Demodulation

continues comparing until the c_ID_H and c_ID_L parameters are the same as the toothbrush

terminal setting, the flag user_fg_demodu_ok will be set high.

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Toothbrush ID Identification

The Holtek wireless rechargeable electric toothbrush solution provides users with a complete

and convenient ID identification function. At the same time, it can also adjust the PWM

operating time, Ping interval time, toothbrush not-present time and other parameter settings. It

also allows the toothbrush to have a customised 8-bit data capacity, where users can have data

transmission for information such as toothbrush status, battery voltage etc. The various

definable parameters of the charging base are explained as follows. Parameters Name Functional Description

c_ID_H User-defined ID code - high byte c_ID_L User-defined ID code - low byte c_PWM_ON_time PWM ping on time, units are 10ms c_ping_cycle PWM ping cycle (ON+OFF time), units are 10ms

c_time_out_after_success The time the toothbrush is not present in the charging base, the charging base will return to the ping state, units are 10ms

Subroutine Name Functional Description Change Flags and Parameters Flag Description

PWM_ping_cycle

The charging base transmits a Ping to check whether the toothbrush is present (call every 50µs)

fg_dac_cal

A DAC calibration will be performed before the charging base transmits power. It will then transmit a Ping to check whether the toothbrush is present. If the toothbrush is present, a DAC calibration will no longer be executed. 0: No calibration required 1. Calibration in progress

time_init Load the user-defined parameters

Load the user-defined parameters: c_PWM_ON_time, c_ping_cycle, c_time_out_after_success

—

OCP_demo_trim The CMP comparison voltage used for decoding is automatically calibrated

OCPDA OCP DAC comparison parameter

Demodulation

Demodulate program and check to see if the demodulation has been successful

user_fg_demodu_ok 0: Demodulation failed 1: Demodulation successful

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Charging Base Flow Chart

After adjusting the above parameters, the charging base subroutine call flow is shown in the

accompanying figure. This flow has been implemented in the program provided by Holtek,

which users can use directly without changing the flow.

Start

Initialization

PWM_ping_cycle

user_fg_demodu_ok

Rx full charge?

No Yes

Re-timing for 2.5sCharging Flag =1

Yes

Yes

No

OCP_demo_trim

Charging Flag =1?

LED on

PWM off0.3s ?

0s ?No

Yes Yes

LED off2.5s finish?YesNo

No

Demodulation

Figure 7. Charging Base Flow Chart

Conclusion

Through this application note, users can quickly debug the demodulation parameters in the program,

and obtain ID information and customised data. Users can choose an appropriate solution according

to their circuit requirements. For any problems encountered with parameter debugging and coil

selection, contact local agents for assistance.

Reference Material

Reference document: BP45F0044 Datasheet.

For more details consult the Holtek website at www.holtek.com.

BP45F0044 Wireless Electric Toothbrush Application Note

AN0569EN V1.00 8 / 8 November 23, 2020

Revision and Modification Information

Date Author Issue Release 2020.05.06 Minhsien, Zheng V1.00

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