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Capacitor IC design and capacitor buck schematic design

March 15, 2019

Capacitive touch button IC design method and precautions:

Capacitive touch button IC works: There is a sensing capacitor between any two conductive objects. A button, that is, a pad and the ground can also form a sensing capacitor. In the case where the surrounding environment does not change, the value of the sensing capacitor It is a fixed small value. When a human finger approaches the touch button, the parallel connection of the sensing capacitor formed by the human finger and the ground and the sensing capacitor formed by the ground increase the total inductance value. The capacitive touch button IC outputs a determination signal that a certain button is pressed after detecting that the value of the sensing capacitance of a certain button has changed. Because capacitive touch buttons have no mechanical construction, all detections are small changes in power, so they are much more sensitive to various disturbances. Touch button design, touch panel design, and touch IC selection are all critical. Here Huaqiang North IC Purchasing Network introduces the design method and precautions of the capacitive touch button IC.

First, touch button design 1. Touch PAD material

The touch PAD can be made of PCB copper foil, metal sheet, flat-top cylindrical spring, conductive cotton, conductive ink, conductive rubber, ITO layer of conductive glass, and the like. Regardless of the material used, the button sensor disc must be snugly attached to the panel with no air gaps in between.

When using a flat-top cylindrical spring, the diameter of the hollowed-out paving of the PCB at the touch line and the spring connection should be slightly larger than the diameter of the spring, ensuring that the spring will not touch the paving even if it is compressed onto the PCB.

2. Touch the PAD shape

In principle, it can be made into any shape with a hole or hollow in the middle. We recommend a rounded shape to avoid tip discharge effects. The general application of circles and squares is more common.

3. Touch the size of the PAD area

Key sensor panel size: minimum 4mm & TImes; 4mm, maximum 30mm & TImes; 30mm. The actual area size depends on the sensitivity requirements, and the area size is proportional to the sensitivity. In general, the diameter of the button sensing disk is greater than 4 times the thickness of the panel, and the size of the electrode is increased to improve the signal to noise ratio. The shape and area of each sensing disc should be the same to ensure consistent sensitivity. Usually, in most applications, 12mm & TImes; 12mm is a typical value.

4. Touch the distance between PADs

The distance between the individual touch PADs should be as large as possible (greater than 5 mm), which can reduce the mutual interference between the electric fields they form. When using PCB copper foil as the touch PAD, if the distance between the touch PADs is relatively close (5mm~10mm), the touch PAD must be isolated by paving, refer to Figure 1. If each touch PAD is far away, it should be isolated as much as possible. Appropriately widening the distance between each touch PAD will help to improve the touch sensitivity.

Second, the choice of touch panel 1. Touch panel material

The panel must be made of insulating material, which can be glass, polystyrene, polyvinyl chloride (PVC), nylon, plexiglass, etc. There must be no metal within 1mm above the button, and the metal within 50mm of the touch button must be grounded. Otherwise, the metal will affect The sensitivity of the case. In the production process, to maintain the material and thickness of the panel, the surface of the panel must be coated with an insulating coating.

2. Touch panel thickness

Usually the panel thickness is set between 0 and 10 mm. Different materials correspond to different typical thicknesses. For example, acrylic materials are generally disposed between 2 mm and 4 mm, and ordinary glass materials are generally disposed between 3 mm and 6 mm.

3. Double-sided tape

The touch button PCB and the touch panel are bonded by double-sided tape, and the thickness of the double-sided tape is preferably 0.1 to 0.15 mm, and 3M468MP is recommended, and the thickness thereof is 0.13 mm. There is no air required between the PCB and the panel because the dielectric constant of the air is 1, which is quite different from the dielectric coefficient of the panel. Air has a great influence on the sensitivity of the touch buttons. Therefore, double-sided adhesive and panel, double-sided adhesive and PCB bonding are the key processes in the production and assembly of touch buttons, and quality must be guaranteed.

PCB and double-panel bonding, PCB with double-sided tape and panel assembly must be assembled with positioning fixtures (http:// copyright), after assembly, manually or with a clamp. In order to ensure that there is no air between the PCB and the panel, it is necessary to open holes and exhaust slots on the double-sided board and to cooperate with the openings in the PCB. When designing the clamping fixture, focus on the part that touches the button to ensure that there is no air in the sensing area.

Third, the choice and application of capacitive touch button IC

At present, the world's leading electronic component suppliers have increased the application research of capacitive touch button ICs, and introduced a number of professional chips (herein referred to as touch chips), there are also many ICs based on MCU integration class, designers choose space Larger, you can choose the IC that suits your design needs according to the needs of the function and the cost performance of the chip. You can also design the touch IC based on the A/D port of the MCU. This article selects the 12-key capacitive touch sensor SC12A with self-correction function. .

The SC12A is a self-correcting capacitive touch sensor that detects if 12 sensors are touched. It senses capacitance changes from any non-conductive medium such as glass and plastic. This capacitive sensing switch can be used in many electronic products.

1. Features

12 fully independent touch-sensitive buttons; maintain automatic correction without external intervention; key output is fully debounced; multi-interface - I2C serial interface / BCD code output; all buttons share a sensitivity capacitor.

2. Pin
At present, the world's leading electronic component suppliers have increased the application research of capacitive touch button ICs, and introduced a number of professional chips (herein referred to as touch chips), there are also many ICs based on MCU integration class, designers choose space Larger, you can choose the IC that suits your design needs according to the needs of the function and the cost performance of the chip. You can also design the touch IC based on the A/D port of the MCU. This article selects the 12-key capacitive touch sensor SC12A with self-correction function. .
3. Chip function

(1) Initialization time: After power-on reset, the chip needs 300ms to initialize, calculate the ambient capacitance of the sensing pin, and then work normally.

(2) Sensitivity: determined by the capacitance value of the CDC port. The smaller the value, the higher the sensitivity.

(3) Self-correction: According to the drift of external environment temperature and humidity, the chip will always adjust the capacitance reference value of each button. From the detection of the button, the chip will stop correcting for a period of time, which is about 15 to 50 seconds. Then the chip will continue to self-calibration, which means that the detection button will not take more than 15 to 50 seconds.

(4) Touch reaction time: Each channel is sampled approximately every 12.5 ms. After the button debounce processing, the reaction time for detecting the button press is about 68 milliseconds, and the reaction time for detecting the button leaving is about 44 milliseconds. So the fastest frequency to detect a button is about 9 times per second.

(5) BCD code output: SC12A can detect multiple buttons at the same time. However, if you use the BCD code output, you cannot output multiple button values at the same time. When there is no button, the BCD[3:0] output is F. The button priority is reduced from CIN0 to CIN11 in order.

4. Application circuit (as shown in Figure 1)

At present, the world's leading electronic component suppliers have increased the application research of capacitive touch button ICs, and introduced a number of professional chips (herein referred to as touch chips), there are also many ICs based on MCU integration class, designers choose space Larger, you can choose the IC that suits your design needs according to the needs of the function and the cost performance of the chip. You can also design the touch IC based on the A/D port of the MCU. This article selects the 12-key capacitive touch sensor SC12A with self-correction function. .

Capacitor step-down principle

The working principle of the capacitor step-down is to limit the maximum operating current by using the capacitive reactance generated by the capacitor at a certain AC signal frequency. For example, at a power frequency of 50 Hz, a 1 uF capacitor produces a capacitive reactance of approximately 3180 ohms.

When an AC voltage of 220V is applied across the capacitor, the maximum current flowing through the capacitor is approximately 70 mA. Although the current flowing through the capacitor is 70 mA, there is no power consumption on the capacitor. If the capacitor is an ideal capacitor, the current flowing through the capacitor is the imaginary current, and the work done is reactive power.

According to this feature, if we connect a resistive component in series with a 1uF capacitor, the voltage obtained across the resistive component and the power dissipation it generates are completely dependent on the characteristics of the resistive component.

For example, we connect a 110V/8W bulb in series with a 1uF capacitor. When connected to an AC voltage of 220V/50Hz, the bulb is illuminated and emits normal brightness without being burned. Because the 110V/8W bulb requires 8W/110V=72mA, it matches the current limiting characteristics of the 1uF capacitor.

Similarly, we can also connect a 5W/65V bulb and a 1uF capacitor in series to 220V/50Hz AC. The bulb will also be lit without being burned. Because the 5W/65V bulb also has an operating current of about 70mA.

Therefore, the capacitor buck is actually using the capacitive reactance current limit. The capacitor actually acts as a limiting current and dynamically distributing the voltage across the capacitor and the load.

The following figure shows the typical application of RC capacitor. C1 is the step-down capacitor. R1 is the bleeder resistance of C1 when the power is off. D1 is the half-wave rectifier diode. D2 provides the discharge circuit for C1 in the negative half of the mains. Otherwise, When capacitor C1 is fully charged, it will not work. Z1 is a Zener diode and C2 is a filter capacitor. The output is a stable voltage value of Zener diode Z1.

At present, the world's leading electronic component suppliers have increased the application research of capacitive touch button ICs, and introduced a number of professional chips (herein referred to as touch chips), there are also many ICs based on MCU integration class, designers choose space Larger, you can choose the IC that suits your design needs according to the needs of the function and the cost performance of the chip. You can also design the touch IC based on the A/D port of the MCU. This article selects the 12-key capacitive touch sensor SC12A with self-correction function. .

In practical applications, the following figure can be used instead of the above figure. Here, the Z1 forward characteristic and the reverse characteristic are used, and the reverse characteristic (that is, its voltage regulation characteristic) is used to stabilize the voltage, and the forward characteristic is used in the commercial power. A negative half cycle provides a discharge loop for C1.

At present, the world's leading electronic component suppliers have increased the application research of capacitive touch button ICs, and introduced a number of professional chips (herein referred to as touch chips), there are also many ICs based on MCU integration class, designers choose space Larger, you can choose the IC that suits your design needs according to the needs of the function and the cost performance of the chip. You can also design the touch IC based on the A/D port of the MCU. This article selects the 12-key capacitive touch sensor SC12A with self-correction function. .

In larger current applications, full-wave rectification can be used, as shown below:

At present, the world's leading electronic component suppliers have increased the application research of capacitive touch button ICs, and introduced a number of professional chips (herein referred to as touch chips), there are also many ICs based on MCU integration class, designers choose space Larger, you can choose the IC that suits your design needs according to the needs of the function and the cost performance of the chip. You can also design the touch IC based on the A/D port of the MCU. This article selects the 12-key capacitive touch sensor SC12A with self-correction function. .

At small voltage full-wave rectified outputs, the maximum output current is:

Capacitance: Xc=1/(2πfC)

Current: Ic = U/Xc=2πfCU

Pay attention to the following points when using capacitor step-down:

1. Select the appropriate capacitor according to the current of the load and the operating frequency of the AC, not the voltage and power of the load.

2, the current limiting capacitor must use non-polar capacitors, absolutely can not use electrolytic capacitors. Moreover, the withstand voltage of the capacitor must be above 400V. The most ideal capacitor is an iron-shell oil-immersed capacitor.

3. Capacitor buck cannot be used for high power conditions because it is not safe.

4. Capacitor step-down is not suitable for dynamic load conditions.

5. Similarly, the capacitor step-down is not suitable for capacitive and inductive loads.

6. When DC operation is required, half-wave rectification should be used as much as possible. Bridge rectification is not recommended. And to meet the conditions of a constant load.

Device selection

1. When designing the circuit, first determine the exact value of the load current, then refer to the example to select the capacity of the step-down capacitor. Since the current Io supplied to the load through the step-down capacitor C1 is actually the charge and discharge current Ic flowing through C1. The larger the capacity of C1 is, the smaller the capacitive reactance Xc is, and the larger the charging and discharging current flowing through C1 is. When the load current Io is less than the charge and discharge current of C1, the excess current will flow through the Zener diode. If the maximum allowable current Idmax of the Zener diode is less than Ic-Io, the regulator tube will burn out.

2. To ensure reliable operation of C1, its withstand voltage should be greater than twice the supply voltage.

3. The selection of the bleeder resistor R1 must ensure that the charge on C1 is released during the required time.

Design example

It is known that C1 is 0.33μF and the AC input is 220V/50Hz. The maximum current that the circuit can supply to the load is obtained. The capacitive reactance Xc of C1 in the circuit is:

Xc=1 /(2 πf C)= 1/(2*3.14*50*0.33*10-6)= 9.65K

The charging current (Ic) flowing through capacitor C1 is:

Ic = U / Xc = 220 / 9.65 = 22mA.

Generally, the relationship between the capacity C of the step-down capacitor C1 and the load current Io can be approximated as follows: C = 14.5 I, wherein the capacity unit of C is μF, and the unit of Io is A. Capacitor buck power supply is a non-isolated power supply. In application, special attention should be paid to isolation to prevent electric shock.


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