SNOSCZ7A December 2015 – January 2016 LDC0851
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The LDC0851 relies on two externally placed sensors (LSENSE and LREF) and a capacitor (CSENSOR) for proper operation. The design and matching of the coils is very critical to ensure a proper switching occurrence. It is also important to note that the parasitic capacitance of the board (CBOARD) and of the LCOM input pin (CIN_COM) are in parallel with CSENSOR, and the sum of all three capacitances create a total capacitance (CTOTAL) which is considered part of the system. CTOTAL must be greater than 33 pF to be considered in the valid design space.
The sensor frequency is calculated with the following equation.
The following procedure should be followed for determining the sensor characteristics:
The LDC0851 can be used for event counting applications such gear tooth detection or rotational speed measurements. An example of gear tooth detection using side by side coils is shown below where the gear is made of a conductive material and rotates over the coils. Two identical coils can be placed such that when one of the coils is covered by a gear tooth, the other is uncovered. The output will toggle when the inductance values of both coils are equal as the gear passes by.
Assume a gear with 8 conductive teeth is used in a system to determine flow rate. Determine the maximum speed that can be reliably detected by the LDC0851 using a sensor frequency of 15 MHz.
To ensure a reliable reading an event must be sampled twice when the gear tooth covers more of the LSENSE coil than the LREF coil (LS<LR) producing a LOW output and equivalently twice when the gear tooth covers more of LREF than LSENSE (LR<LS) producing a HIGH output. The maximum speed can be achieved when the output toggles at a duty cycle rate of 50%. This can be achieved by using a gear where the width of each tooth is the same as the width of the gaps between the teeth. For asymmetric systems, the minimum width of either the gap or the gear tooth determines the maximum detectable speed. For symmetrical systems, the maximum rotational speed that can be reliably detected in revolutions per minute (rpm) for a given number of gear teeth can be determined by the following formula:
A gear with 8 teeth and sensor frequency of 15MHz could reliably measure a gear rotational speed of 6500 rpm.
The metal coverage has an inverse relationship to coil inductance. Figure 27 shows the relationship between the output of the LDC0851 and relative inductance of the coils as the gear is rotating.
The LDC0851 may be used for coarse proximity sensing such as a push button application. A conductive target may be added to the underside of a mechanical push button as shown below.
A push button that is made of flexible material has a conductive target attached to the underside and a contactless solution using the LDC0851 is required for reliability purposes. Determine the coil characteristics as well as the threshold adjust setting if the following conditions are true:
To conserve PCB area, a 4 layer stacked coil approach is used with the sense coil on the top 2 layers and reference on the bottom 2 layers. The LDC0851 switching threshold is then determined by following parameters:
The coil diameter should not exceed the diameter of the conductive target of 10mm in order to keep the target-to-coil coverage ≥100%. Additionally, in order to detect the lightest button pushes where the conductive target rests at a height of 2.5 mm, the coil should be at least 3 times greater giving a minimum size of 7.5 mm. The user may therefore select a coil size between 7.5 mm and 10 mm. A coil diameter of 10mm is chosen for the most flexibility and tuning range. The response versus ADJ code is shown below in Figure 29.
In this example the deflection caused by the button press (∆d) is 2mm. Note that the ∆d must be enough to cross the “Switch ON” threshold and return past “Switch OFF” threshold of the LDC0851 for a given ADJ code to be considered a valid code. Codes 0 through 6 should not be used because the conductive target has already crossed the "Switch ON" thresholds and would always be in the ON state without a button push. Similarly code 15 should not be used because the output would always be in an OFF state regardless of how hard the button is pushed. Therefore codes 8 through 14 are clearly inside the travel distance of the button. Select code 8 to detect light button pushes, code 11 for medium button pushes, or code 14 to only detect strong button pushes. Once the ADJ code is selected based on user preference, set the resistor divider R1 and R2 values according to section Setting the Threshold Adjust Values.
It may be desirable to operate the LDC0851 on battery power and take samples at a very low sample rate, such as portable sensor devices or intruder detection systems. By using a nanotimer (ultra low power timer) such as the TPL5110 or a microcontroller such as the MSP430F5500 it is possible to duty cycle the EN pin of the LDC0851 as shown in the application schematic in Figure 30.
The LDC0851 is used in a low power, battery operated system to detect when a window is opened. Determine the average supply current of the LDC0851 if following requirements exist:
In order to achieve 10 year lifetime out of a single CR2032 battery, the enable pin (EN) of the LDC0851 can be duty cycled to achieve a low average supply current. Refer to Figure 31 to see the three different states of LDC0851 supply current during duty cycle operation. The sum of the Standby, Ramp, and On currents can be used to calculate the average supply current of the LDC0851, which needs to be below 2.5 µA to achieve a 10 year lifetime from a 220 mAh CR2032 battery.
The average supply current can be calculated in the following steps:
For example, using a sensor frequency of 10 MHz, sensor inductance of 10 µH, and 1 sample per second yields a lifetime of 13.5 years for a single CR2032 battery.