SNOSCZ7A December 2015 – January 2016 LDC0851
The LDC0851 is an inductance comparator with push/pull output. It utilizes a sensing coil and a reference coil to determine the relative inductance in a system. The push/pull output (OUT) switches low when the sense inductance drops below the reference and returns high when the reference inductance is higher than the sense inductance. Matching the sense and reference coils is important to maintain a consistent switching distance over temperature and to compensate for other environmental factors. The LDC0851 features internal hysteresis to prevent false switching due to noise or mechanical vibration at the switching threshold. The switching threshold is set by the sensor characteristics and proximity to conductive objects, which is considered Basic Operation Mode described further in section Basic Operation Mode. The LDC0851 also features a Threshold Adjust Mode where an offset is subtracted from the reference inductance to change the effective switching point as described in section Threshold Adjust Mode.
The sensing coil is connected across the LSENSE and LCOM pins and the reference coil is connected across the LREF and LCOM pins. A sensor capacitor is connected from LCOM to GND to set the sensor oscillation frequency. The sensor capacitor is common to both LSENSE and LREF making the inductance measurement differential.
The LDC0851 is configured for Basic Operation mode when the ADJ pin is tied to ground. Two identical coils should be used for LSENSE and LREF. The switching point occurs when the inductances of both coils are equal. Basic Operation mode can be used for a wide variety of applications including event counting or proximity sensing. An example showing gear tooth counting can be found in section Event Counting.
For proximity sensing the switching point can be set by placing a conductive target at a fixed distance from the reference coil as shown in Figure 17. The output will switch when a conductive target approaches LSENSE and reaches the same distance set by the fixed reference target. For reliable and repeatable switching it is recommended to place the reference target at a distance less than 40% of the coil diameter from the reference coil.
In some systems adding a reference target at a fixed height to set the switching distance is not feasible. Therefore to set the switching distance a small amount of mismatch between the sense and reference coils can be introduced. To achieve the maximum switching distance the reference inductance should be approximately 0.4% less than the sense inductance as shown in Figure 18 below. The 0.4% mismatch will ensure that the output will switch off when the target is removed.
In Threshold Adjust mode, an offset inductance is subtracted from LREF to alter the switching threshold without the use of a reference target. In order to configure the LDC0851 for Threshold Adjust mode, place a resistor divider between VDD and GND as shown in Figure 19. The threshold adjust values can then be easily changed as described in section Setting the Threshold Adjust Values. Threshold adjust mode can be used in a variety of applications including coarse proximity sensing and simple button applications as shown in Coarse Position Sensing. Two example coil configurations for proximity sensing are shown below for side by side coil orientation in Figure 19 as well as stacked configuration in Figure 20.
Stacked coils can be utilized in designs where PCB space is a concern or if the user only wants to detect proximity to metal from one side of the PCB such as a button application. The sensing range is slightly reduced due to the fact that both the sense and the reference coil are affected by same conductive target, however since the sense coil is closer to the target its respective inductance decreases more than the reference inductance allowing the output to switch as shown in Figure 20.
To get the most sensing range with stacked coils the spacing between the sensing coil and reference coil (height = h) should be maximized as shown in Figure 21. See section Stacked Coils for more information on the layout of stacked coils.
To configure a threshold setting, connect a 49.9 kΩ resistor (R1) between VDD and the ADJ pin as shown in Figure 20. The threshold is determined by the value of R2 as shown in the Table 1 below. R1 and R2 should be 1% or tighter tolerance resistors with a temperature coefficient of <50 ppm/°C.
|ADJ Code||R2 (kΩ)|
The switching distance for each ADJ code can be approximated with the following formula:
For example, consider a coil with a diameter of 10 mm: An ADJ code of 1 will yield a switching distance of 3.75 mm and for a code of 15 a switching distance of 0.25 mm. This method helps reduce the effort needed to design the coil ratio precisely for a specific switching distance. It should be noted that the maximum sensing distance is determined almost entirely by the diameter of the coil for circular coils or by the minimum outer dimension for rectangular coils.
The LDC0851 includes hysteresis for the switching threshold. The switch point is determined by the inductance ratio between LSENSE and LREF. When the ratio of LSENSE to LREF drops below 99.6%, the device switches ON (output low). When LSENSE/LREF becomes greater than 100.4% it switches OFF (output high). The hysteresis window is therefore specified 0.8% from the switch ON point.
For proximity sensing, hysteresis may also be approximated in terms of distance as shown in Figure 23.
The length of time for the LDC0851 to complete one conversion and update the output is called the conversion time and is a function of sensor frequency. The conversion time is calculated with the following equation:
It is important to note that the frequency of the sensor increases in the presence of conductive objects. Therefore the worst case conversion time is calculated with no target present or when the target is at the maximum distance from the sensor.
This indicates the switch output state when there is no metal target within the switching distance of LDC0851. On power-up the LDC0851 output will be held HIGH until the part performs the sensor test and is ready for normal operation. This remains true even if the enable pin (EN) is pulled low. A HIGH to LOW transition on the OUT line occurs when the metal target comes within the switching distance of LDC0851. In the case of any sensor fault condition the LDC0851 maintains a HIGH state. An example of a sensor fault is if the sensor gets disconnected or damaged.
To save power, the LDC0851 has a shutdown mode. In order to place the LDC0851 in shutdown mode set the EN (Enable) pin low. This mode is useful for low power applications where the EN pin can be duty cycled at a low rate for wake-up applications to achieve a very low average supply current. An example of a duty-cycled application can be found in the applications section Low Power Operation. To resume active operation, set EN high and wait tAMT + tDELAY for valid output data. The current consumption in this mode is given in the electrical table as ISD. Note that the output will remain high (OFF) when EN is low. See Power-Up Conditions for more information on the startup conditions.
When the LDC0851 EN pin is pulled high, the LDC0851 is put into active mode. The active supply current (IDD) is broken up into three pieces: Static current (Istatic), Dynamic current (Idyn), and Sensor current (Isensor).
Static current is the DC device current given in the electrical characteristics and does not vary over frequency.
Dynamic current is the AC device current which varies with both sensor frequency (ƒSENSOR) and board parasitic capacitance (CBOARD). Dynamic current can be computed with the following equation:
Sensor current is the AC current required to drive an external LC sensor. Sensor current varies with both the frequency and inductance of the sensor and is given by the following equation:
The total active supply current is given by the following equation: