8.2.2.1 Quick Start

When a new sensor is being used the registers should be setup using the following procedure. Instructions on how to determine the value to put in each register are described in the Register Maps section.

1. Set RP_MAX in the OSC_CONFIG_0 register.
2. Set PADC_TIMEC and RP_MIN in the OSC_CONFIG_1 register.
3. Set RESONATOR_MIN_FREQ in the OSC_CONFIG_2 register.
4. Set UNDER_RANGE_SWITCH_EN, OSC_AMP, and RESPONSE_TIME in the OSC_CONFIG_3_INIT and OSC_CONFIG_3_FNL registers. The same values should be written to both registers. Setting OSC_AMP to 4V and RESPONSE_TIME to 6144 should give the most accurate results. Note that the power on default for OSC_AMP is 11:Reserved so OSC_AMP must be changed to either 1V, 2V, or 4V.
5. Select the value of the CF capacitor as described in the CF (CFA and CFB Terminals) section.
6. Set the values in the OUT_CONFIG_INIT and OUT_CONFIG_FNL registers as needed. The same values should be written to both registers.
7. Put the sensor at the target distance that the switch is supposed to turn on. Read the PROXIMITY_MSB and Proximity LSB multiple times. If needed, RP_MAX or RP_MIN can be adjusted up or down one step at a time to determine the combination that gives the most accurate setting for this specific sensor.
8. Put the sensor at the target distance that the switch is supposed to turn on. Read the PROXIMITY_MSB and Proximity LSB multiple times, take an average, and write this value into the DET_H_MSB_INIT and DET_H_LSB_INIT and DET_H_MSB_FNL and DET_H_LSB_FNL registers. This value may need to be adjusted.
9. Put the sensor at the target distance that the switch is supposed to turn off. Read the PROXIMITY_MSB and Proximity LSB multiple times, take an average, and write this value into the DET_L_MSB_INIT and DET_L_LSB_INIT and DET_L_MSB_FNL and DET_L_LSB_FNL registers. This value may need to be adjusted.

8.2.2.2 Determining The R_P of an LC Tank

The method in the Quick Start section for setting the values in the LMP91300 registers requires that the R_P of the LC tank be known at the switching point (the point that the switch is changed from the normal condition to the triggered condition). It is best to use an impedance analyzer to characterize the R_P of the LC tank over distance. If an impedance analyzer is not available the R_P of the LC tank can be determined using the method below.

1. Set the target at the switching distance from the sensor.
2. Set PADC_TIMEC = 1, RESONATOR_MIN_FREQ as described in the Register Maps section, OSC_AMP = 4V, and RESPONSE_TIME = 6144.
3. Put a scope probe on INA or INB.
4. Step RP_MIN up one step at a time until the amplitude of the signal on the oscilloscope becomes variable as shown in Figure 21 and Figure 22. Increase RP_MIN by two, for example if the amplitude becomes variable at 17, set RP_MIN to 19.
5. Step RP_MAX up one step at a time until the amplitude of the signal on the oscilloscope becomes steady. Decrease RP_MAX by two steps, for example if amplitude became steady at 16, set RP_MAX to 14.
6. Set the CF capacitor as described in the CF (CFA and CFB Terminals) section.
7. Read the Proximity value in registers 0x7A and 0x7B and convert this value to decimal.
8. Use the formula R_PLCTANK (Ω) = (RP_MAX × RP_MIN ) / ( RP_MIN × (1-Y) + RP_MAX × Y ), where Y = Proximity Data / 2^15 and RP_MAX and RP_MIN are the impedance values shown in Register Maps section.

This value can have a tolerance of ±25% when compared to the value from an impedance analyzer.

8.2.2.3 Component Selection And Layout

The following PCB layout guidelines and suggested components should be used when designing a PCB.

8.2.2.4 CF (CFA and CFB Terminals)

10pF to 100nF, ≥10V, X7R ceramic capacitor. The traces connecting CFA and CFB to the capacitor should be as short as possible to minimize the parasitic capacitance. The value of this capacitor will be based on the time constant and resonating frequency of the LC tank.

For optimal performance, the value of CF, needs to be as small as possible, but large enough such that the active filter does not saturate. The size of this capacitor depends on the time constant of the SENSE coil, which is given by L/R_S, (L = inductance, R_S = series resistance of the inductor at oscillation frequency). The larger this time constant becomes, the larger the value of filter capacitor that is required. Hence, this time constant reaches its maximum when there is no target present in front of the sensing coil.

The following procedure can be used to determine CF:

1. Start with a default value of 10nF for CF.
2. Set RP_MAX, PADC_TIMEC, RP_MIN, RESONATOR_MIN_FREQ, OSC_AMP, and RESPONSE_TIME to the desired values as described in the Register Maps section.
3. Move the metal target far away from the LC tank.
4. Connect a scope probe to the INB (terminal 21) and CFB (terminal 17) terminals. Since the CFB terminal is very sensitive to capacitive loading, it is recommended to use an active probe. As an alternative, a passive probe with a 1kΩ series resistance between the tip and the CFB terminal can be used.
5. Set the time scale of the oscilloscope so that many periods of the signal on the INA terminal can be seen. See Figure 21.
6. Set the CF capacitor value so that the AC portion of the waveform is about 1V_PP maximum. Decreasing the capacitor value will make the AC portion of the waveform larger. This signal scales linearly with the reciprocal of the filter capacitance. For example, if a 100pF filter capacitor is used and the signal observed on the CFB terminal has a peak-to-peak value of 200mV, the desired 1V peak-to-peak value is obtained using a 200mV / 1V × 100pF = 20pF filter capacitor. Figure 21 shows the waveforms on CFB and INA and Figure 22 shows the waveforms using a zoomed in horizontal scale. Note that the waveforms on CFB and INA are not a constant amplitude. The waveform on CFB should be adjusted so that the maximum value is 1V_PP.

Figure 21. Determining the Value of CF

Figure 22. Determining the Value of CF

8.2.2.5 NTC (TEMP+ Terminal)

The thermistor, such as the Murata NCP03WF104F05RL, should be placed as close to the LC tank as possible to minimize error introduced by temperature variation within the operating environment. The NTC should also be close to the LMP91300 to minimize the parasitic capacitance. It is connected between terminals 22 and 23. There should be a ground trace separating the thermistor from the LC tank, to minimize the coupling from the signal on the LC tank. Ideally, the thermistor could be on one side of the PCB and the LC tank on the other side of the PCB with a ground plane between them.

8.2.2.6 C1

0.1µF to 1µF, ≥50V, X7R ceramic capacitor. This is a bypass capacitor for the regulator. The value of this capacitor will also affect the rising and falling edges of the SWIF signal. A good value to start with is 0.1µF.

8.2.2.7 C_{V+/EXT E}

100nF, ≥10V, X7R ceramic capacitor. If the loop voltage is ≤ 8V, 100nF is the maximum value that can be used.

8.2.2.8 CBY (CBY Terminal)

56nF, ≥5V, X7R ceramic capacitor. Connect between the CBY terminal and ground.

8.2.2.9 R_SENSE

The value of this resistor and power rating of the R_SENSE resistor depends on the amount of current allowed through the switch transistor. The LMP91300 has an Over Current Detection Threshold of 310mV typical. When the LMP91300 detects ≥ 310mV across the sense resistor it will go into Overload Protection mode. In this mode it will periodically turn on the switch for 30µs to check if the overload condition is still there. If the LMP91300 detects a value ≥ 310mV (typical) across the sense resistor it will limit the current through the switch so that the voltage across R_SENSE is ≤ 480mV. See the OVERLOAD PROTECTION entries in the ELECTRICAL CHARACTERISTICS section and Figure 7, Figure 12, and Figure 13.

8.2.2.10 R_{EXT B} (EXT B Terminal):

The internal regulator along with the external NPN transistor will develop 5V on the V+/EXT E terminal. The EXT B terminal will be one diode drop above this at about 5.6V. The voltage across R_{EXT B} will be the difference between the loop voltage and the 5.6V on the EXT B terminal. The value of R_{EXT B} depends on the minimum loop voltage and the minimum temperature that the LMP91300 will be used at. The values shown in Figure 23 show the largest typical value for R_{EXT B} that can be used for a specific minimum loop voltage and minimum temperature. For example, if the LMP91300 is being used in a proximity sensor that has a specification of 10V minimum for the loop voltage and an operating temperature minimum of 0°C, a value of 26kΩ or smaller can be used for R_{EXT B}. The data shown in Figure 23 depends on several variables, such as the external transistor used and the value of C_{V+/EXT E}. A SMBTA06 transistor was used when taking the data in Figure 23. The user must ensure that the resistor has the correct power rating and that the regulated 5V on the V+/EXT E terminal comes up correctly and is stable for the entire loop voltage and temperature range in the intended application.

Figure 23. Maximum R_{EXT B} Value

8.2.2.11 R1

33kΩ, 1/8W resistor.

8.2.2.12 SENSE1+ And SENSE2+ Terminals (R_SENSE1+, R_SENSE2+)

If the supply is ≤ 40V these terminals can be shorted to the supply. If the supply is > 40V resistors must be placed between the SENSE1+ and SENSE2+ terminals and the supply. These resistors must drop enough voltage so that the terminals of the LMP91300 are < 40V. The resistors will have 100µA going through them. For example, if the supply is at 50V, 10V will need to be dropped across these resistors so the resistance will be 10V/100µV = 100kΩ. These resistors must be matched resistors, 0.1% or better. Keep the trace between the LMP91300 and the resistors short.

In NPN mode the SENSE1+ terminal is not used. It should be connected to the supply as described above.

8.2.2.13 NPN

A SMBTA06 or similar transistor.

8.2.2.14 PNP

A FMMT593 or similar transistor.

8.2.2.15 LED

The LMP91300 can be programmed to supply 2.5 or 5mA. The LED chosen should have a voltage drop of less than 3V. If an LED is not needed the LED terminal can be connected directly to the V+/EXT E terminal. The LMP91300 uses the LED terminal to talk back to the device controlling the SWIF interface by sinking current into the LED terminal.

8.2.2.16 LC Tank and INA and INB Terminals

There should be a ground trace between the INA and INB terminals and the rest of the terminals of the LMP91300 to decrease the coupling of the signal on the INA and INB terminals to the other terminals of the LMP91300. The trace between the LC tank and the INA and INB terminals should be as short as possible as shown in part A of Figure 24. Longer traces between the LC tank and the INA and INB terminals can cause ringing at the INA and INB terminals which can produce very noisy proximity readings. If longer traces need to be used split the LC tank so that the capacitor is close to the LMP91300 as shown in part B of Figure 24. If both the L an C have to be located a longer distance away from the LMP91300 small capacitors (15 to 22pF) can be connected between INA and ground and INB and ground as shown in part C of Figure 24.

Figure 24. LC Tank Traces

8.2.2.17 SWDRV Terminal

Keep the trace between the SWDRV terminal and the transistor short.

8.2.2.18 P1 To P5 Terminals

Connect to ground using short traces.

8.2.2.19 GND Terminals

Connect to ground using short traces.

8.2.2.20 NC Terminals

These terminals do not connect to the silicon and can be left unconnected.

8.2.2.21 Exposed DAP

Connect to ground. The DAP area on the PCB can be used as the center of a star ground with all other ground terminals connecting to it.

8.2.2.22 SENSE-

In PNP mode the SENSE- terminal is not used. It should be connected to ground.