TIC10024-Q1 Datasheet, PDF (66/81 Pages) Texas Instruments – 24-Input Multiple Switch Detection Interface (MSDI) Device With Adjustable Wetting Current for Automotive Systems

TIC10024-Q1
SCPS268 – SEPTEMBER 2017
www.ti.com
Digital Switch Detection in Automotive Body Control Module (continued)
10.2.1 Design Requirements
Table 23. Example Digital Switch Specification
PARAMETER
V_BAT
V_DIODE (voltage drop across the reverse-blocking diode)
R_ESD
R_SW
R_DIRT
Wetting current requirement
SPECIFICATION
7 V ≤ V_BAT ≤ 16 V
0 V ≤ V_DIODE ≤ 1 V
50 Ω ± 8%
220 Ω Max when
closed ± 8%
5000 Ω Min
10mA Typical
MIN
7V
0V
46 Ω
0Ω
5000 Ω
MAX
16 V
1V
54 Ω
237.6 Ω
∞
An example of digital switch connected to ground shown in Figure 41, with Table 23 summarizing its detailed
requirements. The goal of this design is to utilize the TIC10024-Q1’s integrated comparator to detect and
differentiate between the 2 switch states:
1. State 1: SW open
2. State 2: SW closed
To mimic real automotive systems, the battery is assumed to be fluctuating between 7 V and 16 V. Taking into
account the voltage drop across the reverse-blocking diode, the V_S supply voltage to the TIC10024-Q1 device
can fluctuate between 6 V and 16 V. R_DIRT is introduced to model the small leakage flowing across the switch in
open state. When the switch changes position and the switch state changes from one to another, the TIC10024-
Q1 is required to correctly detect the state transition and issue an interrupt to alert the microcontroller. The switch
information needs to be stored in the status registers for the microcontroller to retrieve.
10.2.2 Detailed Design Procedure
State 1: SW open
State 2: SW closed
Table 24. Detailed Design Procedure
STEP 1
Equivalent Resistance Value (Ω)
MIN
MAX
MIN
5000
∞
>10
0
291.6
0
STEP 2
V_INX (V)
MAX
-
3.32
Use the following procedures to calculate thresholds to program to the TIC10024-Q1 for proper switch detection:
1. Calculate the equivalent resistance values at the 2 switch states, taking into account R_DIRT and the 8%
resistance variation.
2. Estimate the voltage established when wetting current flows through the switch by utilizing the relationship
V_INX = R_{SW_EQU} × I_{WETT_ACT}, where R_{SW_EQU} is the equivalent switch resistance value and I_{WETT_ACT} is the
actual wetting current flowing through the switch. The 10 mA wetting current setting is selected in this design
as required by the specification. The wetting current variation, however, can occur depending on
manufacturing process variation and operating temperature, and needs to be taken into account. Referring to
the electrical table of the TIC10024-Q1 and assuming enough headroom for the current source (CSO) to
operate, the 10mA wetting current setting produces current ranging between 8.4 mA and 11.4 mA (for 6 V ≤
V_S ≤ 35 V condition). The voltage established on the TIC10024-Q1 input pin (V_INX) can be calculated
accordingly.
3. After the V_INX voltage is calculated for the 2 switch states, the proper threshold value needs to be chosen
between minimum V_INX voltage of state 1 (>10 V) and maximum V_INX voltage of state 2 (3.32 V). The
TIC10024-Q1 has 4 thresholds that can be configured for the comparator: 2 V, 2.7 V, 3 V, and 4 V. As a
result, the proper threshold to be used in this design example is 4 V.
4. To properly program the device, follow the below recommend procedure:
– Enable channel IN0 by setting IN_EN_0 bit to 1 in the IN_EN register
– Program the wetting current to source by setting CS_IN0 bit to 0 in the CS SELECT register
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