This is the second blog in a two-part series about ESD protection of OBD-II ports. Read the first part here.
Now that we understand the OBD-II interface and the protocols that it supports, we are in a better position to select appropriate TVS diodes for the ESD protection of GPS trackers via OBD-II pins. Pin 16, which is connected to the battery (+12V), requires a single line TVS diode which is capable of VBus protection. Semtech’s µClamp®1871P is a great solution to safeguard this Vbus pin from ESD threats. The VBus pin needs a TVS diode with more than 12V working voltage, fast response time, low clamping voltage, and no device degradation during the protection of the VBus pin. µClamp1871P’s working voltage is 18V, and it is capable of withstanding extremely high ESD voltages of ±30kV (Air) and ±30kV (Contact) as per IEC 61000-4-2 standard. µClamp1871P comes in a small 1.6 x 1.0 x 0.57mm package with a low on-resistance of 0.05Ω. See Figure 2 for the connection diagram.
Next, let’s explore CAN Bus protection. As explained earlier, CAN High (pin 6) reaches 3.75V and CAN Low (pin 14) drops to 1.25V when transmitting data. So they generate a 2.5V differential signal between the lines. The protection criteria for CAN High and Low are somewhat different than for other pins. These pins carry a differential 1Mbps data signal. Since they are high-speed differential data lines, TVS diodes should protect the OBD port and the circuit during transient events while ensuring signal integrity by maintaining a low line-to-line capacitance. While low capacitance is not a consideration for VBus protection (pin 16), it is among the most important design considerations for Pin 6 and 14 alongside other parameters. Semtech’s µClamp2482SQ, a dual-channel device, can protect CAN High and Low lines. µClamp2482SQ has an operating voltage of 24V with a minimum breakdown voltage of 26.5V. These voltages are sufficient to protect the CAN transceivers if the car’s 12V battery requires a jump-start from a 24V battery in an emergency. This also allows the OBD-II port to remain in a non-conducting state in the event of a short between the battery and signal lines. µClamp2482SQ provides transient protection to ESD events at ±30kV (Air) and ±30kV (Contact) as per the IEC 61000-4-2 standard. It has an extremely low capacitance of 15pF (Typical) and a dynamic resistance of 0.4Ω (Typical). µClamp2482SQ is available in an industry standard SOT23 package.
Figure 2. Possible ESD protection options of vehicle tracking via OBD-II Port
Next comes the protection requirements of ISO 9141 K-Line (pin 7) and ISO 9141 L-Line (pin 15). The maximum voltage of these pins is 12V and maximum data transmission rate is 10.4 Kbps. Considering these requirements, Semtech’s RClamp®1561PQ is a great option to protect the K and L lines together.
Finally, we will discuss the protection requirements of SAE J1850 PWM BUS (+) and BUS (-). The maximum voltage is 5V and maximum data transmission rate is 41.6 Kbps in the case of SAE J1850 (PWM) lines. The maximum voltage of SAE J1850 VPW BUS (+) and BUS (-) is 7V, and maximum data transmission rate is 10.4 Kbps. Semtech’s RClamp1561PQ can be used to protect the SAE J1850 lines as well. RClamp1561PQ has a working voltage of 15V in a very small 0402 package. It provides transient protection at ±15kV (Air) and ±10kV (Contact) as per the IEC 61000-4-2 standard. RClamp1561PQ has a low capacitance of 0.3pF (Maximum) and a dynamic resistance of 3.8Ω (Typical).
As GPS-based vehicle tracking systems become more and more popular, the manufacturers of these systems need to seriously consider how to efficiently protect the tracker from ESD surges and help prevent failures. The last thing a vehicle tracker user would want is to experience damage to the OBD-II port from the use of a GPS device.
Semtech is a leading manufacturer of TVS diodes that protects many of the world’s most popular consumer electronics devices. Its highly efficient and trusted portfolio of TVS products also protect all types of vehicle tracking systems, enabling consumers to enjoy a smooth, problem free and uninterrupted experience in their mobile tracking application.
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