Many CAN transceivers have built-in
ESD protection cells, but to keep the size of the chip down, most of them only
protect up to 8 kV. As it was previously mentioned, depending on the environment,
some ESD strikes can get up to 30 kV. Due to this, an external ESD protection diode
is needed to increase the system-level ESD performance. Below are the key
considerations and parameters needed to select a proper ESD protection diode:
- Working Voltage (Vrwm) and
Polarity
- The Vrwm of the diode is
dependent on the application they will be used in. Under ideal
conditions, the CAN bus voltage levels swing between Vcc (5 V or 3.3 V)
on CANH and 0 V on CANL. However, in vehicles, there is a common mode
voltage present depending on the battery voltage. Smaller vehicles will
use 12 V batteries and larger vehicles like 18-wheelers will use 24 V
batteries. In addition to this common-mode voltage, there is also the
risk of an improper jumpstart if the vehicle’s battery is almost dead.
The proper way to jumpstart a vehicle is to connect a battery of another
car in parallel with the dead battery. A person that does not know this
might connect both batteries in series, doubling the overall voltage of
the car. In the case of a 12 V battery, a 24 V ESD diode is needed to
ensure that it does not burn up in this series jumpstart scenario. In
the case of a 24 V battery that consists of two 12 V cells, a 36 V diode
is needed since the cells are charged individually. All diodes need to
be bidirectional to account for line faults and miswiring.
- IEC 61000-4-2 Rating
- The IEC
61000-4-2 standard defines a waveform that simulates a
real-world ESD strike, contrary to waveforms like human body model (HBM)
and charged device model (CDM) that simulate ESD events in a controlled
environment. Since certain environmental elements such as humidity and
temperature make ESD strikes more strenuous, it is recommended that the
ESD diode has a minimum contact rating of 15 kV.
- ISO 10605 Rating
- The ISO
10605 standard defines a waveform that simulates a real-world
ESD strike in an automotive environment. This waveform defines many
different capacitance and resistance combinations, contrary to IEC
61000-4-2 that only calls for 150 pF/330 Ω. The most strenuous of these
combinations is the 330 pF/330 Ω, which is more strenuous than an IEC
61000-4-2 waveform. To survive ESD strikes in harsh automotive
environments, it is recommended that the ESD diode has a minimum contact
rating of 15 kV.
- Capacitance
- An ESD diode should have
a low capacitance to minimize signal degradation. The maximum allowable
diode capacitance can vary between signal speeds (LS CAN vs CAN FD),
transceiver capacitance, network size, and other components on the CANH
and CANL lines like filtering capacitors. Generally, it is recommended
to keep the diode capacitance below 15 pF. However, system architects
look for diodes with the lowest possible capacitance to maximize their
total capacitance budget for the system.
- Clamping Voltage
- Clamping voltage
requirements can vary depending on the CAN transceiver being used. The
important thing to note is that the clamping voltage should be less than
the abs max rating of the CANH and CANL pins.
- Package
- For CAN applications,
many systems require automatic optical inspection for their boards to
confirm that all components are soldered on properly. To allow for this,
leaded packages like SOT-23 and SC70 are recommended for ESD
diodes.