How to Choose Between TJA1043 and TJA1042 for Standby Power Optimization?

Car batteries drain quickly if electronics stay on. This is a huge problem for design engineers. You must choose the right transceiver¹ to stop this power loss.

The TJA1043² is the correct choice for "always-on" components like Body Control Modules (BCM)³ because it supports sleep and wake-up function⁴s to save power. The TJA1042⁵ is ideal for diagnostic tools like OBD scanners, which only operate when the engine runs, offering a lower cost without complex wake-up logic.

I have worked with many procurement managers and engineers at Nexcir. We see a common mistake in the industry. People often pick a chip based only on price or availability. They ignore the specific power requirements of their application. This leads to dead batteries or wasted budget. I want to share my experience to help you avoid these issues.

Why is the sleep and wake-up function⁴ critical in automotive electronics?

Modern cars have over 70 electronic control units (ECUs). If they all stay on, the battery dies in hours. We need a way to turn them off safely.

The sleep and wake-up function⁴ allows an ECU to enter a low-power mode, consuming micro-amps instead of milli-amps, while still listening for a specific signal to turn back on. This is the only way to meet strict OEM standby current requirements.

Understanding the Power Drain

I remember a project with a client developing a new door control module. They were failing their quiescent current tests⁶. The limit was 100µA (micro-amps), but their board was drawing 5mA (milli-amps). That is 50 times too high. The problem was simple. They used a standard transceiver⁷ that could not put the rest of the board to sleep.

In automotive design, we look at "Key-Off" loads. When you park your car and walk away, the car looks like it is off. But the alarm, the door locks, and the remote receiver are still alive. They are "sleeping." They need to wake up instantly if you press your key fob.

If you use a standard transceiver⁷ without sleep capability, the microcontroller (MCU) must stay awake to check the bus. An active MCU uses a lot of power. However, if you use a transceiver with wake-up capability⁸, the MCU can turn off completely. The transceiver stays in a very low power state. It watches the bus. When it sees a wake-up pattern, it triggers the MCU to start.

Here is a simple breakdown of the current consumption differences⁹ we see in the lab:

At Nexcir, we emphasize this difference. If your device connects directly to the battery (KL30), you cannot ignore the sleep function. It is not just a feature; it is a requirement for the car to function reliably.

When is the TJA1043² the absolute best choice for your design?

You are designing a Body Control Module (BCM) or a gateway. These devices must never fully disconnect from power. They manage the car's security and access.

The TJA1043² is the industry standard for these applications because it features an INH (Inhibit) pin that controls external voltage regulators, allowing the entire board to power down and wake up only when necessary.

The Power of the INH Pin

The TJA1043² is not just a radio for data. It is a power manager. The most critical feature of the TJA1043² is the INH pin¹⁰. I often explain this to hardware engineers who are new to automotive CAN.

Here is how it works. The TJA1043² connects to the battery supply. The INH pin¹⁰ connects to the Enable pin of your main voltage regulator (LDO or DC-DC converter). When the car is running, the INH pin¹⁰ is High. The regulator works, and the MCU has power.

When the car turns off, the MCU tells the TJA1043² to go to "Sleep Mode." The TJA1043² then pulls the INH pin¹⁰ Low. This turns off the regulator. The MCU loses power and turns off. The whole board is dead, except for a tiny part of the TJA1043². This state consumes almost no energy.

Then, two things can happen:

1. Local Wake-up¹¹: A switch connected to the TJA1043² changes state (like a door handle pull).
2. Remote Wake-up¹²: A specific pattern of data appears on the CAN bus wires.

If either happens, the TJA1043² pulls the INH pin¹⁰ High. The regulator turns on. The MCU boots up. The car reacts.

This capability makes the TJA1043² essential for:

• Body Control Modules (BCM)³: Controlling lights, locks, and windows.
• Gateways: Managing data between different networks.
• Keyless Entry Systems¹³: Waiting for a signal.

I always tell my customers: if your module needs to wake up the car, or if the car needs to wake up your module, you must use the TJA1043². There is no cheaper workaround that is safe.

Key Features of TJA1043²

Why should you choose the TJA1042⁵ for diagnostic tools?

Not every device needs to sleep. Some devices only work when the mechanic is fixing the car. In these cases, simplicity and cost are more important.

The TJA1042⁵ is the perfect choice for On-Board Diagnostics (OBD) tools¹⁵ because these devices typically receive power only when the ignition is on, making the complex sleep and wake-up circuitry unnecessary and overpriced.

Simplicity Wins for Diagnostic Tools

I frequently work with customers who manufacture aftermarket tools. These are the OBD scanners, emissions testers, and data loggers you see in repair shops. A common question arises: "Should we use the TJA1043² to be safe?"

My answer is usually "No." Here is why.

An OBD tool usually gets power from the OBDII port. In many cars, this port has power all the time. However, the tool is not a permanent part of the car. A mechanic plugs it in, does the work, and unplugs it. Or, if it is a consumer dongle (like an insurance tracker), it might stay plugged in.

But here is the key difference: The TJA1042⁵ is a basic transceiver. It sends and receives. It has a "Standby" mode, but it does not have the INH pin¹⁰ to control power regulators. It assumes the system power is managed elsewhere or is simply switched on by the ignition key (KL15).

If you use a TJA1043² in a simple OBD tool, you are paying for silicon you do not use. The TJA1043² is more expensive than the TJA1042⁵. It also has more pins and requires a slightly more complex PCB layout.

For an OBD tool, the logic is simple:

1. Ignition On¹⁶: The tool gets power. The TJA1042⁵ starts transmitting.
2. Ignition Off: The tool loses power (or goes into a simple idle). The TJA1042⁵ turns off.

There is no need for the transceiver to wake up the MCU because the user controls the usage.

We also recommend the TJA1042⁵ for:

• Engine Sensors: They only run when the engine runs.
• Infotainment Add-ons: They turn on with the dashboard.
• Industrial CAN systems: Where power is supplied by a main switch.

By choosing the TJA1042⁵, you save money on the Bill of Materials (BOM). You also reduce supply chain risk because the TJA1042⁵ is one of the most common chips in the world. It is easier to source in high volumes.

How do we determine the final selection strategy for your project?

We have looked at the technical details. Now we need a simple rule to make decisions fast. This helps procurement teams and engineers align their goals.

Your selection strategy should follow this rule: Use the TJA1043² for any node connected to "Battery Constant" (KL30), and use the TJA1042⁵ for any node connected to "Ignition Switched" (KL15).

The Decision Matrix¹⁷

At Nexcir, we help clients optimize their supply chain. Part of that is reducing the number of different parts they buy. However, you cannot merge these two chips into one without losing money or performance.

I encourage you to use critical thinking when you review your design. Ask these three questions:

1. Where does the power come from?

   • If it comes directly from the battery and is never cut off physically, you have a high risk of draining the battery. You need the TJA1043².
   • If the power comes from a relay or a switch that the driver controls, the risk is low. You can use the TJA1042⁵.

2. Does the device need to "listen" while off?

   • Does your device need to unlock the doors when a signal comes? Does it need to start the alarm? If yes, it is a "listening" node. It needs the TJA1043².
   • Does your device only send data when the engine is spinning? Then it is a "talking" node. It needs the TJA1042⁵.

3. What is the budget pressure?

   • The TJA1043² costs more. It is a premium chip for a specific task.
   • The TJA1042⁵ is the workhorse. It is cheaper.

Here is a summary table to help you finalize your decision:

We hold stock of both at Nexcir. We source them from authorized distributors to ensure you never get counterfeit parts. Supply chain stability is just as important as the technical specs. If you design in a TJA1043², make sure you have a partner who can deliver it long-term.

Conclusion

To summarize: choose the TJA1043² for always-on nodes like BCMs that require sleep modes, and choose the TJA1042⁵ for ignition-switched tools like OBD scanners to save costs.