By examining the absolute maximum ratings table of current mainstream MCU manufacturers, we find that VDD is limited to 4.0V:

Parameter	STM32 (F1/F4)	GD32 (F1/F3)	National Technology (N32G/N32L)
VDD / VDDIO	-0.3V to +4.0V	-0.3V to +4.0V	-0.3V to +4.0V

Why is this the case? Here are several core factors that determine this voltage value, listed in order of importance:

1. The Most Direct Limitation: Gate Oxide Breakdown Voltage

This is the most critical and direct limiting factor.

• What is the gate oxide layer? In CMOS technology, there is a very thin layer of silicon dioxide insulation between the gate and the channel of the transistor, known as the gate oxide layer.

• Its function: To isolate the gate and control the on/off state of the channel using an electric field.

• Breakdown risk: The electric field strength that the gate oxide layer can withstand is limited. When the voltage difference between the gate and the substrate is too high, a strong electric field can break down this insulating layer, creating a permanent conductive path.

• Why is it fragile? As technology advances, the gate oxide layer becomes thinner (down to a few atomic layers), and its breakdown voltage decreases accordingly. For many modern MCUs, the gate oxide breakdown voltage may be only around 5V ~ 6V.

Therefore, the absolute maximum value (4V) must be set well below the gate oxide breakdown voltage to allow for sufficient safety margin.

2. Forward Conduction of Parasitic Diodes and Latch-Up Effect

As previously discussed, there are inherent parasitic diodes and transistors within the chip.

• Pin protection diodes: Almost all MCU pins have a diode from the pin to VDD and a diode from VSS to the pin, used to resist electrostatic discharge.

• Forward conduction: When the pin voltage exceeds <span>VDD + diode forward voltage drop (approximately 0.7V)</span>, the parasitic diode above will strongly conduct in the forward direction.

• High current risk: This diode is not designed to handle high currents. If the external voltage source is strong, it can generate a huge current through this diode, potentially burning out the metal wire or the diode itself instantly.

• Latch-up effect: Larger current and voltage fluctuations are more likely to trigger the latch-up effect, leading to a low-resistance path between VDD and VSS, which can destroy the entire chip.

The absolute maximum value (4V) must ensure that under any circumstances (even if VDD=3.3V), the pin voltage does not reach the dangerous threshold of <span>VDD + 0.7V</span>.

3. Transistor Reliability: Hot Carrier Injection

Even if the voltage is not high enough to cause immediate breakdown, long-term operation at high voltage can lead to performance degradation.

• Principle: When the electric field in the transistor channel is strong, carriers (electrons or holes) gain high energy and become “hot carriers.” These high-energy carriers may break through the silicon-silicon dioxide interface, injecting into the gate oxide layer and being trapped.

• Consequences: This can lead to threshold voltage drift, reduced transconductance, and gradual performance degradation over time, ultimately resulting in failure.

• Prevention: The absolute maximum value sets a hard limit, ensuring that the chip does not enter a significant HCI degradation zone even under the worst operating conditions.

4. Manufacturing Process Tolerances

There are inevitable deviations in chip manufacturing.

• Transistor characteristics on different areas of the same wafer will have slight differences.

• Different batches of chips will also have variations.

• Temperature will also affect the device’s voltage tolerance.

The absolute maximum value must consider all these worst-case scenarios, ensuring that every chip leaving the factory can safely withstand this voltage across the entire operating temperature range.

How is the number “4V” determined?

It is determined by chip designers and process engineers through the following steps:

1. Process characterization: Measuring test structures in the lab to determine the actual distribution of parameters such as gate oxide breakdown voltage and transistor reliability.

2. Simulation: Using EDA tools to simulate circuit behavior under various stress conditions.

3. Reliability testing: Conducting accelerated life tests on samples to assess their long-term reliability.

4. Setting margins: Based on the above data, setting a conservative absolute maximum value (such as 4.0V) that includes all tolerances and safety margins below the physical limit (such as 5.5V breakdown).

Important tips for hardware engineers:

• The absolute maximum value is not the operating voltage! The normal operating voltage should be well below this value (for example, if the absolute maximum value is 4V, the recommended operating voltage may be 3.3V).

• Transient exceedance can also cause damage! Even a brief voltage spike (such as hot plugging or inductive load back EMF) can cause cumulative or immediate damage to the chip if it exceeds the absolute maximum value.

• Protection circuits must be designed! In scenarios where overvoltage may occur (such as connecting external cables or driving inductive loads), protective measures such as clamp diodes, TVS diodes, and series resistors must be used to ensure that pin voltages never exceed the absolute maximum value specified in the datasheet.

Previous articles:

Factors Determining the Maximum Frequency of MCUs: HVT/SVT/LVT

What is Nano Electrical Measurement?

Why is the sample size 77 in HTOL testing? (Revised)

How to interpret MTTF values after HTOL life tests?

Chip Decap Process and Steps

Are VSS and VSSA internally connected or separated in MCUs? Neither!

What is the typical yield rate of the MCUs you use?

Introduction to Wire Bonding in MCUs

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