In-depth analysis of linear regulators LDO and ASM1117

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In-depth analysis of linear regulators LDO and ASM1117 [Copy link]

Hello to all electronics enthusiasts and engineers.

In the world of embedded development, there is a chip that can be called "legendary." It is cheap, easy to buy, and has a simple circuit. From the first development board we used in school to the small home appliances, routers, and set-top boxes that we see everywhere today, its presence is ubiquitous. It is the classic linear regulator (LDO) - ASM1117-3.3.

Why has this seemingly ordinary chip dominated the market for over a decade? What are the strengths of the new generation of low-dropout LDOs? How should they be selected in practical designs? Today, we'll explain the underlying principles and selection logic in simple terms.

I. What is a linear regulator (LDO)?

If we compare power management to water flow control, then a linear regulator (LDO) can be understood as a "precise variable resistor".

Its core function is to "cut off" excess voltage from a higher input voltage (such as 5V) through an internal regulation mechanism, thereby outputting a stable and clean low voltage (such as 3.3V).

The three major advantages of LDO:

1. Simple structure: The external circuit is extremely simple, and it usually only needs one capacitor to be connected to the input and one capacitor to operate.

2. Low noise: The output voltage ripple is small and very clean.

3. Fast response: It responds quickly to load changes.

Because of these characteristics, LDOs have become the preferred solution for powering devices that are sensitive to power noise, such as microcontrollers (e.g., STM32), sensors, and RF modules.

The inherent weaknesses of LDOs: efficiency and heat generation

However, the law of conservation of energy is fair. The voltage "cut off" by the LDO did not disappear; instead, it was entirely converted into heat.

formula:

Power consumption (P) = (Input voltage - Output voltage) × Output current

For example: Input 5V, output 3.3V, voltage difference 1.7V. If the current is 0.5A, the heating power is as high as 1.7V × 0.5A = 0.85W.

In extreme cases: if the input is 12V and the output is 3.3V, the excess 8.7V will all become heat, resulting in extremely low efficiency and the chip will easily get hot to the touch.

II. Why has ASM1117 been so popular for over a decade?

Despite the inefficiency of LDOs, the ASM1117-3.3 remains the top-selling device and is even considered the "hard currency" of the embedded systems industry. This is not due to technological stagnation, but rather because it perfectly meets the broadest range of essential needs.

1. Ample current and balanced performance.

The ASM1117 can output a maximum current of around 1A. For a typical STM32 minimum system board, including Bluetooth, Wi-Fi modules, and various sensors, 1A is more than enough. Unlike some newer LDOs that sacrifice power handling capacity for low power consumption, it doesn't.

2. Extremely simple, beginner-friendly.

The circuit requires only two components: an input capacitor and an output capacitor. This not only lowers the design threshold but also significantly reduces soldering defects and improves reliability in mass production.

3. A price killer, a boon for cost-sensitive consumers.

On platforms like LCSC, the ASM1117 is priced as low as a few cents to a couple of dimes. For consumer electronics and small appliances with huge sales volumes, every penny saved in cost translates into a huge profit margin. For cost-sensitive products, it's practically the only option.

4. Accurately timed the golden opportunity of the "5V to 3.3V" transition.

The most classic voltage conversion in the embedded field is 5V to 3.3V.

Input: Common USB power supply or 5V adapter.

Output: Mainstream 3.3V logic level (STM32, ESP8266, etc.).

Differential voltage requirement: 5V - 3.3V = 1.7V.

The ASM1117 has a typical dropout voltage of approximately 1.2V, meaning that an input of at least 4.5V is required to stably output 3.3V. In a 5V power supply scenario, this characteristic fully meets the requirements and provides a margin of safety.

III. Challenges of the New Era: Lithium-ion Battery Power and Low-Drop-Count LDO

With the widespread adoption of portable devices, lithium-ion batteries have become the mainstream power source. This exposes a fatal weakness of the ASM1117: excessive voltage drop.

Pain point analysis:

Lithium battery characteristics: nominal voltage 3.7V, fully charged voltage 4.2V, discharge cutoff voltage is usually between 3.0V and 3.3V.

Limitations of the ASM1117: To achieve a stable 3.3V output, the input voltage must be at least 3.3V + 1.2V = 4.5V.

Result: A single lithium battery (maximum 4.2V) is simply insufficient to drive the ASM1117 to output 3.3V. The system will reset or become unstable when the battery level drops even slightly.

Solution: New generation low dropout LDO (such as SGM2036 from Saint-Gobain)

To address this issue, a new generation of LDOs, represented by SGMICRO's SGM2036, has emerged. They offer the following significant advantages:

Ultra-low dropout voltage: The dropout voltage is only tens to hundreds of millivolts. This means that as long as the battery voltage is higher than about 3.4V, it can stably output 3.3V, perfectly matching the full discharge curve of a single lithium battery.

Ultra-low quiescent current: When the device is in standby mode or the downstream circuit is not working, it consumes very little current (microamps), which greatly extends the battery life.

Low noise: It also retains the low noise advantage of LDO, making it suitable for RF and analog circuits.

The cost of the new chip:

Of course, there are trade-offs in new technologies:

Lower current output: The typical output current is around 300mA, and its ability to handle large loads is weaker than that of the ASM1117.

Slightly higher price: The cost is a few cents higher than that of ASM1117.

Minimal packaging: Small packages (such as SOT-23-5) are commonly used, which are slightly more difficult to solder by hand (but this is not a problem in automated production).

IV. Summary and Selection Recommendations

Faced with a dazzling array of power supply chips, how should we choose? The logic is actually quite clear:

V. Conclusion

When we see the black three-pin ASM1117 chip on the development board again, please don't think it's "outdated" or "obsolete".

In the classic embedded system scenario of 5V to 3.3V conversion, it has built an unshakeable competitive advantage thanks to its three key strengths: simplicity, reliability, and affordability. In the engineering field, "usable, easy to use, and inexpensive" are often more important than "the most powerful specifications."

So next time you design an STM32 minimum system board, feel free to use that familiar ASM1117. This isn't about being conservative; it's about paying homage to the optimal solution in engineering practice.

Thank you everyone!