Why does an LDO experience a short voltage overshoot when powered on?  

1. The core cause of overshoot

The core of an LDO is a negative feedback regulation circuit. When the voltage is higher than the set value, it steps down the voltage. When it is lower than the set value, it compensates the voltage to achieve voltage regulation. 

Let's analyze the equivalent circuit of LDO in practical applications:

At power-on, the output capacitor (C_OUT) is considered a short circuit (voltage across both terminals is 0V). If the ESR is too small, the V_OUT voltage will also be temporarily pulled down to 0V. At this point, the LDO's internal negative feedback circuit will generate a significant voltage compensation, causing V_OUT to momentarily increase. At this point, V_DS (MOSFET drain-source voltage) is very small because V_OUT = V_IN - V_DS.

However, the capacitor C_OUT charges very quickly. After it is fully charged, V_OUT reaches 3.3V. However, due to the feedback loop's response time, the MOS tube cannot quickly increase V_DS for regulation. As a result, V_OUT continues to rise due to compensatory inertia, resulting in a brief overshoot (exceeding 3.3V).

2. Solution

The traditional solution of increasing output capacitance has drawbacks: while it can prolong charging time, it can easily cause voltage fluctuations after charging is complete, making it a suboptimal solution.

We can appropriately increase the ESR of the output capacitor

By adding a small series resistor (such as 2.7Ω) to increase ESR_OUT, V_OUT can be prevented from being excessively low during power-on, reducing the compensation amplitude of the negative feedback circuit and alleviating overshoot at the source.

You can also optimize your selection

Some brands of LDOs are not designed to handle extreme operating conditions, making overshoot a common problem. When selecting a model, it is recommended to conduct field testing and prioritize models optimized for transient response to avoid damage to sensitive downstream components (such as chips and sensors) due to overshoot.

Summary

The essence of LDO power-on overshoot is unbalanced feedback regulation caused by insufficient ESR . Properly adjusting the output capacitor's ESR or selecting an appropriate model can effectively solve this problem and ensure circuit stability in both transient and steady-state conditions.