Analysis of ripple condition of LDO power supply in PCB design

In the previous article, we compared the efficiency of switching power supplies and LDOs. It's clear that switching power supplies have a significant advantage, often maintaining an efficiency above 85%. Additionally, they offer the ability to step up or step down voltages, making them highly versatile. At first glance, it seems like LDOs don't stand a chance. However, this isn't entirely true. In fact, many PCB designs still use LDOs, which indicates that they have their own unique advantages. One of the most commonly mentioned benefits is their low ripple, which we will explore in detail in this article. Ripple is a common term used to describe small fluctuations in the output voltage of a power supply. However, there is a more technical way to measure this, especially for LDOs. This is known as the Power Supply Rejection Ratio (PSRR), which measures how well a power supply can suppress variations in the input voltage from appearing at the output. PSRR is typically expressed in decibels (dB) and is calculated using the formula: **PSRR = 20 * log(Vin_ripple / Vout_ripple)** This parameter is crucial for ensuring stable power delivery, and it varies across different frequencies. The higher the PSRR value, the better the power supply is at rejecting noise and interference. To illustrate this, we conducted simulations using two different power supply ICs: LM2941 (an LDO) and TPS5430 (a switching power supply). For simplicity, we introduced a 1V peak-to-peak noise signal at a frequency of 100kHz into the input. The LM2941 circuit was set up as follows: [Image: Circuit diagram of LM2941] The ideal 5V output became distorted, resulting in a ripple of 500µV at the output. This gives us a PSRR of: **PSRR = 20 * log(1V / 500µV) = 66dB** Checking the datasheet for LM2941, we found that its PSRR at 100kHz meets the required specifications, confirming the simulation results. [Image: PSRR curve for LM2941] Now, let’s look at the TPS5430, a switching power supply. The same 1V noise was applied, and the output ripple was measured to be around 13mV. Using the same formula: **PSRR = 20 * log(1V / 13mV) = 37dB** As you can see, the difference between the two is quite significant. The LDO performs much better in terms of ripple suppression. This makes LDOs a preferred choice in applications where clean, stable power is critical, such as in sensitive analog circuits or audio systems. While switching power supplies are more efficient and flexible, they generally cannot match the low-noise performance of LDOs. That’s why both types of power supplies continue to coexist in modern electronics—each serving its own purpose based on design requirements.

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