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, typically maintaining an efficiency above 85%. They also offer the ability to step up or step down the output voltage. 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 means they must have their own unique benefits. One of the most commonly mentioned advantages is their low ripple. This article will focus on exploring the ripple characteristics of both types of power supplies. Ripple is a common term in power supply discussions, but it's actually a serious technical specification. For LDO power supplies, one key parameter is the Power Supply Rejection Ratio (PSRR). PSRR measures how well a power supply can suppress variations in the input voltage from appearing at the output. It’s expressed in decibels (dB) and is calculated as: **PSRR = 20 × log(Vin_ripple / Vout_ripple)** This value is crucial for determining the stability and performance of the power supply. The higher the PSRR, the better the power supply is at rejecting noise and maintaining a clean output voltage. Different IC manufacturers specify PSRR values across various frequencies, and these should be considered when selecting a power supply for sensitive applications. To illustrate this, let's simulate the ripple using two different power supply types: the LM2941 (an LDO) and the TPS5430 (a switching power supply). For simplicity, we'll apply a 1V peak-to-peak noise signal with a frequency of 100kHz. The circuit for the LM2941 is shown below: [Image: Circuit diagram of LM2941] After applying the noise, the ideal 5V output becomes distorted, resulting in a ripple of approximately 500µV. This gives us the following PSRR calculation: **PSRR = 20 × log(1V / 500µV) = 66dB** Checking the datasheet for the LM2941 confirms that the PSRR at 100kHz meets the required specifications. [Image: PSRR vs. frequency graph for LM2941] Now, let's look at the TPS5430 switching power supply under the same conditions. The simulation results show a much higher ripple at the output, around 13mV. Using the same formula: **PSRR = 20 × log(1V / 13mV) = 37dB** As you can see, there's a noticeable difference between the two. While switching power supplies are more efficient, their ripple performance is significantly worse than that of LDOs. This makes LDOs a better choice in applications where low noise and high signal integrity are critical, such as in audio circuits, precision sensors, or RF systems. In conclusion, while switching power supplies excel in efficiency and versatility, LDOs still hold their ground in scenarios where minimal ripple is essential. Understanding the trade-offs between them helps engineers make informed decisions based on the specific requirements of their design.

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