Analysis of the power suppression ratio of LDO

Power Supply Rejection Ratio (PSRR) refers to the ratio of the input ripple voltage to the output ripple voltage, in decibels (dB), and its expression is:

The significance of PSRR lies in quantifying the degree of transmission of changes in the power input to the output. For example, if the PSRR of a certain circuit is 60dB, then for every 1V change in the power supply input voltage, the output will only fluctuate by 1mV. This indicator is particularly important in analog circuits (such as op amps, voltage regulators) because power supply noise can directly lead to signal distortion or system performance degradation.

The factors affecting the PSRR of the LDO power supply are not only related to the ripple frequency, but also to the load current, input and output voltage difference, and output capacitor capacitance value. Specifically, it is reflected in: the smaller the voltage drop, the larger the load current, the higher the frequency, and the smaller the output capacitor, the lower the PSRR. Therefore, when a high common mode rejection ratio is required, it is necessary to appropriately increase the voltage difference, increase the output capacitor and reduce the load current. Figure 1 shows a schematic diagram of the relationship between the PSRR and frequency of the AP2210 ultra-low dropout difference power supply chip. From the diagram, it can be seen that its PSRR can reach 75dB at low frequency.

Picture 1  PSRR vs Frequency

Usually, the LDO power supply has a high PSRR, so the output ripple is low. Therefore, when designing the power supply circuit of the sensor, especially the sensor with analog output, in order to ensure the accuracy of the sensor, a low-noise and small ripple LDO is usually used to power the sensor. The output ripple of the switching power supply is large and is greatly affected by the load size. In terms of efficiency, the switching power supply is higher than that of LDO and has a smaller heat generation. Therefore, the switching power supply is generally used in combination with LDO, and the high power supply suppression ratio of LDO is used to suppress the output ripple of the switching power supply.

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