POWER Voltage accuracy of
switch-mode power supplies By Frederik Dostal, power management expert, Analog Devices
N
o switch-mode power supply is perfectly accurate. There are different tolerances
that influence the precision of the output voltage regulation. This article explains the different sources of inaccuracy and shows how the total tolerance range is established. Switch-mode power supplies have a feedback pin with which an output voltage can be set. This voltage is regulated by the power supply circuit. However, the question arises as to how precisely the voltage is regulated. Figure 1 shows a switch-mode power supply according to the step-down principle.
An existing, usually unregulated, supply voltage (input voltage) is used to generate an output voltage, which is regulated as accurately as possible.
The accuracy of the output voltage is determined by several influences and tolerances.
DC tolerance
The DC regulation deviation includes the accuracy of the reference voltage
Figure 1. Structure of a switch-mode power supply circuit according to the step-down principle.
contained in the voltage converter (Figure 1). This has been created by the designer of the integrated circuit in such a way that only minor deviations occur within the supply voltage range, the manufacturing tolerances of the IC, and the operating temperature. The existing tolerance is
included in the data sheets of the voltage converter ICs. This value can be specified in different ways. It can be communicated as reference voltage line regulation, as output voltage load regulation, and as regulated feedback voltage. In addition to the accuracy of the pure reference voltage, the latter also includes other IC-specific DC tolerances at the feedback pin. The DC tolerance also includes the deviation of the resistance values of the resistor divider between the regulated output voltage and the feedback pin (Rfb1 and Rfb2 in Figure 1). These resistors can be purchased with different tolerance ranges. Usually, a resistor has a resistance value close to the maximum specified tolerance. Better resistors, with a smaller tolerance, are detected during manufacturing of these resistors and are typically sold as more accurate resistors.
Output voltage ripple Figure 2. Measurement of load transients in the time domain with an LTpowerAnalyzer.
The regulated output voltage always has a certain voltage ripple in a switch-mode power supply. This ripple is determined by the rise and fall of the inductor current of a buck regulator and the value of the output capacitor and its equivalent series resistance (ESR). The voltage ripple can be reduced by a high inductance value of the coil and a large output capacitor with a low ESR, but the output voltage ripple cannot
12 DECEMBER/JANUARY 2026 | ELECTRONICS FOR ENGINEERS
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