Understanding Output Voltage Ripple in DCM Operation of D-CAP Buck Converters

Abstract

Output voltage ripple is a key parameter for buck converter, which can make great impact on system application and should be paid more attention on. The principle for generation of output voltage ripple for buck converter in CCM operation is well illustrated in many text books and application notes. However, when focusing on the output voltage ripple in DCM operation, not many detailed description on it. This application note can introduce a method for estimating output voltage ripple in DCM operation for buck converter with D-CAP, D-CAP2 and D-CAP3 mode. Results of bench tests can be given based on TPS56837. Error analysis is also included to illustrate what kind of factors can affect test results. In the conclusion of this paper, a comparison between output voltage ripple in DCM operation and CCM operation in D-CAP buck converters can be made.

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1 Review of CCM Output Voltage Ripple in Buck Converter

Output voltage ripple is a key parameter for buck converter, which could make great impact on system application and should be paid more attention on. The principle for generation of output voltage ripple for buck converter is well illustrated in many text books and application notes. A brief review of CCM output voltage ripple in buck converter will be given first.

For CCM operation, inductor current increases from its valley value at the beginning of switch cycle, reaching its peak value when high-side FET is OFF and going back to its valley value when the switch cycle is finished. The average inductor current is loading current.

When only considering impacts of output capacitors, output voltage ripple is raised by charging and discharging process of output capacitors. Excessive energy marked with Q1 in Figure 1-1 will be charged to C_OUT, leading to the rise of voltage on C_OUT. When the inductor current is lower than loading current, C_OUT will discharge to maintain V_OUT stable. So output voltage ripple will be calculated by Equation 1 and Equation 2.

Equation 1.

Q 1 = \frac{∆ I L \times T_{S W}}{8}

Equation 2.

∆ V_{O U T} = \frac{Q_{1}}{C_{O U T}} = \frac{∆ I L \times T_{S W}}{8 \times C_{O U T}}

Q1: Excessive energy

T_SW: Switching period

ΔV_OUT: Output voltage ripple

C_OUT: Value of output capacitors

Figure 1-1 Inductor Current Illustration for CCM Operation