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LT8331 Datasheet(HTML) 14 Page  Analog Devices 

LT8331 Datasheet(HTML) 14 Page  Analog Devices 
LT8331 14 Rev. C For more information www.analog.com inductance forms a high quality (under damped) tank cir cuit. If the LT8331 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8331’s voltage rating. This situation is easily avoided (see Application Note 88). BOOST CONVERTER: OUTPUT CAPACITOR SELECTION Low ESR (equivalent series resistance) capacitors should be used at the output to minimize the output ripple volt age. Multilayer ceramic capacitors are an excellent choice, as they are small and have extremely low ESR. Use X5R or X7R types. This choice will provide low output ripple and good transient response. A 10µF to 47µF output capacitor is sufficient for most applications, but systems with very low output currents may need only a 1µF or 2.2µF out put capacitor. Solid tantalum or OSCON capacitor can be used, but they will occupy more board area than a ceramic and will have a higher ESR. Always use a capacitor with a sufficient voltage rating. Contributions of ESR (equivalent series resistance), ESL (equivalent series inductance) and the bulk capacitance must be considered when choosing the correct output capacitors for a given output ripple voltage. The effect of these three parameters (ESR, ESL and bulk C) on the out put voltage ripple waveform for a typical boost converter is illustrated in Figure 4. between ∆VESR and ∆VCOUT. This percentage ripple will change, depending on the requirements of the applica tion, and the following equations can easily be modi fied. For a 1% contribution to the total ripple voltage, the ESR of the output capacitor can be determined using the following equation: ESRCOUT ≤ 0.01 • VOUT ID(PEAK) For the bulk C component, which also contributes 1% to the total ripple: COUT ≥ IO(MAX) 0.01 • VOUT • fOSC The output capacitor in a boost regulator experiences high RMS ripple currents, as shown in Figure 4. The RMS ripple current rating of the output capacitor can be deter mined using the following equation: IRMS(COUT) ≥ IO(MAX) • DMAX 1 − DMAX Multiple capacitors are often paralleled to meet ESR requirements. Typically, once the ESR requirement is sat isfied, the capacitance is adequate for filtering and has the required RMS current rating. Additional ceramic capaci tors in parallel are commonly used to reduce the effect of parasitic inductance in the output capacitor, which reduces high frequency switching noise on the converter output. CERAMIC CAPACITORS Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT8331 due to their piezoelectric nature. When in Burst Mode operation, the LT8331’s switching frequency depends on the load current, and at very light loads the LT8331 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT8331 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance Figure 4. The Output Ripple Waveform of a Boost Converter VOUT (AC) tON ∆VESR RINGING DUE TO TOTAL INDUCTANCE (BOARD + CAP) ∆VCOUT 8331 F04 tOFF The choice of component(s) begins with the maximum acceptable ripple voltage (expressed as a percentage of the output voltage), and how this ripple should be divided between the ESR step ∆VESR and the charging/discharg ing ∆VCOUT. For the purpose of simplicity, we will choose 2% for the maximum output ripple, to be divided equally APPLICATIONS INFORMATION 
