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LT8331 Datasheet(HTML) 21 Page - Analog Devices |
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LT8331 Datasheet(HTML) 21 Page - Analog Devices |
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LT8331 21 Rev. C For more information www.analog.com CDC has nearly a rectangular current waveform. During the switch off-time, the current through CDC is IIN, while approximately –IO flows during the on-time. The RMS rating of the coupling capacitor is determined by the fol- lowing equation: IRMS(CDC) > IO(MAX) • VOUT + VD VIN(MIN) A low ESR and ESL, X5R or X7R ceramic capacitor works well for CDC. INVERTING CONVERTER APPLICATIONS The LT8331 can be configured as a dual-inductor invert- ing topology, as shown in Figure 10. The VOUT to VIN ratio is: VOUT+ VD VIN = D 1– D Conversely, the minimum duty cycle (DMIN) occurs when the converter operates at the maximum input voltage : DMIN = VOUT + VD VOUT + VD+ VIN(MAX) Be sure to check that DMAX and DMIN obey : DMAX < 1 – Minimum Off-Time(MAX) • fOSC(MAX) and DMIN > Minimum On-Time(MAX) • fOSC(MAX) where Minimum Off-Time, Minimum On-Time and fOSC are specified in the Electrical Characteristics table. Inverting Converter: Inductor, Output Diode and Input Capacitor Selections The selections of the inductor, output diode and input capacitor of an inverting converter are similar to those of the SEPIC converter. Please refer to the corresponding SEPIC converter sections. Inverting Converter: Output Capacitor Selection The inverting converter requires much smaller output capacitors than those of the boost, flyback and SEPIC converters for similar output ripples. This is due to the fact that, in the inverting converter, the inductor L2 is in series with the output, and the ripple current flowing through the output capacitors are continuous. The output ripple voltage is produced by the ripple current of L2 flow- ing through the ESR and bulk capacitance of the output capacitor: ΔVOUT(P–P) = ΔIL2 • ESRCOUT + 1 8 • fOSC • COUT ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ After specifying the maximum output ripple, the user can select the output capacitors according to the preceding equation. Figure 10. A Simplified Inverting Converter CDC VIN CIN L1 D1 COUT VOUT 8331 F10 + GND LT8331 SW L2 + – + – + APPLICATIONS INFORMATION in continuous conduction mode (CCM). Inverting Converter: Switch Duty Cycle and Frequency For an inverting converter operating in CCM, the duty cycle of the main switch can be calculated based on the negative output voltage (VOUT) and the input voltage (VIN). The maximum duty cycle (DMAX) occurs when the con- verter has the minimum input voltage: DMAX = VOUT + VD VOUT + VD + VIN(MIN) |
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