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LT8331 Datasheet(HTML) 18 Page - Analog Devices |
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LT8331 Datasheet(HTML) 18 Page - Analog Devices |
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LT8331 18 Rev. C For more information www.analog.com LLK is the leakage inductance of the primary winding, which is usually specified in the transformer character- istics. LLK can be obtained by measuring the primary inductance with the secondary windings shorted. The snubber capacitor value (CSN) can be determined using the following equation: CSN = VSN ΔVSN • RSN • fOSC where ∆VSN is the voltage ripple across CSN. A reasonable ∆VSN is 5% to 10% of VSN. The reverse voltage rating of DSN should be higher than the sum of VSN and VIN(MAX). Flyback Converter: Output Diode Selection The output diode in a flyback converter is subject to large RMS current and peak reverse voltage stresses. A fast switching diode with a low forward drop and a low reverse leakage is desired. Schottky diodes are recommended if the output voltage is below 100V. Approximate the required peak repetitive reverse voltage rating VRRM using: VRRM > NS NP • VIN(MAX) + VOUT The power dissipated by the diode is: PD = IO(MAX) • VD and the diode junction temperature is: TJ = TA + PD • RθJA The RθJA to be used in this equation normally includes the RθJC for the device, plus the thermal resistance from the board to the ambient temperature in the enclosure. TJ must not exceed the diode maximum junction tem- perature rating. Flyback Converter: Output Capacitor Selection The output capacitor of the flyback converter has a similar operation condition as that of the boost converter. Refer to the Boost Converter: Output Capacitor Selection section for the calculation of COUT and ESRCOUT. The RMS ripple current rating of the output capacitors in continuous operation can be determined using the fol- lowing equation: IRMS(COUT),CONTINUOUS ≈ IO(MAX) • DMAX 1 − DMAX Flyback Converter: Input Capacitor Selection The input capacitor in a flyback converter is subject to a large RMS current due to the discontinuous primary current. To prevent large voltage transients, use a low ESR input capacitor sized for the maximum RMS current. The RMS ripple current rating of the input capacitors in continuous operation can be determined using the fol- lowing equation: IRMS(CIN),CONTINUOUS ≈ POUT(MAX) VIN(MIN) • η • 1−DMAX DMAX SEPIC CONVERTER APPLICATIONS The LT8331 can be configured as a SEPIC (single-ended primary inductance converter), as shown in Figure 8. This topology allows for the input to be higher, equal, or lower than the desired output voltage. The conversion ratio as a function of duty cycle is: VOUT + VD VIN = D 1 − D in continuous conduction mode (CCM). APPLICATIONS INFORMATION Figure 8. LT8331 Configured in a SEPIC Topology L1 L2 VOUT VIN SW FBX GND EN/UVLO LT8331 VIN VCC INT D1 CIN COUT CDC 8331 F08 |
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