Datasheet LT8331 (Analog Devices) - 18

link to page 18 LT8331
APPLICATIONS INFORMATION
LLK is the leakage inductance of the primary winding, The RMS ripple current rating of the output capacitors in which is usually specified in the transformer character- continuous operation can be determined using the fol- istics. LLK can be obtained by measuring the primary lowing equation: inductance with the secondary windings shorted. The snubber capacitor value (C D SN) can be determined using I MAX RMS(COUT),CONTINUOUS ≈ IO(MAX) • the following equation: 1 − DMAX V C SN SN =
Flyback Converter: Input Capacitor Selection
ΔVSN • RSN • fOSC The input capacitor in a flyback converter is subject to where ∆VSN is the voltage ripple across CSN. A reasonable a large RMS current due to the discontinuous primary ∆VSN is 5% to 10% of VSN. The reverse voltage rating of current. To prevent large voltage transients, use a low DSN should be higher than the sum of VSN and VIN(MAX). ESR input capacitor sized for the maximum RMS current. The RMS ripple current rating of the input capacitors in
Flyback Converter: Output Diode Selection
continuous operation can be determined using the fol- The output diode in a flyback converter is subject to large lowing equation: RMS current and peak reverse voltage stresses. A fast POUT(MAX) 1−DMAX switching diode with a low forward drop and a low reverse IRMS(CIN),CONTINUOUS ≈ • VIN(MIN) • η DMAX leakage is desired. Schottky diodes are recommended if the output voltage is below 100V. Approximate the required peak repetitive reverse voltage
SEPIC CONVERTER APPLICATIONS
rating VRRM using: The LT8331 can be configured as a SEPIC (single-ended primary inductance converter), as shown in Figure 8. This N V S RRM > • V topology allows for the input to be higher, equal, or lower N IN(MAX) + VOUT P than the desired output voltage. The conversion ratio as a function of duty cycle is: The power dissipated by the diode is: V D P OUT + VD = D = IO(MAX) • VD VIN 1 − D and the diode junction temperature is: in continuous conduction mode (CCM). TJ = TA + PD • RθJA L1 CDC D1 The R VIN VOUT θJA to be used in this equation normally includes C the R IN COUT L2 θJC for the device, plus the thermal resistance from the board to the ambient temperature in the enclosure. V T IN SW J must not exceed the diode maximum junction tem- LT8331 perature rating. EN/UVLO FBX
Flyback Converter: Output Capacitor Selection
V INT CC GND 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 8331 F08 for the calculation of COUT and ESRCOUT.
Figure 8. LT8331 Configured in a SEPIC Topology
Rev. C 18 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts