Datasheet LT3958 (Analog Devices) - 19

LT3958
applicaTions inForMaTion
The snubber resistor value (RSN) can be calculated by the
Flyback Converter: Output Capacitor Selection
following equation: The output capacitor of the flyback converter has a similar N operation condition as that of the boost converter. Refer to V2 P SN − VSN • VOUT • N the Boost Converter: Output Capacitor Selection section R S SN = 2 • I2 for the calculation of COUT and ESRCOUT. SW(PEAK) • LLK • ƒ The RMS ripple current rating of the output capacitors LLK is the leakage inductance of the primary winding, in continuous operation can be determined using the which is usually specified in the transformer character- following equation: istics. LLK can be obtained by measuring the primary inductance with the secondary windings shorted. The D I MAX snubber capacitor value (C RMS(COUT),CONTINUOUS ≈ IO(MAX) • SN) can be determined using 1−DMAX the following equation: V C SN
Flyback Converter: Input Capacitor Selection
SN = ∆VSN •RSN • ƒ The input capacitor in a flyback converter is subject to where ∆V a large RMS current due to the discontinuous primary SN is the voltage ripple across CSN. A reasonable ∆V current. To prevent large voltage transients, use a low SN is 5% to 10% of VSN. The reverse voltage rating of D ESR input capacitor sized for the maximum RMS current. SN should be higher than the sum of VSN and VIN(MAX). The RMS ripple current rating of the input capacitors in
Flyback Converter: Output Diode Selection
continuous operation can be determined using the fol- lowing equation: The output diode in a flyback converter is subject to large 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 LT3958 can be configured as a SEPIC (single-ended N primary inductance converter), as shown in Figure 1. This V S RRM > • VIN(MAX) + V topology allows for the input to be higher, equal, or lower OUT NP 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 = V 1−D and the diode junction temperature is: IN T in continuous conduction mode (CCM). J = TA + PD • RθJA The R In a SEPIC converter, no DC path exists between the input θJA to be used in this equation normally includes the R and output. This is an advantage over the boost converter θJC for the device, plus the thermal resistance from the board to the ambient temperature in the enclosure. T for applications requiring the output to be disconnected J must not exceed the diode maximum junction temperature rating. from the input source when the circuit is in shutdown. 3958fa For more information www.linear.com/LT3958 19 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Applications Related Parts