Datasheet LT3999 (Analog Devices) - 10

LT3999
APPLICATIONS INFORMATION Turns Ratio Winding Resistance
The turns ratio of the transformer determines the output Resistance in either the primary or secondary winding voltage. The following equation is used as a first pass to reduces overall efficiency and degrades load regulation. calculate the turns ratio: If efficiency or load regulation is unsatisfactory, verify N that the voltage drops in the transformer windings are S V = OUT + VF ( ) not excessive. NP 2 VIN – VSW DC
Capacitors
where VF is the forward voltage of the output diode, VSW In applications with full duty cycle operation, the input is the voltage drop across the internal switches (see the supply current is approximately constant. Therefore, large Typical Performance curves) and DC is the duty cycle. input “hold-up type” capacitors are not necessary. A low Sufficient margin should be added to the turns ratio to value (>4.7µF), low ESR ceramic will be adequate to filter account for voltage drops due to transformer winding high frequency noise at the input. The output capacitors resistance. supply energy to the output load only during switch
Magnetizing Current
transitions. Therefore, large capacitance values are not necessary on the output. The magnetizing inductance of the transformer causes a ripple current that is independent of load current. This Transformer winding capacitance between the isolated ripple current is calculated by: primary and secondary has parasitic currents that can cause noise on the grounds. Providing a high frequency, ∆I= VIN •DC low impedance path between the primary and secondary gives the parasitic currents a local return path. A 2.2nF, fSW •LM 1kV ceramic capacitor is recommended. where ∆I and LM are primary ripple current and magnetizing inductance referred to the primary side of the transformer,
Optional LC Filter
respectively. Increasing the transformer magnetizing in- An optional LC filter, as shown on the Typical Application ductance, LM, reduces the ripple current. The ripple current on the first page of this data sheet, should be included if formula shows the effect of the switching frequency on ultralow noise and ripple are required. It is recommended the magnetizing inductance. Setting the LT3999 at high that the corner frequency of the filter should be set a switching frequency reduces the ripple current for the decade below the switching frequency so that the switch same magnetizing inductance. Therefore, it is possible to noise is attenuated by a factor of 100. For example, if the reduce the transformer turns and still achieve low ripple f current. This helps to reduce the power converter footprint OSC = 100kHz, then fCORNER = 10kHz where: as well. The transformer magnetizing inductance should f be designed for the worst-case duty cycle and input line CORNER = 1 2•π LC voltage combination.
Switching Diode Selection
A good rule of thumb is to set the primary current ripple A fast recovery, surface mount diode such as a Schottky amplitude 10% to 30% of the average primary current, IP: is recommended. The proximity of the diodes to the P transformer outputs is important and should be as close I OUT P = as possible with short, wide traces connecting them. VIN •eff where POUT is the output power of the converter and eff is the converter efficiency, typically around 85%. 3999fa 10 For more information www.linear.com/LT3999 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts