Datasheet LT3439 (Analog Devices) - 10

LT3439
U U W U APPLICATIO S I FOR ATIO
The primary inductance is then calculated:
Optional LC Filter
1 1 An optional LC filter, as shown on the Typical Application f on the first page of this data sheet, should be included if L = V OSC kHz = 5 100 = 467 H PRI IN µ ultralow noise and ripple are required. It is recommended I ∆ 0 107 . A PRI that the corner frequency of the filter should be set a ∆I = 15% of IPRI = 0 1 . 5 • 0 714 . A = 0 107 . A decade below the switching frequency so that the switch Next, build a transformer with the calculated values of noise is attenuated by a factor of 100. For example, if the turns ratio and primary inductance. Minimize resistance in fOSC = 100kHz, then fCORNER = 10kHz where: the windings. The turns ratio can be tweaked to get the 1 specified output voltage. fCORNER 2 • π LC
Capacitors Output Voltage Regulation
The DC transformer topology runs effectively at 100% duty cycle (50% each side). This means that the input The output voltage of the DC transformer topology is supply current is approximately constant. Therefore, large unregulated. Variations in the input voltage will cause the “hold-up type” capacitors are not necessary. A low value output voltage to vary because the output voltage is a (> 4.7µF), low ESR ceramic will be adequate to filter high function of the input voltage and the transformer turn frequency noise at the input. ratio. Also, variations in the output load will cause the output voltage to change because of circuit parasitics, The output capacitors supply energy to the output load only such as the transformer DC resistance and power switch during switch transitions. Therefore, large capacitance on resistance. If regulation is necessary, a post regulator values are not necessary. Low ESR, surface mount capaci- such as a linear regulator can be added to the output of the tors such as ceramic, OS-CON of POSCAPs are recom- supply. See the Typical Applications for examples of mended. An additional LC filter can be added in addition to adding a linear regulator. the output capacitor to further reduce output noise. Transformer winding capacitance between the isolated
More Help
primary and secondary have parasitic currents that can AN70: “A Monolithic Switching Regulator with 100µV cause noise on the grounds. Providing a high frequency, Output Noise” contains much information concerning low impedance path between the primary and secondary applications and noise measurement techniques. gives the parasitic currents a local return path. A 2.2nF, 1kV ceramic capacitor is recommended. AN19: “LT1070 Design Manual” AN29: “Some Thoughts on DC-DC Converters” also have
Switching Diode Selection
general knowledge on switching regulators. A fast recovery, surface mount diode such as a Schottky An LTC SwitcherCADTM model is available to verify design is recommended. The proximity of the diodes to the performance. transformer outputs is important and should be as close as possible with short, wide traces connecting them. The LTC Applications department is always ready to lend a helping hand. SwitcherCAD is a trademark of Linear Technology Corporation. sn3439 3439fs 10