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LTC1773EMS-TRPBF Datasheet(PDF) 8 Page - Linear Technology |
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LTC1773EMS-TRPBF Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  LTC1773 8 1773fb inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Molypermalloy (from Magnetics, Inc.) is a very good, low loss core material for toroids, but it is more expensive than ferrite. A reasonable compromise from the same manu- facturer is Kool M µ. Toroids are very space efficient, especially when you can use several layers of wire. Be- cause they generally lack a bobbin, mounting is more difficult. However, new designs for surface mount are available which do not increase the height significantly. Power MOSFET and Schottky Diode Selection Two external power MOSFETs must be selected for use with the LTC1773: a P-channel MOSFET for the top (main) switch, and an N-channel MOSFET for the bottom (syn- chronous) switch. The peak-to-peak gate drive levels are set by the VIN voltage. Therefore, for VIN > 5V, logic-level threshold MOSFETs should be used. But, for VIN < 5V, sub-logic level threshold MOSFETs (VGS(TH) < 3V) should be used. In these applications, make sure that the VIN to the LTC1773 is less than 8V because the absolute maximum VGS rating of the majority of these sub-logic threshold MOSFETs is 8V. Selection criteria for the power MOSFETs include the “ON” resistance RDS(ON), reverse transfer capacitance CRSS, input voltage, maximum output current, and total gate charge. When the LTC1773 is operating in continuous mode the duty cycles for the top and bottom MOSFETs are given by: Main Switch Duty Cycle = VOUT/VIN Synchronous Switch Duty Cycle = (VIN – VOUT)/VIN The MOSFET power dissipations at maximum output current are given by: P V V IR KV I C f MAIN OUT IN MAX DSON IN MAX RSS = () + () + () ( )( )( ) 2 2 1 δ APPLICATIONS INFORMATION The operating frequency and inductor selection are inter- related in that higher operating frequencies allow the use of smaller inductor and capacitor values. However, oper- ating at a higher frequency generally results in lower efficiency because of external MOSFET gate charge losses. The inductor value has a direct effect on ripple current. The ripple current, ∆IL, decreases with higher inductance or frequency and increases with higher VIN or VOUT. ∆I fL V V V L OUT OUT IN = ()( ) ⎛ ⎝⎜ ⎞ ⎠⎟ 1 1– (1) Accepting larger values of ∆IL allows the use of lower inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is 30% to 40% of IMAX. Remember, the maximum ∆IL occurs at the maximum input voltage. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 1/3 its original value. Lower inductor values (higher ∆IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot af- ford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy, or Kool M µ® cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will in- crease. Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard”, which means that inductance collapses abruptly when the peak design cur- rent is exceeded. This results in an abrupt increase in |
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