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LTC1627 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1627 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LTC1627 frequency. The actual reduction in average current is less than for peak current. The basic LTC1627 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. Inductor Value Calculation The inductor selection will depend on the operating fre- quency of the LTC1627. The internal preset frequency is 350kHz, but can be externally synchronized up to 525kHz. 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 internal 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 low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is ∆IL = 0.4(IMAX). 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 200mA. 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 afford 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 increase. Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can con- centrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that induc- tance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Kool M µ (from Magnetics, Inc.) is a very good, low loss core material for toroids with a “soft” saturation character- istic. Molypermalloy is slightly more efficient at high (>200kHz) switching frequencies but quite a bit more expensive. Toroids are very space efficient, especially when you can use several layers of wire, while inductors wound on bobbins are generally easier to surface mount. New designs for surface mount are available from Coiltronics, Coilcraft and Sumida. CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CI VV V V IN MAX OUT IN OUT IN required IRMS ≅ − () []12/ This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. Always consult the manufacturer if there is any question. Kool M µ is a registered trademark of Magnetics, Inc. APPLICATIO S I FOR ATIO |
Similar Part No. - LTC1627_15 |
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Similar Description - LTC1627_15 |
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