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LTC1877 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1877 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 18 page  LTC1877 10 1877fb APPLICATIONS INFORMATION Accepting larger values of ΔIL allows the use of low in- ductance, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is ΔIL = 0.4(IMAX). The inductor value also has an effect on Burst Mode opera- tion. The transition to low current operation begins when the inductor current peaks fall to approximately 250mA. Lower inductor values (higher ΔIL) 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 Mμ 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 pre- ferred 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 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 Mμ (from Magnetics, Inc.) is a very good, low loss core material for toroids with a soft saturation characteristic. Molypermalloy is slightly more efficient at high (>200kHz) switching frequencies but quite a bit more expensive. To- roids 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 inductors are available from Coiltronics, Coilcraft, Dale 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: CIN required IRMS ≅ IOMAX VOUT VIN − VOUT () ⎡⎣ ⎤⎦ 1/2 VIN This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note the 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. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- ment is satisfied, the capacitance is adequate for filtering. The output ripple ΔVOUT is determined by: ΔVOUT ≅ΔIL ESR+ 1 8fCOUT ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. The output ripple is highest at maximum input voltage since ΔIL increases with input voltage. For the LTC1877, the general rule for proper operation is: COUT required ESR < 0.25Ω The choice of using a smaller output capacitance increases the output ripple voltage due to the frequency dependent term but can be compensated for by using capacitor(s) of very low ESR to maintain low ripple voltage. The ITH pin compensation components can be optimized to provide stable high performance transient response regardless of the output capacitor selected. ESR is a direct function of the volume of the capacitor. Manufacturers such as Taiyo Yuden, AVX, Sprague, Kemet and Sanyo should be considered for high performance ca- |
Similar Part No. - LTC1877_15 |
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Similar Description - LTC1877_15 |
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