Datasheet LTC1878 (Analog Devices) - 9

LTC1878
U U W U APPLICATIO S I FOR ATIO
1  V  New designs for surface mount inductors are available ∆I OUT L = ( V 1 from Coiltronics, Coilcraft, Dale and Sumida. f)(L) OUT −  V (1) IN  
C
Accepting larger values of ∆I
IN and COUT Selection
L allows the use of low inductance, but results in higher output voltage ripple and In continuous mode, the source current of the top MOSFET greater core losses. A reasonable starting point for setting is a square wave of duty cycle VOUT/VIN. To prevent large ripple current is ∆I voltage transients, a low ESR input capacitor sized for the L = 0.4(IMAX). maximum RMS current must be used. The maximum The inductor value also has an effect on Burst Mode RMS capacitor current is given by: operation. The transition to low current operation begins when the inductor current peaks fall to approximately / V [ (V −V )]1 2 250mA. Lower inductor values (higher ∆I OUT IN OUT L) will cause this C required I ≅ I IN RMS OMAX to occur at lower load currents, which can cause a dip in VIN efficiency in the upper range of low current operation. In This formula has a maximum at V Burst Mode operation, lower inductance values will cause IN = 2VOUT, where I the burst frequency to increase. RMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations do not offer much relief. Note the capacitor manufacturer’s
Inductor Core Selection
ripple current ratings are often based on 2000 hours of life. Once the value for L is known, the type of inductor must be This makes it advisable to further derate the capacitor, or selected. High efficiency converters generally cannot choose a capacitor rated at a higher temperature than afford the core loss found in low cost powdered iron cores, required. Several capacitors may also be paralleled to forcing the use of more expensive ferrite, molypermalloy, meet size or height requirements in the design. Always or Kool Mµ® cores. Actual core loss is independent of core consult the manufacturer if there is any question. size for a fixed inductor value, but it is very dependent on The selection of C inductance selected. As inductance increases, core losses OUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- go down. Unfortunately, increased inductance requires ment is satisfied, the capacitance is adequate for filtering. more turns of wire and therefore copper losses will The output ripple ∆V increase. OUT is determined by: Ferrite designs have very low core losses and are pre-  1  ferred at high switching frequencies, so design goals can ∆VOUT ≅ ∆I ESR L +  8fCOUT  concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that where f = operating frequency, COUT = output capacitance inductance collapses abruptly when the peak design cur- and ∆IL = ripple current in the inductor. The output ripple rent is exceeded. This results in an abrupt increase in is highest at maximum input voltage since ∆IL increases inductor ripple current and consequent output voltage with input voltage. For the LTC1878, the general rule for ripple. Do not allow the core to saturate! proper operation is: Kool Mµ (from Magnetics, Inc.) is a very good, low loss COUT required ESR < 0.25Ω core material for toroids with a “soft” saturation character- The choice of using a smaller output capacitance istic. Molypermalloy is slightly more efficient at high increases the output ripple voltage due to the frequency (>200kHz) switching frequencies but quite a bit more dependent term but can be compensated for by using expensive. Toroids are very space efficient, especially capacitor(s) of very low ESR to maintain low ripple when you can use several layers of wire, while inductors voltage. The I wound on bobbins are generally easier to surface mount. TH pin compensation components can be Kool Mµ is a registered trademark of Magnetics, Inc. 9