Datasheet LTC1877 (Analog Devices) - 10

LTC1877
APPLICATIONS INFORMATION
Accepting larger values of ΔIL allows the use of low in-
CIN and COUT Selection
ductance, 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 V ripple current is ΔI OUT/VIN. To prevent large L = 0.4(IMAX). voltage transients, a low ESR input capacitor sized for the The inductor value also has an effect on Burst Mode opera- maximum RMS current must be used. The maximum RMS tion. The transition to low current operation begins when capacitor current is given by: the inductor current peaks fall to approximately 250mA. ⎡ 1/2 V ⎣ ⎤ OUT ( VIN − VOUT )⎦ Lower inductor values (higher ΔIL) will cause this to occur CIN required IRMS ≅ IOMAX at lower load currents, which can cause a dip in effi ciency VIN in the upper range of low current operation. In Burst Mode This formula has a maximum at VIN = 2VOUT, where IRMS operation, lower inductance values will cause the burst = IOUT/2. This simple worst-case condition is commonly frequency to increase. used for design because even signifi cant deviations do not offer much relief. Note the capacitor manufacturer’s
Inductor Core Selection
ripple current ratings are often based on 2000 hours of Once the value for L is known, the type of inductor must life. This makes it advisable to further derate the capacitor, be selected. High effi ciency converters generally cannot or 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 meet forcing the use of more expensive ferrite, molypermalloy, size or height requirements in the design. Always consult or Kool Mμ cores. Actual core loss is independent of core the manufacturer if there is any question. size for a fi xed inductor value, but it is very dependent The selection of C on inductance selected. As inductance increases, core OUT is driven by the required effective series resistance (ESR). Typically, once the ESR require- losses go down. Unfortunately, increased inductance ment is satisfi ed, the capacitance is adequate for fi ltering. requires more turns of wire and therefore copper losses The output ripple ΔV will increase. OUT is determined by: ⎛ 1 ⎞ Ferrite designs have very low core losses and are pre- ΔVOUT ≅ ΔIL ES ⎜ R + ⎟ ferred at high switching frequencies, so design goals can ⎝ 8fC OUT ⎠ concentrate on copper loss and preventing saturation. where f = operating frequency, COUT = output capacitance Ferrite core material saturates hard, which means that and ΔIL = ripple current in the inductor. The output ripple inductance collapses abruptly when the peak design current is highest at maximum input voltage since ΔIL increases is exceeded. This results in an abrupt increase in inductor with input voltage. For the LTC1877, the general rule for ripple current and consequent output voltage ripple. Do proper operation is: not allow the core to saturate! COUT required ESR < 0.25Ω Kool Mμ (from Magnetics, Inc.) is a very good, low loss core material for toroids with a soft saturation characteristic. The choice of using a smaller output capacitance increases Molypermalloy is slightly more effi cient at high (>200kHz) the output ripple voltage due to the frequency dependent switching frequencies but quite a bit more expensive. To- term but can be compensated for by using capacitor(s) of roids are very space effi cient, especially when you can use very low ESR to maintain low ripple voltage. The ITH pin several layers of wire, while inductors wound on bobbins compensation components can be optimized to provide are generally easier to surface mount. New designs for stable high performance transient response regardless of surface mount inductors are available from Coiltronics, the output capacitor selected. Coilcraft, Dale and Sumida. 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- 1877fb 10