Datasheet LTC1701, LTC1701B (Analog Devices) - 6

LTC1701/LTC1701B
U U W U APPLICATIO S I FOR ATIO
Accepting larger values of ∆IL allows the use of low
Catch Diode Selection
inductances, but results in higher output voltage ripple The diode D1 shown in Figure 1 conducts during the off- and greater core losses. A reasonable starting point for time. It is important to adequately specify the diode peak setting ripple current is ∆IL = 0.4A. current and average power dissipation so as not to exceed The inductor value also has an effect on low current the diode ratings. operation. Lower inductor values (higher ∆IL) will cause Losses in the catch diode depend on forward drop and Burst Mode operation to begin at higher load currents, switching times. Therefore, Schottky diodes are a good which can cause a dip in efficiency in the upper range of choice for low drop and fast switching times. low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to de- Since the catch diode carries the load current during the crease. off-time, the average diode current is dependent on the switch duty cycle. At high input voltages, the diode con-
Inductor Core Selection
ducts most of the time. As VIN approaches VOUT, the diode conducts only a small fraction of the time. The most Once the value for L is selected, the type of inductor must stressful condition for the diode is when the regulator be chosen. Basically, there are two kinds of losses in an output is shorted to ground. inductor —core and copper losses. Under short-circuit conditions (V Core losses are dependent on the peak-to-peak ripple OUT = 0V), the diode must safely handle I current and core material. However, it is independent of SC(PK) at close to 100% duty cycle. Under normal load conditions, the average current con- the physical size of the core. By increasing inductance, the ducted by the diode is simply: peak-to-peak inductor ripple current will decrease, there- fore reducing core loss. Unfortunately, increased induc-  V V  tance requires more turns of wire and, therefore, copper I = − I IN OUT DIODE(avg) LOAD(avg) +  losses will increase. V V IN D High efficiency converters generally cannot afford the core Remember to keep lead lengths short and observe proper loss found in low cost powdered iron cores, forcing the grounding (see Board Layout Considerations) to avoid use of more expensive ferrite, molypermalloy or Kool Mµ® ringing and increased dissipation. cores. Ferrite designs have very low core loss and are The forward voltage drop allowed in the diode is calculated preferred at high switching frequencies. Ferrite core ma- from the maximum short-circuit current as: terial saturates “hard,” which means that inductance col- lapses abruptly when the peak design current is exceeded.  P  V + V  This results in an abrupt increase in inductor ripple current V D IN D D ≈ ( )    and consequent output voltage ripple. Do not allow the I  V SC avg IN core to saturate! where PD is the allowable diode power dissipation and will Molypermalloy (from Magnetics, Inc.) is a very good, low be determined by efficiency and/or thermal requirements loss core material for toroids, but it is more expensive than (see Efficiency Considerations). ferrite. A reasonable compromise from the same manu- Most LTC1701 circuits will be well served by either an facturer is Kool Mµ core material. Toroids are very space MBR0520L or an MBRM120L. An MBR0520L is a good efficient, expecially when you can use several layers of choice for I wire. Because they generally lack a bobbin, mounting is OUT(MAX) ≤ 500mA, as long as the output doesn’t need to sustain a continuous short. more difficult. However, surface mount designs that do not increase the height significantly are available Kool Mµ is a registered trademark of Magnetics, Inc. 6