Datasheet LTC1779 (Analog Devices) - 8

LTC1779
applicaTions inForMaTion
A smaller value than LMIN could be used in the circuit; and to understand how it is going to work over the entire however, the inductor current will not be continuous input voltage range. during burst periods.
Inductor Core Selection RSENSE Selection for Output Current
Once the value for L is known, the type of inductor must The selection of RSENSE determines the output current be selected. High efficiency converters generally cannot limit, the maximum possible output current before the afford the core loss found in low cost powdered iron cores, internal current limit threshold is reached. IOUT(MAX), the forcing the use of more expensive ferrite, molypermalloy maximum specified output current in a design, must be or Kool Mu® cores. Actual core loss is independent of core less than ICL. With the current comparator monitoring size for a fixed inductor value, but it is very dependent the voltage developed across RSENSE, the threshold of the on inductance selected. As inductance increases, core comparator determines the inductor’s peak current. The losses go down. Unfortunately, increased inductance re- maximum output current, ICL, the LTC1779 can provide quires more turns of wire and therefore copper losses will is given by: increase. Ferrite designs have very low core losses and are ⎛ ⎞ preferred at high switching frequencies, so design goals I ⎛ ⎞ 0.12V CL =M SF ⎜ ⎟⎜ ⎟– IRIPPLE can concentrate on copper loss and preventing satura- ⎝100 ⎠⎝RSENSE +2Ω⎠ 2 tion. Ferrite core material saturates “hard,” which means where SF and M are as defined in the previous section, that inductance collapses abruptly when the peak design Figures 2 and 3. Typically, R current is exceeded. This results in an abrupt increase in SENSE is chosen between 0Ω and 20Ω. Current limit is at a minimum at minimum inductor ripple current and consequent output voltage input voltage and maximum at maximum input voltage. ripple. Do not allow the core to saturate! Both conditions should be considered in a design where Molypermalloy (from Magnetics, Inc.) is a very good, low current limit is important. loss core material for toroids, but it is more expensive To calculate several current limit conditions and choose than ferrite. A reasonable compromise from the same the best sense resistor for your design, first use minimum manufacturer is Kool Mu. Toroids are very space effi- input voltage. Calculate the duty cycle at minimum input cient, especially when you can use several layers of wire. voltage. Because they generally lack a bobbin, mounting is more difficult. However, new designs for surface mount that do DC V = OUT not increase the height significantly are available. VIN(MIN)
Output Diode Selection
Choose the slope factor, SF, from Figure 2 based on the duty cycle. The ripple current calculated at minimum input The catch diode carries load current during the off-time. voltage and the chosen L should be used in the current The average diode current is therefore dependent on limit equation (see Inductor Value Calculation). Figure 3 the internal P-channel switch duty cycle. At high input provides several values of R voltages the diode conducts most of the time. As V SENSE and their corresponding IN ap- M values at different input voltages. Select the minimum proaches VOUT the diode conducts only a small fraction input voltage and calculate the resulting minimum current of the time. The most stressful condition for the diode is limit settings. when the output is short-circuited. Under this condition the diode must safely handle I The process must be repeated for maximum current PK at close to 100% duty cycle. Therefore, it is important to adequately specify the limit using duty cycle, slope factor, ripple current and diode peak current and average power dissipation so as mirror ratio based on maximum input voltage in order not to exceed the diode ratings. to choose the best sense resistor for a particular design Kool Mu is a registered trademark of Magnetics, Inc. 1779fa 8 For more information www.linear.com/LTC1779