Electronic Components Datasheet Search |
|
LTC1779 Datasheet(PDF) 7 Page - Linear Technology |
|
LTC1779 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page 7 LTC1779 A smaller value than LMIN could be used in the circuit; however, the inductor current will not be continuous during burst periods. RSENSE Selection for Output Current The selection of RSENSE determines the output current limit, the maximum possible output current before the internal current limit threshold is reached. IOUT(MAX), the maximum specified output current in a design, must be less than ICL. With the current comparator monitoring the voltage developed across RSENSE, the threshold of the comparator determines the inductor’s peak current. The maximum output current, ICL, the LTC1779 can provide is given by: IM SF V R I CL SENSE RIPPLE = +Ω 100 012 22 . – where SF and M are as defined in the previous section, Figures 2 and 3. Typically, RSENSE is chosen between 0Ω and 20 Ω. Current limit is at a minimum at minimum input voltage and maximum at maximum input voltage. Both conditions should be considered in a design where current limit is important. To calculate several current limit conditions and choose the best sense resistor for your design, first use minimum input voltage. Calculate the duty cycle at minimum input voltage. DC V V OUT IN MIN = () Choose the slope factor, SF, from Figure 2 based on the duty cycle. The ripple current calculated at minimum input voltage and the chosen L should be used in the current limit equation (see Inductor Value Calculation). Figure 3 provides several values of RSENSE and their corresponding M values at different input voltages. Select the minimum input voltage and calculate the resulting minimum current limit settings. The process must be repeated for maximum current limit using duty cycle, slope factor, ripple current and mirror ratio based on maximum input voltage in order to choose the best sense resistor for a particular design and to understand how it is going to work over the entire input voltage range. 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 Mu ® 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 preferred 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 cur- rent is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Molypermalloy (from Magnetics, Inc.) is a very good, low loss core material for toroids, but it is more expensive than ferrite. A reasonable compromise from the same manu- facturer is Kool Mu. Toroids are very space efficient, especially when you can use several layers of wire. Because they generally lack a bobbin, mounting is more difficult. However, new designs for surface mount that do not increase the height significantly are available. Output Diode Selection The catch diode carries load current during the off-time. The average diode current is therefore dependent on the internal P-channel switch duty cycle. At high input volt- ages the diode conducts most of the time. As VIN ap- proaches VOUT the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short-circuited. Under this condition the diode must safely handle IPK at close to 100% duty cycle. Therefore, it is important to adequately specify the diode peak current and average power dissipation so as not to exceed the diode ratings. Kool Mu is a registered trademark of Magnetics, Inc. APPLICATIO S I FOR ATIO |
Similar Part No. - LTC1779 |
|
Similar Description - LTC1779 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |