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LTC1159-3.3 数据表(PDF) 8 Page - Linear Technology |
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LTC1159-3.3 数据表(HTML) 8 Page - Linear Technology |
8 / 20 page 8 LTC1159/LTC1159-3.3/LTC1159-5 APPLICATIO S I FOR ATIO 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 which can cause Burst Mode operation to be falsely triggered in the LTC1159. Do not allow the core to saturate! Molypermalloy (from Magnetics, Inc.) is a low loss core material for toroids, but it is more expensive than ferrite. A reasonable compromise from the same manufacturer is Kool M µ. 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 surface mount designs available from Coiltronics do not increase the height significantly. Power MOSFET Selection Two external power MOSFETs must be selected for use with the LTC1159: a P-channel MOSFET for the main switch and an N-channel MOSFET for the synchronous switch. The peak-to-peak drive levels are set by the VCC voltage on the LTC1159. This voltage is typically 4.5V during start-up and 5V to 7V during normal operation (see EXTVCC Pin Connection). Consequently, logic-level threshold MOSFETs must be used in most LTC1159 family applica- tions. The only exception is applications in which EXTVCC is powered from an external supply greater than 8V, in which standard threshold MOSFETs (VGS(TH) < 4V) may be used. Pay close attention to the BVDSS specification for the MOSFETs as well; many of the logic-level MOSFETs are limited to 30V. Selection criteria for the power MOSFETs include the “ON” resistance RDS(ON), reverse transfer capacitance CRSS, input voltage, and maximum output current. When the LTC1159 is operating in continuous mode, the duty cycle for the P-channel MOSFET is given by: P-Ch Duty Cycle = VOUT VIN N-Ch Duty Cycle = VIN – VOUT VIN The MOSFET dissipations at maximum output current are given by: f = 1 tOFF ) )1– VOUT VIN where tOFF = 1.3 × 104 × CT Once the frequency has been set by CT, the inductor L must be chosen to provide no more than 0.025V/RSENSE of peak-to-peak inductor ripple current. This results in a minimum required inductor value of: LMIN = 5.1 × 105 × RSENSE × CT × VREG As the inductor value is increased from the minimum value, the ESR requirements for the output capacitor are eased at the expense of efficiency. If too small an inductor is used, the LTC1159 may not enter Burst Mode operation and efficiency will be severely degraded at low currents. Inductor Core Selection Once the minimum 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 M µ® cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on the inductance selected. As induc- tance increases, core losses go down but copper (I2R) losses will increase. Ferrite designs have very low core loss, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that FREQUENCY (kHz) 0 0 200 400 600 1400 1000 50 100 1200 800 150 200 250 VOUT = 5V VIN = 48V VIN=12V VIN = 24V LTC1159 • F03 Figure 3. Timing Capacitor Selection Kool M µ is a registered trademark of Magnetics, Inc. |
类似零件编号 - LTC1159-3.3 |
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类似说明 - LTC1159-3.3 |
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