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LTC1149CN-5 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1149CN-5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC1149 LTC1149-3.3/LTC1149-5 APPLICATIO S I FOR ATIO 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 inductance selected. As induc- tance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses 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 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 LTC1149 series. 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 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. P-Channel MOSFET Selection Two external power MOSFETs must be selected for use with the LTC1149 series: a P-channel MOSFET for the main switch, and an N-channel MOSFET for the synchro- nous switch. The minimum input voltage determines whether standard threshold or logic-level threshold MOSFETs must be used. For VIN > 8V, standard threshold MOSFETs (VGS(TH) < 4V) may be used. If VIN is expected to drop below 8V, logic- level threshold MOSFETs (VGS(TH) < 2.5V) are strongly recommended. When logic-level MOSFETs are used, the absolute maximum VGS rating for the MOSFETs must be greater than the LTC1149 series internal regulator voltage VCC. Selection criteria for the P-channel MOSFET include the on-resistance RDS(ON), reverse transfer capacitance CRSS, input voltage and maximum output current. When the LTC1149 is operating in continuous mode, the duty cycle for the P-channel MOSFET is given by: P-Ch Duty Cycle = VOUT VIN The P-channel MOSFET dissipation at maximum output current is given by: P-Ch PD = VOUT VIN + K(VIN) 2(IMAX)(CRSS)(f) (IMAX) 2(1 + ∂P) RDS(ON) where ∂ is the temperature dependency of RDS(ON) and K is a constant related to the gate drive current. Note the two distinct terms in the equation. The first gives the I2R losses, which are highest at low input voltages, while the second gives the transition losses, which are highest at high input voltages. For VIN < 24V, the high current efficiency generally improves with larger MOSFETs (although gate charge losses begin eating into the gains. See Efficiency Considerations). For VIN > 24V, the transi- tion losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actually provides higher efficiency. This is illustrated in the Design Example section. The term (1 + ∂) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve, but ∂ = 0.007/°C can be used as an approximation for low voltage MOSFETs. CRSS is usually specified in the MOSFET electrical characteristics. The constant K is much harder to pin down, but K = 5 can be used for the LTC1149 series to estimate the relative contributions of the two terms in the P-channel dissipation equation. N-Channel MOSFET and D1 Selection The same input voltage constraints apply to the N-channel MOSFET as to the P-channel with regard to when logic- level devices are required. However, the dissipation calcu- lation is quite different. The duty cycle and dissipation for Kool M µ is a registered trademark of Magnetics, Inc. |
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