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LTC1148L Datasheet(PDF) 9 Page - Linear Technology |
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LTC1148L Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC1148 LTC1148-3.3/LTC1148-5 APPLICATIO S I FOR ATIO Inductor Core Selection Once the minimum value for L is known, the type of inductor must be selected. The highest efficiency will be obtained using ferrite, Kool M µ® on molypermalloy (MPP) cores. Lower cost powdered iron cores provide suitable performance but cut efficiency by 3% to 7%. 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. Do not allow the core to saturate! Kool M µ (from Magnetics, Inc.) is a very good, low loss core material for toroids, with a “soft” saturation charac- teristic. Molypermalloy is slightly more efficient at high (>200kHz) switching frequencies, but quite a bit more expensive. Toroids are very space efficient, especially when you can use several layers of wire. Because they generally lack a bobbin, mounting is more difficult. How- ever, new designs for surface mount are available from Coiltronics and Beckman Industrial Corp. which do not increase the height significantly. Power MOSFET and D1 Selection Two external power MOSFETs must be selected for use with the LTC1148 series: a P-channel MOSFET for the main switch, and an N-channel MOSFET for the synchro- nous switch. The main selection criteria for the power MOSFETs are the threshold voltage VGS(TH) and on resis- tance RDS(ON). 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- FREQUENCY (kHz) 0 0 200 400 600 100 200 LTC1148 • F03 800 1000 300 VSENSE– = VOUT = 5V VIN=12V VIN = 10V VIN = 7V Figure 3. Timing Capacitor Value f = 1 tOFF ) )1– VOUT VIN where: tOFF = 1.3(104)CT ) )VREG VOUT VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT is the measured output voltage. Thus VREG/VOUT = 1 in regulation. Note that as VIN decreases, the frequency decreases. When the input to output voltage differential drops below 1.5V, the LTC1148 series reduces tOFF by in- creasing the discharge current in CT. This prevents audible operation prior to dropout. Once the frequency has been set by CT, the inductor L must be chosen to provide no more than 25mV/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 inductor current will decrease past zero and change polarity. A consequence of this is that the LTC1148 series may not enter Burst Mode operation and efficiency will be severely degraded at low currents. Kool M µis a registered trademark of Magnetics, Inc. |
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