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LTC1624 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1624 Datasheet(HTML) 7 Page - Linear Technology |
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7 / 28 page  7 LTC1624 APPLICATIONS INFORMATION The LTC1624 can be used in a wide variety of switching regulator applications, the most common being the step- down converter. Other switching regulator architectures include step-up, SEPIC and positive-to-negative converters. The basic LTC1624 step-down application circuit is shown in Figure 1 on the first page. External component selection is driven by the load requirement and begins with the selection of RSENSE. Once RSENSE is known, the inductor can be chosen. Next, the power MOSFET and D1 are selected. Finally, CIN and COUT are selected. The circuit shown in Figure 1 can be configured for operation up to an input voltage of 28V (limited by the external MOSFETs). Step-Down Converter: RSENSE Selection for Output Current RSENSE is chosen based on the required output current. The LTC1624 current comparator has a maximum thresh- old of 160mV/RSENSE. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current IMAX equal to the peak value less half the peak-to-peak ripple current, ∆IL. Allowing a margin for variations in the LTC1624 and external component values yields: R mV I SENSE MAX = 100 The LTC1624 works well with values of RSENSE from 0.005 Ω to 0.5Ω. Step-Down Converter: Inductor Value Calculation With the operating frequency fixed at 200kHz smaller inductor values are favored. Operating at higher frequen- cies generally results in lower efficiency because of MOSFET gate charge losses. In addition to this basic trade-off, the effect of inductor value on ripple current and low current operation must also be considered. The inductor value has a direct effect on ripple current. The inductor ripple current ∆IL decreases with higher induc- tance and increases with higher VIN or VOUT: ∆I VV fL VV VV L IN OUT OUT D IN D = − ()( ) + + where VD is the output Schottky diode forward drop. Accepting larger values of ∆IL allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is ∆IL = 0.4(IMAX). Remember, the maximum ∆IL occurs at the maximum input voltage. The inductor value also has an effect on low current operation. Lower inductor values (higher ∆IL) will cause Burst Mode operation to begin at higher load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation lower inductance values will cause the burst frequency to decrease. In general, inductor values from 5 µH to 68µH are typical depending on the maximum input voltage and output current. See also Modifying Burst Mode Operation section. Step-Down Converter: 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 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 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 loss and are preferred at high switching frequencies, so design goals can con- centrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that induc- tance collapses abruptly when the peak design current 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 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, designs for surface mount that do not increase the height significantly are available. Kool Mu is a registered trademark of Magnetics, Inc. |
Similar Part No. - LTC1624_15 |
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Similar Description - LTC1624_15 |
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