Datasheet LTC1624 (Analog Devices) - 7

LTC1624
U U W U APPLICATIONS INFORMATION
The LTC1624 can be used in a wide variety of switching Accepting larger values of ∆IL allows the use of low regulator applications, the most common being the step- inductances, but results in higher output voltage ripple down converter. Other switching regulator architectures and greater core losses. A reasonable starting point for include step-up, SEPIC and positive-to-negative converters. setting ripple current is ∆IL = 0.4(IMAX). Remember, the maximum ∆I The basic LTC1624 step-down application circuit is shown L occurs at the maximum input voltage. in Figure 1 on the first page. External component selection The inductor value also has an effect on low current is driven by the load requirement and begins with the operation. Lower inductor values (higher ∆IL) will cause selection of RSENSE. Once RSENSE is known, the inductor Burst Mode operation to begin at higher load currents, can be chosen. Next, the power MOSFET and D1 are which can cause a dip in efficiency in the upper range of selected. Finally, CIN and COUT are selected. The circuit low current operation. In Burst Mode operation lower shown in Figure 1 can be configured for operation up to an inductance values will cause the burst frequency to input voltage of 28V (limited by the external MOSFETs). decrease. In general, inductor values from 5µH to 68µH are typical depending on the maximum input voltage and
Step-Down Converter: RSENSE Selection for
output current. See also Modifying Burst Mode Operation
Output Current
section. RSENSE is chosen based on the required output current. The LTC1624 current comparator has a maximum thresh-
Step-Down Converter: Inductor Core Selection
old of 160mV/RSENSE. The current comparator threshold Once the value for L is known, the type of inductor must be sets the peak of the inductor current, yielding a maximum selected. High efficiency converters generally cannot average output current IMAX equal to the peak value less afford the core loss found in low cost powdered iron cores, half the peak-to-peak ripple current, ∆IL. forcing the use of more expensive ferrite, molypermalloy or Kool Mµ® cores. Actual core loss is independent of core Allowing a margin for variations in the LTC1624 and size for a fixed inductor value, but it is very dependent on external component values yields: inductance selected. As inductance increases, core losses mV go down. Unfortunately, increased inductance requires RSENSE = 100 I more turns of wire and, therefore, copper losses will MAX increase. The LTC1624 works well with values of RSENSE from Ferrite designs have very low core loss and are preferred 0.005Ω to 0.5Ω. at high switching frequencies, so design goals can con-
Step-Down Converter: Inductor Value Calculation
centrate on copper loss and preventing saturation. Ferrite With the operating frequency fixed at 200kHz smaller core material saturates “hard,” which means that induc- inductor values are favored. Operating at higher frequen- tance collapses abruptly when the peak design current is cies generally results in lower efficiency because of exceeded. This results in an abrupt increase in inductor MOSFET gate charge losses. In addition to this basic ripple current and consequent output voltage ripple. Do trade-off, the effect of inductor value on ripple current and not allow the core to saturate! low current operation must also be considered. Molypermalloy (from Magnetics, Inc.) is a very good, low The inductor value has a direct effect on ripple current. The loss core material for toroids, but it is more expensive than inductor ripple current ∆I ferrite. A reasonable compromise from the same manu- L decreases with higher induc- tance and increases with higher V facturer is Kool Mµ. Toroids are very space efficient, IN or VOUT: especially when you can use several layers of wire.   ∆ V V V + V Because they generally lack a bobbin, mounting is more I IN OUT OUT D L = − f L  V + V ( )( ) difficult. However, designs for surface mount that do not IN D  increase the height significantly are available. where VD is the output Schottky diode forward drop. Kool Mu is a registered trademark of Magnetics, Inc. 7