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LTC3604 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3604 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3621/LTC3621-2 10 3621f For more information www.linear.com/LTC3621 applicaTions inForMaTion Lower ripple current reduces power losses in the inductor, ESR losses in the output capacitors and output voltage ripple. Highest efficiency operation is obtained at low frequency with small ripple current. However, achieving this requires a large inductor. There is a trade-off between component size, efficiency and operating frequency. A reasonable starting point is to choose a ripple current that is about 40% of IOUT(MAX). To guarantee that ripple current does not exceed a specified maximum, the induc- tance should be chosen according to: L = VOUT f •∆IL(MAX) 1– VOUT VIN(MAX) Once the value for L is known, the type of inductor must be selected. Actual core loss is independent of core size for a fixed inductor value, but is very dependent on the inductance selected. As the inductance or frequency in- creases, core losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Copper losses also increase as frequency increases. Ferrite designs have very low core losses and are pre- ferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard”, which means that inductancecollapsesabruptlywhenthepeakdesigncurrent is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/ currentandprice/currentrelationshipofaninductor.Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price versus size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Toko, Vishay, NEC/Tokin, Cooper, TDK and Würth Electronik. Refer to Table 1 for more details. Checking Transient Response Theregularloopresponsecanbecheckedbylookingatthe load transient response. Switching regulators take several cyclestorespondtoastepinloadcurrent.Whenaloadstep occurs, VOUT immediately shifts by an amount equal to the ∆ILOAD • ESR, where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The initial output voltage step may not be within the bandwidth of the feedback loop, so the standard second order overshoot/DC ratio cannot be used to determine phase margin. In addition, a feedforward capacitor can be added to improve the high frequency response, as shown in Figure 1. Capacitor CFF provides phase lead by creating a high frequency zero with R2, which improves the phase margin. Theoutputvoltagesettlingbehaviorisrelatedtothestability of the closed-loop system and will demonstrate the actual overall supply performance. For a detailed explanation of optimizing the compensation components, including a review of control loop theory, refer to application Note 76. Insomeapplications,amoreseveretransientcanbecaused by switching in loads with large (>1µF) input capacitors. The discharge input capacitors are effectively put in paral- lel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the switchconnectingtheloadhaslowresistanceandisdriven quickly. The solution is to limit the turn-on speed of the load switch driver. A Hot Swap™ controller is designed specifically for this purpose and usually incorporates current limiting, short-circuit protection and soft-starting. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would |
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