Datasheet LTC3418 (Analog Devices) - 10

LTC3418
applicaTions inForMaTion
The basic LTC3418 application circuit is shown on the Having a lower ripple current reduces the core losses in front page of this data sheet. External component selection the inductor, the ESR losses in the output capacitors and is determined by the maximum load current and begins the output voltage ripple. Highest efficiency operation is with the selection of the operating frequency and inductor achieved at low frequency with small ripple current. This, value followed by CIN and COUT. however, requires a large inductor.
Operating Frequency
A reasonable starting point for selecting the ripple current is ∆IL = 0.4(IMAX). The largest ripple current occurs at the Selection of the operating frequency is a trade-off between highest VIN. To guarantee that the ripple current stays efficiency and component size. High frequency operation below a specified maximum, the inductor value should allows the use of smaller inductor and capacitor values. be chosen according to the following equation: Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger  V  V  L OUT  1– OUT  inductance values and/or capacitance to maintain low =  fΔI    L(MAX) V  IN(MAX) output ripple voltage.   The operating frequency of the LTC3418 is determined The inductor value will also have an effect on Burst Mode by an external resistor that is connected between the R operation. The transition from low current operation be- T pin and ground. The value of the resistor sets the ramp gins when the peak inductor current falls below a level current that is used to charge and discharge an internal set by the burst clamp. Lower inductor values result in timing capacitor within the oscillator and can be calculated higher ripple current which causes this to occur at lower by using the following equation: load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency R 7.3 • 1010 OSC = Ω ⎡⎣ ⎤⎦– 2.5kΩ f to increase. Although frequencies as high as 4MHz are possible, the
Inductor Core Selection
minimum on-time of the LTC3418 imposes a minimum Once the value for L is known, the type of inductor must limit on the operating duty cycle. The minimum on-time be selected. Actual core loss is independent of core size is typically 80ns. Therefore, the minimum duty cycle is for a fixed inductor value, but it is very dependent on the equal to: inductance selected. As the inductance increases, core 100 • 80ns • f(Hz) losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses
Inductor Selection
will increase. For a given input and output voltage, the inductor value Ferrite designs have very low core losses and are pre- and operating frequency determine the ripple current. The ferred at high switching frequencies, so design goals can ripple current ∆IL increases with higher VIN or VOUT and concentrate on copper loss and preventing saturation. decreases with higher inductance: Ferrite core material saturates “hard,” which means that  V  V  inductance collapses abruptly when the peak design current ΔI OUT OUT L = 1–  is exceeded. This results in an abrupt increase in inductor  fL  VIN  ripple current and consequent output voltage ripple. Do not allow the core to saturate! 3418fc 10 For more information www.linear.com/LTC3418 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts