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LTC3602EUF Datasheet(PDF) 9 Page - Linear Technology |
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LTC3602EUF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC3602 9 3602fb APPLICATIONS INFORMATION The basic LTC3602 application circuit is shown on the front page of this data sheet. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. Operating Frequency Selection of the operating frequency is a tradeoff between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge and switching losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage. The operating frequency of the LTC3602 is determined by an external resistor that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by using the following equation: R fHz k OSC = 115 10 10 11 .• () – Although frequencies as high as 3MHz are possible, the minimum on-time of the LTC3602 imposes a minimum limit on the operating duty cycle. The minimum on-time is typically 90ns. Therefore, the minimum duty cycle is equal to 100 • 90ns • f(Hz). Inductor Selection For a given input and output voltage, the inductor value and operating frequency determine the ripple current. The ripple current ΔIL increases with higher VIN and decreases with higher inductance. ΔI L = V OUT fL ⎛ ⎝⎜ ⎞ ⎠⎟ •1– V OUT V IN ⎛ ⎝⎜ ⎞ ⎠⎟ Having a lower ripple current reduces the ESR losses in the output capacitors and the output voltage ripple. Highest efficiency operation is achieved at low frequency with small ripple current. This, however, requires a large inductor. A reasonable starting point for selecting the ripple current is ΔIL = 0.4(IMAX), where IMAX is the maximum output current. The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L = V OUT f ΔI L(MAX) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ •1– V OUT V IN(MAX) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this to occur at lower 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 to increase. 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 the more expensive ferrite cores. Actual core loss is independent of core size for a fixed inductor value but it is very dependent on the inductance selected. As the inductance increases, core losses decrease. Un- fortunately, increased inductance requires more turns of wire and therefore copper losses will increase. 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 inductance 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! Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy ma- terials are small and do not radiate energy but generally cost more than powdered iron core inductors with similar |
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