Datasheet LTC3414 (Analog Devices) - 8

LTC3414
U U W U APPLICATIO S I FOR ATIO
The basic LTC3414 application circuit is shown in Figure 1. A reasonable starting point for selecting the ripple current External component selection is determined by the maxi- is ΔIL = 0.4(IMAX). The largest ripple current occurs at the mum load current and begins with the selection of the highest VIN. To guarantee that the ripple current stays operating frequency and inductor value followed by CIN below a specified maximum, the inductor value should be and COUT. chosen according to the following equation:
Operating Frequency
⎛ V ⎞ ⎛ V ⎞ L OUT OUT = 1– Selection of the operating frequency is a tradeoff between ⎝⎜ f I Δ ⎠⎟ ⎝⎜ V L MAX IN MAX ⎠⎟ ( ) ( ) efficiency and component size. High frequency operation The inductor value will also have an effect on Burst Mode allows the use of smaller inductor and capacitor values. operation. The transition to low current operation begins Operation at lower frequencies improves efficiency by when the peak inductor current falls below a level set by reducing internal gate charge losses but requires larger the burst clamp. Lower inductor values result in higher inductance values and/or capacitance to maintain low ripple current which causes this to occur at lower load output ripple voltage. currents. This causes a dip in efficiency in the upper range The operating frequency of the LTC3414 is determined by of low current operation. In Burst Mode operation, lower an external resistor that is connected between pin RT and inductance values will cause the burst frequency to in- ground. The value of the resistor sets the ramp current that crease. is used to charge and discharge an internal timing capaci-
Inductor Core Selection
tor within the oscillator and can be calculated by using the following equation: Once the value for L is known, the type of inductor must be selected. Actual core loss is independent of core size for a 3 08 1011 . • fixed inductor value, but it is very dependent on the R – k ( ) OSC = Ω 10 Ω f inductance selected. As the inductance increases, core losses decrease. Unfortunately, increased inductance re- Although frequencies as high as 4MHz are possible, the quires more turns of wire and therefore copper losses will minimum on-time of the LTC3414 imposes a minimum increase. limit on the operating duty cycle. The minimum on-time is typically 110ns; therefore, the minimum duty cycle is Ferrite designs have very low core losses and are preferred equal to 100 • 110ns • f(Hz). at high switching frequencies, so design goals can con- centrate on copper loss and preventing saturation. Ferrite
Inductor Selection
core material saturates “hard,” which means that induc- For a given input and output voltage, the inductor value tance collapses abruptly when the peak design current is and operating frequency determine the ripple current. The exceeded. This results in an abrupt increase in inductor ripple current ΔI ripple current and consequent output voltage ripple. Do L increases with higher VIN or VOUT and decreases with higher inductance. not allow the core to saturate! ⎛ Different core materials and shapes will change the size/ V ⎞ Δ V I OUT OUT current and price/current relationship of an inductor. L = ⎛ ⎞ 1– ⎝⎜ f • L ⎠⎟ ⎝⎜ V ⎠⎟ IN Toroid or shielded pot cores in ferrite or permalloy mate- Having a lower ripple current reduces the core losses in rials are small and don’t radiate much energy, but gener- the inductor, the ESR losses in the output capacitors, and ally cost more than powdered iron core inductors with the output voltage ripple. Highest efficiency operation is similar characteristics. The choice of which style inductor achieved at low frequency with small ripple current. This, to use mainly depends on the price verus size require- however, requires a large inductor. ments and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, Coilcraft, Toko, and Sumida. 3414fb 8