Datasheet LTC3416 (Analog Devices) - 8

LTC3416
U U W U APPLICATIO S I FOR ATIO
The basic LTC3416 application circuit is shown in Figure 1a. achieved at low frequency with small ripple current. This, External component selection is determined by the maxi- however, requires a large inductor. mum load current and begins with the selection of the A reasonable starting point for selecting the ripple current operating frequency and inductor value followed by CIN is ∆I and C L = 0.4(IMAX). The largest ripple current occurs at the OUT. highest VIN. To guarantee that the ripple current stays below a specified maximum, the inductor value should be
Operating Frequency
chosen according to the following equation: Selection of the operating frequency is a tradeoff between ⎛ ⎞⎛ ⎞ efficiency and component size. High frequency operation V V L OUT OUT = 1– allows the use of smaller inductor and capacitor values. ⎝⎜ f∆I ⎠⎟⎝⎜ V L MAX IN MAX ⎠⎟ ( ) ( ) Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger
Inductor Core Selection
inductance values and/or capacitance to maintain low Once the value for L is known, the type of inductor must be output ripple voltage. selected. Actual core loss is independent of core size for a The operating frequency of the LTC3416 is determined by fixed inductor value, but it is very dependent on the an external resistor that is connected between the R inductance selected. As the inductance increases, core T pin and ground. The value of the resistor sets the ramp current losses decrease. Unfortunately, increased inductance re- that is used to charge and discharge an internal timing quires more turns of wire and therefore copper losses will capacitor within the oscillator and can be calculated by increase. using the following equation: Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can con- 3 08 1011 . • R ( )– k centrate on copper loss and preventing saturation. Ferrite OSC = Ω 10 Ω f core material saturates “hard,” which means that induc- Although frequencies as high as 4MHz are possible, the tance collapses abruptly when the peak design current is minimum on-time of the LTC3416 imposes a minimum exceeded. This results in an abrupt increase in inductor limit on the operating duty cycle. The minimum on-time is ripple current and consequent output voltage ripple. Do typically 110ns. Therefore, the minimum duty cycle is not allow the core to saturate! equal to 100 • 110ns • f(Hz). Different core materials and shapes will change the size/ current and price/current relationship of an inductor.
Inductor Selection
Toroid or shielded pot cores in ferrite or permalloy mate- For a given input and output voltage, the inductor value rials are small and don’t radiate much energy, but gener- and operating frequency determine the ripple current. The ally cost more than powdered iron core inductors with ripple current ∆I similar characteristics. The choice of which style inductor L increases with higher VIN or lower VOUT and decreases with higher inductance. to use mainly depends on the price vs size requirements and any radiated field/EMI requirements. New designs for surface mount inductors are available from Coiltronics, ∆ V V I OUT OUT L = ⎛ ⎞⎛ ⎞ ⎝⎜ 1– fL ⎠⎟ ⎝⎜ V Coilcraft, Toko and Sumida. IN ⎠ ⎟ Having a lower ripple current reduces the core losses in
CIN and COUT Selection
the inductor, the ESR losses in the output capacitors and The input capacitance, CIN, is needed to filter the trapezoi- the output voltage ripple. Highest efficiency operation is dal wave current at the source of the top MOSFET. To 3416fa 8