Datasheet LTC3602 (Analog Devices) - 9

LTC3602
APPLICATIONS INFORMATION
The basic LTC3602 application circuit is shown on the front A reasonable starting point for selecting the ripple current page of this data sheet. External component selection is is ΔIL = 0.4(IMAX), where IMAX is the maximum output determined by the maximum load current and begins with current. The largest ripple current occurs at the highest the selection of the inductor value and operating frequency VIN. To guarantee that the ripple current stays below a followed by CIN and COUT. specifi ed maximum, the inductor value should be chosen according to the following equation:
Operating Frequency
⎛ ⎞ ⎛ ⎞ Selection of the operating frequency is a tradeoff between V V L = OUT ⎜ ⎟ • 1– OUT ⎜ ⎟ effi ciency and component size. High frequency operation f ⎝ ΔI ⎠ V ⎝ ⎠ L(MAX) IN(MAX) allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves effi ciency by The inductor value will also have an effect on Burst Mode reducing internal gate charge and switching losses but operation. The transition from low current operation requires larger inductance values and/or capacitance to begins when the peak inductor current falls below a level maintain low output ripple voltage. The operating frequency set by the burst clamp. Lower inductor values result in of the LTC3602 is determined by an external resistor that is higher ripple current which causes this to occur at lower connected between the RT pin and ground. The value of the load currents. This causes a dip in effi ciency in the upper resistor sets the ramp current that is used to charge and range of low current operation. In Burst Mode operation, discharge an internal timing capacitor within the oscillator lower inductance values will cause the burst frequency and can be calculated by using the following equation: to increase. 11 . •
Inductor Core Selection
R – k OSC = 1 15 10 10 f H ( z) Once the value for L is known, the type of inductor must be selected. High effi ciency converters generally cannot Although frequencies as high as 3MHz are possible, the afford the core loss found in low cost powdered iron cores, minimum on-time of the LTC3602 imposes a minimum forcing the use of the more expensive ferrite cores. Actual limit on the operating duty cycle. The minimum on-time core loss is independent of core size for a fi xed inductor is typically 90ns. Therefore, the minimum duty cycle is value but it is very dependent on the inductance selected. equal to 100 • 90ns • f(Hz). As the inductance increases, core losses decrease. Un- fortunately, increased inductance requires more turns of
Inductor Selection
wire and therefore copper losses 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 ΔIL increases with higher VIN and decreases concentrate on copper loss and preventing saturation. with higher inductance. Ferrite core material saturates “hard,” which means that ⎛ V ⎛ V ⎞ inductance collapses abruptly when the peak design current Δ ⎞ I = OUT OUT L ⎝⎜ is exceeded. This results in an abrupt increase in inductor fL ⎠⎟ • 1– ⎝⎜ V ⎠⎟ IN ripple current and consequent output voltage ripple. Do not allow the core to saturate! Having a lower ripple current reduces the ESR losses in the output capacitors and the output voltage ripple. Different core materials and shapes will change the Highest effi ciency operation is achieved at low frequency size/current and price/current relationship of an inductor. with small ripple current. This, however, requires a large Toroid or shielded pot cores in ferrite or permalloy ma- inductor. terials are small and do not radiate energy but generally cost more than powdered iron core inductors with similar 3602fb 9