Datasheet LT1676 (Analog Devices) - 8

LT1676
U U W U APPLICATIONS INFORMATION
Once the inductance value is decided, inductor peak internal switch will ramp up VIN current into the diode in an current rating and resistance need to be considered. Here, attempt to get it to recover. Then, when the diode has the inductor peak current rating refers to the onset of finally turned off, some tens of nanoseconds later, the VSW saturation in the core material, although manufacturers node voltage ramps up at an extremely high dV/dt, per- sometimes specify a “peak current rating” which is haps 5 to even 10V/ns ! With real world lead inductances, derived from a worst-case combination of core saturation the VSW node can easily overshoot the VIN rail. This can and self-heating effects. Inductor winding resistance alone result in poor RFI behavior and if the overshoot is severe limits the inductor’s current carrying capability as the I2R enough, damage the IC itself. power threatens to overheat the inductor. If applicable, remember to include the condition of output short circuit.
Selecting Bypass Capacitors
Although the peak current rating of the inductor can be The basic topology as shown in Figure 1 uses two bypass exceeded in short-circuit operation, as core saturation per capacitors, one for the VIN input supply and one for the se is not destructive to the core, excess resistive self- VOUT output supply. heating is still a potential problem. User selection of an appropriate output capacitor is rela- The final inductor selection is generally based on cost, tively easy, as this capacitor sees only the AC ripple current which usually translates into choosing the smallest physi- in the inductor. As the LT1676 is designed for Buck or cal size part that meets the desired inductance value, step-down applications, output voltage will nearly always resistance and current carrying capability. An additional be compatible with tantalum type capacitors, which are factor to consider is that of physical construction. Briefly generally available in ratings up to 35V or so. These stated, “open” inductors built on a rod- or barrel-shaped tantalum types offer good volumetric efficiency and many core generally offer the smallest physical size and lowest are available with specified ESR performance. The product cost. However their open construction does not contain of inductor AC ripple current and output capacitor ESR will the resulting magnetic field, and they may not be accept- manifest itself as peak-to-peak voltage ripple on the output able in RFI-sensitive applications. Toroidal style induc- node. (Note: If this ripple becomes too large, heavier tors, many available in surface mount configuration, offer control loop compensation, at least at the switching fre- improved RFI performance, generally at an increase in quency, may be required on the VC pin.) The most cost and physical size. And although custom design is demanding applications, requiring very low output ripple, always a possibility, most potential LT1676 applications may be best served not with a single extremely large can be handled by the array of standard, off-the-shelf output capacitor, but instead by the common technique of inductor products offered by the major suppliers. a separate L/C lowpass post filter in series with the output. (In this case, “Two caps are better than one.”)
Selecting Freewheeling Diode
The input bypass capacitor is normally a more difficult Highest efficiency operation requires the use of a Schottky choice. In a typical application e.g., 48V type diode. DC switching losses are minimized due to its IN to 5VOUT, relatively heavy V low forward voltage drop, and AC behavior is benign due IN current is drawn by the power switch for only a small portion of the oscillator period (low ON to its lack of a significant reverse recovery time. Schottky duty cycle). The resulting RMS ripple current, for which diodes are generally available with reverse voltage ratings the capacitor must be rated, is often several times the DC of 60V and even 100V, and are price competitive with other average V types. IN current. Similarly, the “glitch” seen on the VIN supply as the power switch turns on and off will be related The use of so-called “ultrafast” recovery diodes is gener- to the product of capacitor ESR, and the relatively high ally not recommended. When operating in continuous instantaneous current drawn by the switch. To compound mode, the reverse recovery time exhibited by “ultrafast” these problems is the fact that most of these applications diodes will result in a slingshot type effect. The power will be designed for a relatively high input voltage, for 8