Datasheet LT1776 (Analog Devices) - 9

LT1776
U U W U APPLICATIONS INFORMATION
limits the inductor’s current carrying capability as the I2R result in poor RFI behavior and if the overshoot is severe power threatens to overheat the inductor. If applicable, enough, damage the IC itself. remember to include the condition of output short circuit. Although the peak current rating of the inductor can be
Selecting Bypass Capacitors
exceeded in short-circuit operation, as core saturation per The basic topology as shown in Figure 1 uses two bypass se is not destructive to the core, excess resistive self- capacitors, one for the VIN input supply and one for the heating is still a potential problem. VOUT output supply. The final inductor selection is generally based on cost, User selection of an appropriate output capacitor is rela- which usually translates into choosing the smallest physi- tively easy, as this capacitor sees only the AC ripple current cal size part that meets the desired inductance value, in the inductor. As the LT1776 is designed for buck or resistance and current carrying capability. An additional step-down applications, output voltage will nearly always factor to consider is that of physical construction. Briefly be compatible with tantalum type capacitors, which are stated, “open” inductors built on a rod- or barrel-shaped generally available in ratings up to 35V or so. These core generally offer the smallest physical size and lowest tantalum types offer good volumetric efficiency and many cost. However their open construction does not contain are available with specified ESR performance. The product the resulting magnetic field, and they may not be accept- of inductor AC ripple current and output capacitor ESR will able in RFI-sensitive applications. Toroidal style induc- manifest itself as peak-to-peak voltage ripple on the output tors, many available in surface mount configuration, offer node. (Note: If this ripple becomes too large, heavier improved RFI performance, generally at an increase in control loop compensation, at least at the switching fre- cost and physical size. And although custom design is quency, may be required on the VC pin.) The most de- always a possibility, most potential LT1776 applications manding applications, requiring very low output ripple, can be handled by the array of standard, off-the-shelf may be best served not with a single extremely large inductor products offered by the major suppliers. output capacitor, but instead by the common technique of a separate L/C lowpass post filter in series with the output.
Selecting Freewheeling Diode
(In this case, “Two caps are better than one”.) Highest efficiency operation requires the use of a Schottky The input bypass capacitor is normally a more difficult type diode. DC switching losses are minimized due to its choice. In a typical application e.g., 40V low forward voltage drop, and AC behavior is benign due IN to 5VOUT, relatively heavy V to its lack of a significant reverse recovery time. Schottky IN current is drawn by the power switch for only a small portion of the oscillator period (low ON diodes are generally available with reverse voltage ratings duty cycle). The resulting RMS ripple current, for which of 60V and even 100V, and are price competitive with other the capacitor must be rated, is often several times the DC types. average VIN current. Similarly, the “glitch” seen on the VIN The use of so-called “ultrafast” recovery diodes is gener- supply as the power switch turns on and off will be related ally not recommended. When operating in continuous to the product of capacitor ESR, and the relatively high mode, the reverse recovery time exhibited by “ultrafast” instantaneous current drawn by the switch. To compound diodes will result in a slingshot type effect. The power these problems is the fact that most of these applications internal switch will ramp up VIN current into the diode in an will be designed for a relatively high input voltage, for attempt to get it to recover. Then, when the diode has which tantalum capacitors are generally unavailable. Rela- finally turned off, some tens of nanoseconds later, the VSW tively bulky “high frequency” aluminum electrolytic types, node voltage ramps up at an extremely high dV/dt, per- specifically constructed and rated for switching supply haps 5 to even 10V/ns ! With real world lead inductances, applications, may be the only choice. the VSW node can easily overshoot the VIN rail. This can 9