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LT1777

APPLICATIONS INFORMATION

through the main inductor has most of its energy concen-

trated in the fundamental and lower harmonics.) Toroidal

style inductors, many available in surface mount configu-

ration, offer a reduced external magnetic field, generally at

an increase in cost and physical size. Although custom

design is always a possibility, most potential LT1777 ap-

plications can be handled by the array of standard, off-the-

shelf inductor products offered by the major suppliers.

Selecting Bypass Capacitors

The basic topology as shown in the Typical Application on

the first page uses two bypass capacitors, one for the VIN

input supply and one for the VOUT output supply.

User selection of an appropriate output capacitor is rela-

tively easy, as this capacitor sees only the AC ripple current

in the inductor L1. As the LT1777 is designed for buck or

step-down applications, output voltage will nearly always

be compatible with tantalum type capacitors, which are

generally available in ratings up to 35V or so. These

tantalum types offer good volumetric efficiency, and many

are available with specified ESR performance. The product

of inductor AC ripple current and output capacitor ESR will

manifest itself as peak-to-peak voltage ripple on the output

node. (Note: If this ripple becomes too large, heavier

control loop compensation, at least at the switching fre-

quency, may be required on the VC pin.)

The input bypass capacitor can present a more difficult

choice. In a typical application e.g., 24VIN to 5VOUT,

relatively heavy VIN current is drawn by the power switch

for only a small portion of the oscillator period (low ON

duty cycle). The resulting RMS ripple current, for which

the capacitor must be rated, can be several times the DC

average VIN current. The straightforward choice for a low

volume, surface mountable electrolytic capacitor with

good ESR/ripple current ratings is a tantalum type. How-

ever, worst-case (high) input voltage coupled with stan-

dard capacitor voltage derating may exceed the 35V or so

for which tantalum capacitors are generally available.

Relatively bulky “high frequency” aluminum electrolytic

types, specifically constructed and rated for switching

supply applications, may then be the only choice.

Additionally, it may be advantageous to parallel the input

and output capacitors with 0.1

µF ceramic bypass capaci-

500ns/DIV

1777 F06

GND

Figure 6. LT1676 VSW Node Voltage Behavior

for Comparison Purposes Only, VIN = 36V

Selecting Main Inductor

There are several parameters to consider when selecting

a main inductor. These include inductance value, peak

current rating (to avoid core saturation), DC resistance,

construction type, physical size, and of course, cost.

Once the inductance value is decided, inductor peak

current rating and resistance need to be considered. Here,

the inductor peak current rating refers to the onset of

saturation in the core material, although manufacturers

sometimes specify a “peak current rating” which is de-

rived from a worst-case combination of core saturation

and self-heating effects. Inductor winding resistance alone

limits the inductor’s current carrying capability as the I2R

power threatens to overheat the inductor. Remember to

include the condition of output short circuit, if applicable.

Although the peak current rating of the inductor can be

exceeded in short-circuit operation, as core saturation per

se is not destructive to the core, excess resistive self-

heating is still a potential problem.

The final inductor selection is generally based on cost,

which usually translates into choosing the smallest physi-

cal size part which meets the desired inductance value,

resistance and current carrying capability. An additional

factor to consider is that of physical construction. Briefly

stated, “open” inductors built on a rod- or barrel-shaped

core generally offer the smallest physical size and lowest

cost. However their open construction does not contain

the resulting magnetic field, and they may not be accept-

able in RFI-sensitive applications. (A mitigating factor is

that, as mentioned previously, the AC current passing

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