LT1777 Linear Technology, LT1777 Datasheet - Page 13

• Manufacturer Part Number: LT1777
• Description: Low Noise Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
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 I
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
GND
Figure 6. LT1676 V
for Comparison Purposes Only, V
U
INFORMATION
500ns/DIV
SW
U
Node Voltage Behavior
W
IN
= 36V
1777 F06
U
2
R
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 V
input supply and one for the V
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 V
The input bypass capacitor can present a more difficult
choice. In a typical application e.g., 24V
relatively heavy V
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 V
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 F ceramic bypass capaci-
IN
current. The straightforward choice for a low
IN
current is drawn by the power switch
OUT
C
output supply.
pin.)
IN
LT1777
to 5V
13
OUT
IN
,