LTC1504IS8 Datasheet, PDF (7/12 Pages) Linear Technology – 500mA Low Voltage Step-Down Synchronous

LTC1504A
APPLICATIONS INFORMATION
Ideally, the LTC1504A requires a low impedance bypass
right at the chip and a larger reservoir capacitor that can be
located somewhat farther away. This requirement usually
can be met with a ceramic capacitor right next to the
LTC1504A and an electrolytic capacitor (usually 10µF to
100µF, depending on expected load current) located some-
where nearby. In certain cases, the bulk capacitance
requirement can be met by the output bypass of the input
supply. Applications running at very high load currents or
at input supply voltages greater than 6V may require the
local ceramic capacitor to be 1µF or greater. In some
cases, both the low impedance and bulk capacitance
requirements can be met by a single capacitor, mounted
very close to the LTC1504A. Low ESR organic semicon-
ductor (OS-CON) electrolytic capacitors or surge tested
surface mount tantalum capacitors can have low enough
impedance to keep the LTC1504A happy in some circuits.
Often the RMS current capacity of the input bypass capaci-
tors is more important to capacitor selection than value.
Buck converters like the LTC1504A are hard on input
capacitors, since the current flow alternates between the
full load current and near zero during every clock cycle. In
the worst case (50% duty cycle or V_OUT = 0.5V_IN) the RMS
current flow in the input capacitor is half of the total load
current plus half the ripple current in the inductor—
perhaps 300mA in a typical 500mA load current applica-
tion. This current flows through the ESR of the input
bypass capacitor, heating it up and shortening its life,
sometimes dramatically. Many ordinary electrolytic ca-
pacitors that look OK at first glance are not rated to
withstand such currents—check the RMS current rating
before you specify a device! If the RMS current rating isn’t
specified, it should not be used as an input bypass capaci-
tor. Again, low ESR electrolytic and surge tested tantalums
usually do well in LTC1504A applications and have high
RMS current ratings. The local ceramic bypass capacitor
usually has negligible ESR, allowing it to withstand large
RMS currents without trouble. Table 1 shows typical
surface mount capacitors that make acceptable input
bypass capacitors in LTC1504A applications.
Inductor
The LTC1504A requires an external inductor to be con-
nected from the switching node SW to the output node
Table 1. Representative Surface Mount Input Bypass Capacitors
PART
VALUE ESR MAX RMS TYPE HEIGHT
AVX
TPSC226M016R0375
TPSD476M016R0150
TPSE107M016R0125
1206YC105M
1210YG106Z
22µF
47µF
100µF
1µF
10µF
0.38Ω
0.15Ω
0.13Ω
Low
Low
0.54A
0.86A
1.15A
>1A
>1A
Tantalum 2.6mm
Tantalum 2.9mm
Tantalum 4.1mm
X7R Ceramic 1.5mm
Y5V Ceramic 1.7mm
Sanyo
16SN33M
16SN68M
16CV100GX
16CV220GX
33µF
68µF
100µF
220µF
0.15Ω
0.1Ω
0.44Ω
0.34Ω
1.24A
1.65A
0.23A*
0.28A*
OS-CON
OS-CON
Electrolytic
Electrolytic
7mm
7mm
6mm
7.7mm
Sprague
593D476X0016D2W
593D107X0016E2W
47µF 0.17Ω
100µ 0.15Ω
0.93A
1.05A
Tantalum 2.8mm
Tantalum 4mm
*Note: Use multiple devices in parallel or limit output current to prevent capacitor overload.
where the load is connected. Inductor requirements are
fairly straightforward; it must be rated to handle continu-
ous DC current equal to the maximum load current plus
half the ripple current and its value should be chosen
based on the desired ripple current and/or the output
current transient requirements. Large value inductors
lower ripple current and decrease the required output
capacitance, but limit the speed that the LTC1504A can
change the output current, limiting output transient re-
sponse. Small value inductors result in higher ripple
currents and increase the demands on the output capaci-
tor, but allow faster output current slew rates and are often
smaller and cheaper for the same DC current rating. A
typical inductor used in an LTC1504A application might
have a maximum current rating between 500mA and 1A
and an inductance between 33µH and 220µH.
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally cost more than powdered iron rod core inductors with
similar electrical characteristics. The choice of which style
inductor to use often depends more on the price vs size
requirements and any radiated field/EMI requirements
than on what the LTC1504A requires to operate. Table 2
shows some typical surface mount inductors that work
well in LTC1504A applications.
7