Datasheet

6
LT1615/LT1615-1
sn16151 16151fas
APPLICATIO S I FOR ATIO
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input voltages greater than 5V), the current limit over-
shoot can be quite high. Although it is internally current
limited to 350mA, the power switch of the LT1615 can
handle larger currents without problem, but the overall
efficiency will suffer. Best results will be obtained when
I
PEAK
is kept below 700mA for the LT1615 and below
400mA for the LT1615-1.
Capacitor Selection
Low ESR (Equivalent Series Resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are the best choice, as they
have a very low ESR and are available in very small
packages. Their small size makes them a good companion
to the LT1615’s SOT-23 package. Solid tantalum capaci-
tors (like the AVX TPS, Sprague 593D families) or OS-CON
capacitors can be used, but they will occupy more board
area than a ceramic and will have a higher ESR. Always use
a capacitor with a sufficient voltage rating.
Ceramic capacitors also make a good choice for the input
decoupling capacitor, which should be placed as close as
possible to the LT1615. A 4.7µF input capacitor is suffi-
cient for most applications. Table 2 shows a list of several
capacitor manufacturers. Consult the manufacturers for
more detailed information and for their entire selection of
related parts.
Diode Selection
For most LT1615 applications, the Motorola MBR0520
surface mount Schottky diode (0.5A, 20V) is an ideal
choice. Schottky diodes, with their low forward voltage
drop and fast switching speed, are the best match for the
LT1615. For higher output voltage applications the 30V
MBR0530 can be used. Many different manufacturers
make equivalent parts, but make sure that the component
is rated to handle at least 0.35A. For LT1615-1 applica-
tions, a Philips BAT54 or Central Semiconductor CMDSH-3
works well.
Lowering Output Voltage Ripple
Using low ESR capacitors will help minimize the output
ripple voltage, but proper selection of the inductor and the
output capacitor also plays a big role. The LT1615 pro-
vides energy to the load in bursts by ramping up the
inductor current, then delivering that current to the load.
If too large of an inductor value or too small of a capacitor
value is used, the output ripple voltage will increase
because the capacitor will be slightly overcharged each
burst cycle. To reduce the output ripple, increase the
output capacitor value or add a 4.7pF feed-forward capaci-
tor in the feedback network of the LT1615 (see the circuits
in the Typical Applications section). Adding this small,
inexpensive 4.7pF capacitor will greatly reduce the output
voltage ripple.
Table 2. Recommended Capacitors
CAPACITOR TYPE VENDOR
Ceramic Taiyo Yuden
[phone]
www.t-yuden.com
Ceramic AVX
[phone]
www.avxcorp.com
Ceramic Murata
[phone]
www.murata.com
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