LT1944 Ver la hoja de datos (PDF) - Linear Technology
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LT1944 Datasheet PDF : 8 Pages
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LT1944
APPLICATIO S I FOR ATIO
limited to 350mA, the power switch of the LT1944 can
handle larger currents without problem, but the overall
efficiency will suffer. Best results will be obtained when
IPEAK is kept below 700mA for the LT1944.
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 LT1944’s MS10 package. Solid tantalum capacitors
(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 LT1944. 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.
Table 2. Recommended Capacitors
CAPACITOR TYPE
VENDOR
Ceramic
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
Ceramic
AVX
(803) 448-9411
www.avxcorp.com
Ceramic
Murata
(714) 852-2001
www.murata.com
Setting the Output Voltage
Set the output voltage for each switching regulator by
choosing the appropriate values for feedback resistors R1
and R2 (see Figure 1).
R1 =
R2
VOUT
1.23V
− 1
Diode Selection
For most LT1944 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
LT1944. For higher output voltage applications the 30V
MBR0530 or 40V MBR0540 can be used. Many different
manufacturers make equivalent parts, but make sure that
the component is rated to handle at least 0.35A.
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 LT1944 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 LT1944 (see the circuits
in the Typical Applications section). Adding this small,
inexpensive 4.7pF capacitor will greatly reduce the output
voltage ripple.
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