LT1374
U
W U U
APPLICATIONS INFORMATION
saturation),averagecurrent(tolimitheating),andfault
current(iftheinductorgetstoohot, wireinsulationwill
melt and cause turn-to-turn shorts). Keep in mind that
allgoodthingslikehighefficiency,lowprofile,andhigh
temperature operation will increase cost, sometimes
dramatically. Get a quote on the cheapest unit first to
calibrate yourself on price, then ask for what you really
want.
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true, and type TPS capacitors are
speciallytestedforsurgecapability,butsurgeruggedness
is not a critical issue with the output capacitor. Solid
tantalum capacitors fail during very high turn-on surges,
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
5. After making an initial choice, consider the secondary
things like output voltage ripple, second sourcing, etc.
Use the experts in the Linear Technology’s applica-
tions department if you feel uncertain about the final
choice. They have experience with a wide range of
inductor types and can tell you about the latest devel-
opments in low profile, surface mounting, etc.
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
triangular with a typical value of 200mARMS. The formula
to calculate this is:
Output Capacitor Ripple Current (RMS):
Output Capacitor
0.29 V
V − V
IN OUT
(
OUT)(
)
The output capacitor is normally chosen by its Effective
Series Resistance (ESR), because this is what determines
output ripple voltage. At 500kHz, any polarized capacitor
is essentially resistive. To get low ESR takes volume, so
physically smaller capacitors have high ESR. The ESR
range for typical LT1374 applications is 0.05Ω to 0.2Ω. A
typical output capacitor is an AVX type TPS, 100µF at 10V,
with a guaranteed ESR less than 0.1Ω. This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical, and values from
22µF to greater than 500µF work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22µF solid
tantalumcapacitor, itwillhavehighESR, andoutputripple
voltage will be terrible. Table 3 shows some typical solid
tantalum surface mount capacitors.
IRIPPLE RMS
=
(
)
L f V
( )( )( )
IN
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becomingavailableinsmallercasesizes.Thesearetempt-
ing for switching regulator use because of their very low
ESR. Unfortunately, the ESR is so low that it can cause
loop stability problems. Solid tantalum capacitor’s ESR
generatesaloop“zero”at5kHzto50kHzthatisinstrumen-
tal in giving acceptable loop phase margin. Ceramic
capacitors remain capacitive to beyond 300kHz and usu-
allyresonatewiththeirESLbeforeESRbecomeseffective.
They are appropriate for input bypassing because of their
highripplecurrentratingsandtoleranceofturn-onsurges.
Linear Technology plans to issue a design note on the use
of ceramic capacitors in the near future.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
OUTPUT RIPPLE VOLTAGE
E Case Size
ESR (Max.,
Ω
)
Ripple Current (A)
0.7 to 1.1
0.4
AVX TPS, Sprague 593D
AVX TAJ
0.1 to 0.3
0.7 to 0.9
Figure 3 shows a typical output ripple voltage waveform
for the LT1374. Ripple voltage is determined by the high
frequency impedance of the output capacitor, and ripple
current through the inductor. Peak-to-peak ripple current
through the inductor into the output capacitor is:
D Case Size
AVX TPS, Sprague 593D
C Case Size
0.1 to 0.3
0.2 (typ)
0.7 to 1.1
0.5 (typ)
AVX TPS
1374fb
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