LT1370_15 Datasheet, PDF (9/16 Pages) Linear Technology – 500kHz High Efficiency 6A Switching Regulator

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APPLICATIO S I FOR ATIO
) ^{IPEAK = (IOUT)}
V_OUT
V_IN
+
V_IN(V_OUT – V_IN)
2(f)(L)(V_OUT)
V_IN = Minimum input voltage
f = 500kHz switching frequency
3. Decide if the design can tolerate an “open” core geom-
etry, like a rod or barrel, which has high magnetic field
radiation, or whether it needs a closed core, like a
toroid, to prevent EMI problems. One would not want an
open core next to a magnetic storage media, for
instance! This is a tough decision because the rods or
barrels are temptingly cheap and small and there are no
helpful guidelines to calculate when the magnetic field
radiation will be a problem.
4. Start shopping for an inductor that meets the
requirements of core shape, peak current (to avoid
saturation), average current (to limit heating) and fault
current. If the inductor gets too hot, wire insulation will
melt and cause turn-to-turn shorts. Keep in mind that
all good things like high efficiency, low profile and high
temperature operation will increase cost, sometimes
dramatically.
5. After making an initial choice, consider the secondary
things like output voltage ripple, second sourcing, etc.
Use the experts in the LTC Applications 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 developments in low profile,
surface mounting, etc.
Output Capacitor
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 needed for typical LT1370 applications is 0.025Ω
to 0.2Ω. A typical output capacitor is an AVX type TPS,
22µF at 25V (two each), with a guaranteed ESR less than
0.2Ω. This is a “D” size surface mount solid tantalum
capacitor. TPS capacitors are specially constructed and
LT1370
tested for low ESR, so they give the lowest ESR for a given
volume. To further reduce ESR, multiple output capaci-
tors can be used in parallel. 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 tantalum
capacitor, it will have high ESR and output ripple voltage
will be terrible. Table 1 shows some typical solid tantalum
surface mount capacitors.
Table 1. Surface Mount Solid Tantalum Capacitor
ESR and Ripple Current
E CASE SIZE
ESR (MAX Ω) RIPPLE CURRENT (A)
AVX TPS, Sprague 593D
AVX TAJ
0.1 to 0.3
0.7 to 0.9
0.7 to 1.1
0.4
D CASE SIZE
AVX TPS, Sprague 593D
AVX TAJ
0.1 to 0.3
0.9 to 2.0
0.7 to 1.1
0.36 to 0.24
C CASE SIZE
AVX TPS
AVX TAJ
0.2 (Typ)
1.8 to 3.0
0.5 (Typ)
0.22 to 0.17
B CASE SIZE
AVX TAJ
2.5 to 10
0.16 to 0.08
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true and AVX type TPS capacitors are
specially tested for surge capability, but surge ruggedness
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.
Single inductor boost regulators have large RMS ripple
current in the output capacitor, which must be rated to
handle the current. The formula to calculate this is:
Output Capacitor Ripple Current (RMS)
DC
I_RIPPLE (RMS) = I_OUT 1 – DC
V_OUT – V_IN
= I_OUT
V_IN
DC = Switch duty cycle
sn1370 1370fs
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