Datasheet

12
LT1576/LT1576-5
APPLICATIONS INFORMATION
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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.
Table 2
SERIES CORE
VENDOR/ VALUE DC CORE RESIS- MATER- HEIGHT
PART NO. (
µ H) (Amps) TYPE TANCE( Ω ) IAL (mm)
Coiltronics
CTX15-2 15 1.7 Tor 0.059 KMµ 6.0
CTX33-2 33 1.4 Tor 0.106 KMµ 6.0
CTX68-4 68 1.2 Tor 0.158 KMµ 6.4
CTX15-1P 15 1.4 Tor 0.087 52 4.2
CTX33-2P 33 1.3 Tor 0.126 52 6.0
CTX68-4P 68 1.1 Tor 0.238 52 6.4
Sumida
CDRH74-150 15 1.47 SC 0.081 Fer 4.5
CDH115-330 33 1.68 SC 0.082 Fer 5.2
CDRH125-680 68 1.5 SC 0.12 Fer 6
CDH74-330 33 1.45 SC 0.17 Fer 5.2
Coilcraft
DO3308P-153 15 2 SC 0.12 Fer 3
DO3316P-333 33 2 SC 0.1 Fer 5.21
DO3316P-683 68 1.4 SC 0.18 Fer 5.21
Pulse
PE-53602 35 1.4 Tor 0.166 Fer 9.1
PE-53604 73 1.3 Tor 0.290 Fer 9.1
PE-53632 22 2.7 Tor 0.063 Fer 9.1
PE-53633 40 2.7 Tor 0.085 Fer 10
Gowanda
SMP3316-152K 15 3.5 SC 0.041 Fer 6
SMP3316-332K 33 2.3 SC 0.092 Fer 6
SMP3316-682K 68 1.7 SC 0.178 Fer 6
Tor = Toroid
SC = Semi-closed geometry
Fer = Ferrite core material
52 = Type 52 powdered iron core material
KMµ = Kool Mµ
Output Capacitor
The output capacitor is normally chosen by its Effective
Series Resistance (ESR), because this is what determines
output ripple voltage. To get low ESR takes volume , so
physically smaller capacitors have high ESR. The ESR
range for typical LT1576 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
tantalum capacitor, it will have high ESR, and output ripple
voltage will be terrible. Table 3 shows some typical solid
tantalum surface mount capacitors.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size ESR (Max., Ω ) Ripple Current (A)
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
AVX TAJ 0.7 to 0.9 0.4
D Case Size
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
C Case Size
AVX TPS 0.2 (typ) 0.5 (typ)
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
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.
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 200mA
RMS
. The formula
to calculate this is: