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