LTC3122_15 Datasheet, PDF (13/26 Pages) Linear Technology – 15V, 2.5A Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3122
Applications Information
Output and Input Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used to minimize the output voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
extremely low ESR and are available in small footprints.
X5R and X7R dielectric materials are preferred for their
ability to maintain capacitance over wide voltage and tem-
perature ranges. Y5V types should not be used. Although
ceramic capacitors are recommended, low ESR tantalum
capacitors may be used as well.
When selecting output capacitors, the magnitude of the
peak inductor current, together with the ripple voltage
specification, determine the choice of the capacitor. Both
the ESR (equivalent series resistance) of the capacitor and
the charge stored in the capacitor each cycle contribute
to the output voltage ripple.
The ripple due to the charge is approximately:
V_{RIPPLE(CHARGE)}
≈
I_P • V_IN
C_OUT • V_OUT
•
ƒ
where I_P is the peak inductor current.
The ESR of C_OUT is usually the most dominant factor for
ripple in most power converters. The ripple due to the
capacitor ESR is:
V_RIPPLE(ESR)
= I_LOAD
• R_ESR
•
V_OUT
V_IN
where R_ESR = capacitor equivalent series resistance.
The input filter capacitor reduces peak currents drawn from
the input source and reduces input switching noise. A low
ESR bypass capacitor with a value of at least 4.7µF should
be located as close to the V_IN pin as possible.
Low ESR and high capacitance are critical to maintain low
output voltage ripple. Capacitors can be used in parallel
for even larger capacitance values and lower effective
ESR. Ceramic capacitors are often utilized in switching
converter applications due to their small size, low ESR and
low leakage currents. However, many ceramic capacitors
experience significant loss in capacitance from their rated
value with increased DC bias voltage. It is not uncommon
for a small surface mount capacitor to lose more than 50%
of its rated capacitance when operated near its rated volt-
age. As a result it is sometimes necessary to use a larger
capacitor value or a capacitor with a larger value and case
size, such as 1812 rather than 1206, in order to actually
realize the intended capacitance at the full operating volt-
age. Be sure to consult the vendor’s curve of capacitance
vs DC bias voltage. Table 2 shows a sampling of capacitors
suited for LTC3122 applications.
Table 2. Representative Output Capacitors
MANUFACTURER,
PART NUMBER
VALUE VOLTAGE SIZE L × W × H (mm)
(µF)
(V)
TYPE, ESR (mΩ)
AVX,
12103D226MAT2A
22
25
3.2 × 2.5 × 2.79,
X5R Ceramic
Kemet,
22
25
5.7 × 5.0 × 2.4,
C2220X226K3RACTU
X7R Ceramic
Kemet,
22
A700D226M016ATE030
16
7.3 × 4.3 × 2.8,
Alum. Polymer, 30mΩ
Murata,
22
25
3.2 × 2.5 × 2.5,
GRM32ER71E226KE15L
X7R Ceramic
Nichicon,
PLV1E121MDL1
82
25
8 × 8 × 12,
Alum. Polymer, 25mΩ
Panasonic,
ECJ-4YB1E226M
22
25
3.2 × 2.5 × 2.5,
X5R Ceramic
Sanyo,
25TQC22MV
22
25
7.3 × 4.3 × 3.1,
POSCAP, 50mΩ
Sanyo,
16TQC100M
100
16
7.3 × 4.3 × 1.9,
POSCAP, 45mΩ
Sanyo,
25SVPF47M
47
25
6.6 × 6.6 × 5.9,
OS-CON, 30mΩ
Taiyo Yuden,
22
25
3.2 × 2.5 × 2.5,
TMK325BJ226MM-T
X5R Ceramic
TDK,
47
25
6.5 × 5.5 × 5.5,
CKG57NX5R1E476M
X5R Ceramic
Cap-XX
GS230F
1.2Farads 4.5
39 × 17 × 3.8
28mΩ
Cooper
A1030-2R5155
1.5Farads 2.5
Ø = 10, L = 30
60mΩ
Maxwell
BCAP0050-P270
50Farads 2.5
Ø = 18, L = 40
20mΩ
For applications requiring a very low profile and very large
capacitance, the GS, GS2 and GW series from Cap-XX
and PowerStor Aerogel Capacitors from Cooper all offer
very high capacitance and low ESR in various low profile
packages.
A method for improving the converter’s transient response
uses a small feed-forward series network of a capacitor and
3122fa
For more information www.linear.com/LTC3122
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