Datasheet

LTC3114-1
21
Rev. C
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APPLICATIONS INFORMATION
threshold defined by VC. However, in boost mode, especially
at large step-up ratios, the output current capability can
also be limited by the total resistive losses in the power
stage. These losses include, switch resistances, inductor
DC resistance and PCB trace resistance. Avoid inductors
with a high DC resistance (DCR) as they can degrade the
maximum output current capability from what is shown
in the Typical Performance Characteristics section and
from the Typical Application circuits. As a guideline, the
inductor DCR should be significantly less than the typical
power switch resistance of 250mΩ. The only exceptions
are applications that have a maximum output current much
less than what the LTC3114-1 is capable of delivering.
Different inductor core materials and styles have an impact
on the size and price of an inductor at any given current
rating. Shielded construction is generally preferred as it
minimizes the chances of interference with other circuitry.
The choice of inductor style depends upon the price, sizing,
and EMI requirements of a particular application. Table1
provides a small sampling of inductors that are well suited
to many LTC3114-1 applications.
Output Capacitor Selection
A low effective series resistance (ESR) output capacitor
should be connected at the output of the buck-boost
converter in order to minimize output voltage ripple. Mul
-
tilayer ceramic capacitors are an excellent option as they
have low ESR and are available in small footprints. The
capacitor value should be chosen large enough to reduce
the output voltage ripple to acceptable levels. Neglecting
the capacitor’s ESR and ESL (effect series inductance),
the peak-to-peak output voltage ripple can be calculated
by the following formula, where f is the frequency in MHz
(1.2MHz for the LTC3114 -1), C
OUT
is the capacitance in µF,
t
LOW
is the switch pin minimum low time in us (0.1µs for
the LTC3114-1) and I
LOAD
is the output current in Amps.
ΔV
P-P(BUCK)
=
I
LOAD
t
LOW
C
OUT
Volts
ΔV
P-P(BOOST)
=
I
LOAD
fC
OUT
V
OUT
– V
IN
+ t
LOW
fV
IN
V
OUT
⎛
⎝
⎜
⎞
⎠
⎟
Volts
Examining the previous equations reveal that the output
voltage ripple increases with load current and is gener-
ally higher in boost mode than in buck mode. Note that
these equations only take into account the voltage ripple
that occurs from the inductor current to the output being
discontinuous.
They provide a good approximation to the
ripple at any significant load current but underestimate the
output voltage ripple at very light loads where the output
voltage ripple is dominated by the inductor current ripple.
In addition to the output voltage ripple generated across
the output capacitance, there is also output voltage ripple
produced across the internal resistance of the output
capacitor. The ESR-generated output voltage ripple is
proportional to the series resistance of the output capacitor
and is given by the following expressions where R
ESR
is
Table1. Representative Surface Mount Inductors
PART NUMBER
VALUE
(µH)
DCR
(mΩ)
MAX DC
CURRENT (A)
SIZE (mm)
W × L × H
Coilcraft
LPS6225
LPS6235
MSS1038
D03316P
4.7
6.8
22
15
65
75
70
50
3.2
2.8
3.3
3.0
6.2 × 6.2 × 2.5
6.2 × 6.2 × 3.5
10.2 × 10.5 × 3.8
12.9 × 9.4 × 5.2
Cooper
-Bussmann
CD1-150-R
DR1030-100-R
FP3-8R2-R
DR1040-220-R
15
10
8.2
22
50
40
74
54
3.6
3.18
3.4
2.9
10.5 × 10.4 × 4.0
10.3 × 10.5 × 3.0
7.3 × 6.7 × 3.0
10.3 × 10.5 × 4.0
Panasonic
ELLCTV180M
ELLATV100M
18
10
30
23
3.0
3.3
12 × 12 × 4.2
10 × 10 × 4.2
Sumida
CDRH8D28/HP
CDR10D48MNNP
CDRH8D28NP
10
39
4.7
78
105
24.7
3.0
3.0
3.4
8.3 × 8.3 × 3
10.3 × 10.3 × 5
8.3 × 8.3 × 3
T
aiyo-Yuden
NR10050T150M 15 46 3.6 9.8 × 9.8 × 5
TOKO
B1047AS-6R8N
B1179BS-150M
892NAS-180M
6.8
15
18
36
56
42
2.9
3.3
3.0
7.6 × 7.6 × 5
10.3 × 10.3 × 4
12.3 × 12.3 × 4.5
W
ürth
7447789004
7440650068
744771133
744066150
4.7
6.8
33
15
33
33
49
40
2.9
3.6
2.7
3.2
7.3 × 7.3 × 3.2
10 × 10 × 3
12 × 12 × 6
10 × 10 × 3.8
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