LTC3115-2_15 Datasheet, PDF (19/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Applications Information
provides a small sampling of inductors that are well suited
to many LTC3115-2 applications.
In applications with V_OUT ≥ 20V, it is recommended that
a minimum inductance value, L_MIN, be utilized where f is
the switching frequency:
L_MIN
=
(
12H
f /Hz)
Table 1. Representative Surface Mount Inductors
PART NUMBER
VALUE DCR MAX DC
(µH) (mΩ) CURRENT (A)
SIZE (mm)
W×L×H
Coilcraft
LPS6225
LPS6235
MSS1038
D03316P
4.7 65
6.8 75
22 70
15 50
3.2
6.2 × 6.2 × 2.5
2.8
6.2 × 6.2 × 3.5
3.3
10.2 × 10.5 × 3.8
3.0
12.9 × 9.4 × 5.2
Cooper-Bussmann
CD1-150-R
15 50
DR1030-100-R
10 40
FP3-8R2-R
8.2 74
DR1040-220-R
22 54
3.6
10.5 × 10.4 × 4.0
3.18
10.3 × 10.5 × 3.0
3.4
7.3 × 6.7 × 3.0
2.9
10.3 × 10.5 × 4.0
Panasonic
ELLCTV180M
18 30
3.0
ELLATV100M
10 23
3.3
12 × 12 × 4.2
10 × 10 × 4.2
Sumida
CDRH8D28/HP
10 78
3.0
CDR10D48MNNP 39 105
3.0
CDRH8D28NP
4.7 24.7
3.4
8.3 × 8.3 × 3
10.3 × 10.3 × 5
8.3 × 8.3 × 3
Taiyo-Yuden
NR10050T150M 15 46
3.6
9.8 × 9.8 × 5
TOKO
B1047AS-6R8N
B1179BS-150M
892NAS-180M
6.8 36
15 56
18 42
2.9
7.6 × 7.6 × 5
3.3
10.3 × 10.3 × 4
3.0
12.3 × 12.3 × 4.5
Würth
7447789004
744771133
744066150
4.7 33
2.9
33 49
2.7
15 40
3.2
7.3 × 7.3 × 3.2
12 × 12 × 6
10 × 10 × 3.8
capacitance, t_LOW is the switch pin minimum low time, and
I_LOAD is the output current. Curves for the value of t_LOW
as a function of switching frequency and temperature can
be found in Typical Performance Characteristics section
of this data sheet.
∆V_P-P(BUCK)
=
I_LOADt_LOW
C_OUT
∆V_P-P(BOOST)
=
I_LOAD
fC_OUT
⎛
⎝⎜
V_OUT
–
V_IN + t_LOWfV_IN
V_OUT
⎞
⎠⎟
The output voltage ripple increases with load current and
is generally higher in boost mode than in buck mode.
These expressions only take into account the output
voltage ripple that results from the output current 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 output
voltage ripple is dominated by the inductor current ripple.
In addition to 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
the series resistance of the output capacitor and all other
terms are as previously defined.
∆V_P-P(BUCK)
= ^ILOADRESR
1– t_LOWf
≅ I_LOADR_ESR
( ) ^{∆VP-P(BOOST)}
= ^{ILOADRESRVOUT}
V_IN 1– t_LOWf
≅
I_LOADR_ESR
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
Output Capacitor Selection
Input Capacitor Selection
A low ESR output capacitor should be utilized at the buck-
boost converter output in order to minimize output voltage
ripple. Multilayer 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 ESR and ESL, the peak-to-peak
output voltage ripple can be calculated by the following
formulas, where f is the switching frequency, C_OUT is the
The PV_IN pin carries the full inductor current and provides
power to internal control circuits in the IC. To minimize
input voltage ripple and ensure proper operation of the IC,
a low ESR bypass capacitor with a value of at least 4.7µF
should be located as close to this pin as possible. The
traces connecting this capacitor to PV_IN and the ground
plane should be made as short as possible. The V_IN pin
provides power to the V_CC regulator and other internal
circuitry. If the PCB trace connecting V_IN to PV_IN is long, it
31152fa
For more information www.linear.com/LTC3115-2
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