LTC3118_15 Datasheet, PDF (26/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Applications Information
Table 1. Representative Buck-Boost Surface Mount Inductors
PART NUMBER
VALUE
(µH)
DCR
(mΩ)
MAX DC
CURRENT (A)
MSS7341T
3.3
18
3.7
XAL7030
6.8
42
4.4
SD8328
3.3
14
4.0
4.7
19
3.6
LQH88PN
LQH88PN
LQH88PN
3.3
16
5
4.7
22
4.2
6.8
28
3.8
CDRH8D28NP
3.3
18
4
4.7
25
3.4
VLP840
3.3
15
5.2
6.8
24
3.6
FDSD0603
3.3
23
5.6
6.8
51
3.7
7447789003
3.3
30
4.2
7447789004
4.7
35
3.9
7447779006
6.8
35
3.3
SIZE
(W × L × H) mm
7×7×4
8×8×3
8×8×3
8×8×3
8×8×4
8×8×4
8×8×4
8×8×3
8×8×3
8×8×4
8×8×4
7×7×3
7×7×3
7×7×3
7×7×3
7 × 7 × 4.5
MANUFACTURER
Coilcraft
www.coilcraft.com
Coiltronics
www.coiltronics.com
Murata
www.murata.com
Sumida
www.sumida.com
TDK Electronics
www.tdk.co.jp
Toko
www.toko.co.jp
Würth Elektronik
www.we-online.com
value should be chosen large enough to reduce the output
voltage ripple to acceptable levels. Neglecting the capaci-
tor’s ESR and ESL, the peak-to-peak output voltage ripple
can be calculated by the following formula, where f is the
frequency in MHz (1.2MHz), C_OUT is the capacitance in
µF, t_LOW is the switch pin minimum low time in µs (0.1µs)
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
V_OUT
fV_IN
⎞
⎟
Volts
⎠
Examining the previous equations reveals 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 pro-
portional 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 as previously defined.
ΔV_P-P(BUCK)
= ^{ILOAD RESR}
1− t_LOW f
≅ I_LOAD R_ESRVolts
ΔV_P-P(BOOST) =
I_LOAD R_ESR V_OUT
V_{IN (}1− t_LOW f₎
≅
I_LOAD
R_ESR
⎛
⎜
⎝
V_OUT
V_IN
⎞
⎟ ^Volts
⎠
In most LTC3118 applications, an output capacitor between
47µF and 100µF will work well.
Input Capacitor Selection
The V_IN1 or V_IN2 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 10µF should be located as close to the pin as possible.
3118f
26
For more information www.linear.com/LTC3118