Datasheet

LTC3129-1
19
31291fa
For more information www.linear.com/LTC3129-1
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 750mΩ
each. The only exceptions are applications that have a
maximum output current requirement much less than
what the LTC3129-1 is capable of delivering. Generally
speaking, inductors with a DCR in the range of 0.15Ω to
0.3Ω are recommended. Lower values of DCR will improve
the efficiency at the expense of size, while higher DCR
values will reduce efficiency (typically by a few percent)
while allowing the use of a physically smaller inductor.
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 2
provides a wide sampling of inductors that are well suited
to many LTC3129-1 applications.
Table 2. Recommended Inductors
VENDOR PART
Coilcraft
www.coilcraft.com
EPL2014, EPL3012, EPL3015, XFL3012
LPS3015, LPS3314
Coiltronics
www.cooperindustries.com
SDH3812, SD3814
SD3114, SD3118
Murata
www.murata.com
LQH3NP
LQH32P
LQH44P
Sumida
www.sumida.com
CDRH2D16, CDRH2D18
CDRH3D14, CDRH3D16
Taiyo-Y
uden
www.t-yuden.com
NR3012T, NR3015T, NRS4012T
BRC2518
TDK
www.tdk.com
VLS3012, VLS3015
VLF302510MT, VLF302512MT
Toko
www.tokoam.com
DB3015C, DB3018C, DB3020C
DP418C, DP420C, DEM2815C,
DFE322512C, DFE252012C
Würth
www.we-online.com
WE-TPC 2813, WE-TPC 3816,
WE-TPC 2828
applicaTions inForMaTion
Recommended inductor values for different operating
voltage ranges are given in Table 3. These values were
chosen to minimize inductor size while maintaining an
acceptable amount of inductor ripple current for a given
V
IN
and V
OUT
range.
Table 3. Recommended Inductor and Output Capacitor Values
V
IN
AND V
OUT
RANGE RECOMMENDED
INDUCTOR
VALUES
MAXIMUM RECOMMENDED
TOTAL OUTPUT CAPACITOR
VALUE FOR PWM MODE
OPERATION AT LIGHT LOAD
(<15mA, PWM PIN
HIGH)
V
IN
and V
OUT
Both < 4.5V 3.3µH to 4.7µH 10µF
V
IN
and V
OUT
Both < 8V 4.7µH to 6.8µH 10µF
V
IN
and V
OUT
Both < 11V 6.8µH to 8.2µH 10µF
V
IN
and V
OUT
Up to 15V 8.2µH to 10µH 10µF
Due to the fixed, internal loop compensation and feedback
divider provided by the LTC3129-1, there are limitations to
the maximum recommended total output capacitor value in
applications that must operate in PWM mode at light load
(PWM pin pulled high with minimum load currents less
than ~15mA). In these applications, a maximum output
capacitor value, shown in Table 3, is recommended. For
applications that must operate in PWM mode at light load
with higher values of output capacitance, the LTC3129 is
recommended. Its external feedback pin allows the use
of additional feedforward compensation for improved
light-load stability under these conditions.
Note that for applications where Burst Mode operation
is enabled (PWM pin grounded), the output capacitor
value can be increased without limitation regardless of
the minimum load current or
inductor value.
Output Capacitor Selection
A low effective series resistance (ESR) output capacitor
of 4.7µF minimum should be connected at the output of
the buck-boost converter in order to minimize output volt
-
age 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’s ESR and ESL (effec
-
tive series
inductance), the peak-to-peak output voltage
ripple
in PWM mode can be calculated by the following