LTC3129_15 Datasheet, PDF (18/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129
APPLICATIONS INFORMATION
It should be noted that the worst-case peak-to-peak in-
ductor ripple current occurs when the duty cycle in buck
mode is minimum (highest V_IN) and in boost mode when
the duty cycle is 50% (V_OUT = 2 • V_IN). As an example, if
V_IN (minimum) = 2.5V and V_IN (maximum) = 15V, V_OUT
= 5V and L = 10µH, the peak-to-peak inductor ripples at
the voltage extremes (15V V_IN for buck and 2.5V V_IN for
boost) are:
Buck = 248mA peak-to-peak
Boost = 93mA peak-to-peak
One half of this inductor ripple current must be added to
the highest expected average inductor current in order to
select the proper saturation current rating for the inductor.
To avoid the possibility of inductor saturation during load
transients, an inductor with a saturation current rating of
at least 600mA is recommended for all applications.
In addition to its influence on power conversion efficiency,
the inductor DC resistance can also impact the maximum
output current capability of the buck-boost converter
particularly at low input voltages. In buck mode, the
output current of the buck-boost converter is primarily
limited by the inductor current reaching the average cur-
rent limit threshold. 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 750mΩ
each. The only exceptions are applications that have a
maximum output current requirement much less than what
the LTC3129 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 ef-
ficiency 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 1
provides a wide sampling of inductors that are well suited
to many LTC3129 applications.
Table 1. Recommended Inductors
VENDOR
PART
Coilcraft
www.coilcraft.com
EPL2014, EPL3012, EPL3015, LPS3015,
LPS3314, XFL3012
Coiltronics
SDH3812, SD3814, SD3114, SD3118
www.cooperindustries.com
Murata
www.murata.com
LQH3NP, LQH32P, LQH44P
Sumida
www.sumida.com
CDRH2D16, CDRH2D18, CDRH3D14,
CDRH3D16
Taiyo-Yuden
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
Recommended inductor values for different operating
voltage ranges are given in Table 2. 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 2. Recommended Inductor Values
V_IN AND V_OUT RANGE
V_IN and V_OUT Both < 4.5V
V_IN and V_OUT Both < 8V
V_IN and V_OUT Both < 11V
V_IN and V_OUT Up to 15.75V
RECOMMENDED INDUCTOR VALUES
3.3µH to 4.7µH
4.7µH to 6.8µH
6.8µH to 8.2µH
8.2µH to 10µH
Feedforward Capacitor
The use of a voltage feedforward capacitor, as shown in
Figure 5, offers a number of performance advantages. A
feedforward capacitor will reduce output voltage ripple in
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