LTC3536_15 Datasheet, PDF (13/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Applications Information
reducing the peak current to be closer to the average out-
put current and therefore minimize resistive losses due to
high RMS currents. However, a larger inductor within any
given inductor family will generally have a greater series
resistance, thereby counteracting this efficiency advantage.
An inductor used in LTC3536 applications should have a
saturation current rating that is greater than the worst-case
average inductor current plus half the ripple current. The
peak-to-peak inductor current ripple for each operational
mode can be calculated from the following formula, where
f is the switching frequency in MHz, L is the inductance
in µH.
∆I_{L(P-P )(BUCK )}
=
V_OUT
f •L


V_IN
– V_OUT
V_IN


∆I_{L(P-P)(BOOST)}
=
V_IN
f •L


V_OUT – V_IN
V_OUT


In addition to its influence on power conversion efficiency,
the inductor DC resistance can also impact the maximum
output capability of the buck-boost converter particularly
at low input voltages. In buck mode, the output current of
the buck-boost converter is limited only by the inductor
current reaching the current 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 include switch resis-
tances, inductor resistance and PCB trace resistance. Use
of an inductor with high DC resistance can degrade the
output current capability from that shown in the Typical
Performance Characteristics section of this data sheet.
As a guideline, in most applications the inductor DC re-
sistance should be significantly smaller than the typical
power switch resistance of 120mΩ.
The minimum inductor value must guarantee that the
worst-case average input current plus half the ripple
current don’t reach the input current limit threshold.
For a switching frequency of 1MHz the recommended
typical inductor value is 4.7µH. For a higher and lower
switching frequency the inductor value should be changed
accordingly in order to have the same current ripple
(2.2µH for 2MHz, 15µH for 300kHz).
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 LTC3536 applications.
Table 1. Recommended Inductors
VENDOR
PART/STYLE
Coilcraft
847-639-6400
www.coilcraft.com
LPO2506
LPS4012, LPS4018
MSS6122
MSS4020
MOS6020
DS1605, DO1608
XPL4020
XAL4040
XFL4020
Coiltronics
www.cooperet.com
SD52, SD53
SD3114, SD311B
Murata
714-852-2001
www.sumida.com
LQH55D
Sumida
847-956-0666
www.sumida.com
CDH40D11
Taiyo Yuden
www.t-yuden.com
NP04S8
NR3015
NR4018
TDK
847-803-6100
www.component.tdk.com
VLP, LTF
VLF, VLCF
Würth Elektronik
201-785-8800
www.we-online.com
WE-TPC Type S, M, MH
Output 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.
3536fa
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