LTC3605_15 Datasheet, PDF (12/22 Pages) Linear Technology – 15V, 5A Synchronous Step-Down Regulator

LTC3605
Operation
inductance selected. As the inductance or frequency in-
creases, core losses decrease. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses will increase.
Table 1. Inductor Selection Table
INDUCTANCE DCR MAX CURRENT DIMENSIONS
Vishay IHLP-2525CZ-01 Series
0.33µH 4.1mW
18A
6.7mm × 7mm
0.47µH 6.5mW
13.5A
0.68µH 9.4mW
11A
0.82µH 11.8mW
10A
1.0µH 14.2mW
9A
Vishay IHLP-1616BZ-11 Series
0.22µH 4.1mW
12A
4.3mm × 4.7mm
0.47µH 15mW
7A
Toko FDV0620 Series
0.20µH 4.5mW
12.4A
7mm × 7.7mm
0.47µH 8.3mW
9A
1µH 18.3mW
5.7A
NEC/Tokin MLC0730L Series
0.47µH 4.5mW
16.6A
6.9mm × 7.7mm
0.75µH 7.5mW
12.2A
1µH
9mW
10.6A
Cooper HCP0703 Series
0.22µH 2.8mW
23A
7mm × 7.3mm
0.47µH 4.2mW
17A
0.68µH 5.5mW
15A
0.82µH 8mW
13A
1µH
10mW
11A
1.5µH 14mW
9A
TDK RLF7030 Series
1µH
8.8mW
6.4A
6.9mm × 7.3mm
1.5µH 9.6mW
6.1A
2.2µH 12mW
5.4A
Würth Elektronik WE-HC 744312 Series
0.25µH 2.5mW
18A
7mm × 7.7mm
0.47µH 3.4mW
16A
0.72µH 7.5mW
12A
1µH
9.5mW
11A
1.5µH 10.5mW
9A
HEIGHT
3mm
2.0mm
2.0mm
3.0mm
3.0mm
3.2mm
3.8mm
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price versus size requirements
and any radiated field/EMI requirements. New designs for
surface mount inductors are available from Toko, Vishay,
NEC/Tokin, Cooper, TDK and Würth Elektronik. Refer to
Table 1 for more details.
Checking Transient Response
The OPTI-LOOP compensation allows the transient re-
sponse to be optimized for a wide range of loads and
output capacitors. The availability of the ITH pin not
only allows optimization of the control loop behavior but
also provides a DC-coupled and AC-filtered closed-loop
response test point. The DC step, rise time and settling
at this test point truly reflects the closed-loop response.
Assuming a predominantly second order system, phase
margin and/or damping factor can be estimated using the
percentage of overshoot seen at this pin.
The ITH external components shown in the circuit on the
first page of this data sheet provides an adequate starting
point for most applications. The series R-C filter sets the
dominant pole zero loop compensation. The values can
be modified slightly (from 0.5 to 2 times their suggested
values) to optimize transient response once the final PC
layout is done and the particular output capacitor type
and value have been determined. The output capacitors
need to be selected because their various types and values
determine the loop feedback factor gain and phase. An
output current pulse of 20% to 100% of full load current
12
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