Datasheet

LTC3605
12
3605fc
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!
OPERATION
Table 1. Inductor Selection Table
INDUCTANCE DCR MAX CURRENT DIMENSIONS HEIGHT
Vishay IHLP-2525CZ-01 Series
0.33µH
4.1mW
18A
6.7mm × 7mm
3mm
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
2.0mm
0.47µH
15mW
7A
Toko FDV0620 Series
0.20µH
4.5mW
12.4A
7mm × 7.7mm
2.0mm
0.47µH
8.3mW
9A
1µH
18.3mW
5.7A
NEC/Tokin MLC0730L Series
0.47µH
4.5mW
16.6A
6.9mm × 7.7mm
3.0mm
0.75µH
7.5mW
12.2A
1µH
9mW
10.6A
Cooper HCP0703 Series
0.22µH
2.8mW
23A
7mm × 7.3mm
3.0mm
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
3.2mm
1.5µH
9.6mW
6.1A
2.2µH
12mW
5.4A
Wurth Electronik WE-HC 744312 Series
0.25µH
2.5mW
18A
7mm × 7.7mm
3.8mm
0.47µH
3.4mW
16A
0.72µH
7.5mW
12A
1µH
9.5mW
11A
1.5µH
10.5mW
9A
Different core materials and shapes will change the size/cur-
rent 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 Wurth Electronik. 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 deter-
mine the loop feedback factor gain and phase. An output
current pulse of 20% to 100% of full load current having
a rise time of 1µs to 10µs will produce output voltage and
ITH pin waveforms that will give a sense of the overall loop
stability without breaking the feedback loop.
Switching regulators take several cycles to respond to a
step in load current. When a load step occurs, V
OUT
im-
mediately shifts by an amount equal to DI
LOAD
• ESR, where
ESR is the effective series resistance of C
OUT
. DI
LOAD
also
begins to charge or discharge C
OUT
generating a feedback
error signal used by the regulator to return V
OUT
to its