LTC3626_15 Datasheet, PDF (14/28 Pages) Linear Technology – 20V, 2.5A Synchronous Monolithic Step-Down Regulator with Current and Temperature Monitoring

LTC3626
Applications Information
target regulation voltage. To guarantee the ripple current
does not exceed a specified maximum the inductance
should be chosen according to:
L
=
⎛
⎝⎜⎜
f
•
V_OUT
∆I_{L(MAX )}
⎞⎛
⎠⎟⎟⎝⎜⎜^1–
V_OUT
V_{IN(MAX )}
⎞
⎠⎟⎟
Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fixed inductor value but is very dependent on the
inductance selected. As the inductance increases, core loss
decreases. Unfortunately, increased inductance requires
more turns of wire leading to increased copper loss.
Ferrite designs exhibit very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core materials saturate “hard,” meaning the inductance
collapses abruptly when the peak design current is ex-
ceeded. This collapse will result in an abrupt increase in
inductor ripple current, so it is important to ensure the
core will not saturate.
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
Toroidal 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, Coilcraft, TDK and
Würth Elektronik. Table 1 gives a sampling of available
surface mount inductors.
C_IN and C_OUT Selection
The input capacitance, C_IN, is needed to filter the trapezoi-
dal wave current at the drain of the top power MOSFET.
To prevent large voltage transients from occurring, a low
ESR input capacitor sized for the maximum RMS current
is recommended. The maximum RMS current is given by:
( ) _{IRMS = IOUT(MAX)}
V_OUT V_IN – V_OUT
V_IN
Table 1. Inductor Selection Table
INDUCTANCE DCR MAX CURRENT DIMENSIONS
Vishay IHLP-2525CZ-01 Series
0.33µH 3.5mW
20A
6.5mm × 7mm
0.47µH 4.0mW
17.5A
0.68µH 5.0mW
15.5A
0.82µH 6.7mW
13A
1.0µH 9.0mW
11A
1.5µH 14mΩ
9A
2.2µH 18mΩ
8A
3.3µH 28mΩ
6A
4.7µH 37mΩ
5.5A
6.8µH 54mΩ
4.5A
Toko FDV0620 Series
0.47µH 8.3mW
9A
7mm × 7.7mm
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
0.47µH 4.2mW
17A
7mm × 7.3mm
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
0.47µH 3.4mW
16A
7mm × 7.7mm
0.72µH 7.5mW
12A
1µH
9.5mW
11A
1.5µH 10.5mW
9A
HEIGHT
3mm
2.0mm
3.0mm
3.0mm
3.2mm
3.8mm
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS ≅ I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that ripple current ratings
3626fa
14
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