LTC3633A-3_15 Datasheet, PDF (13/28 Pages) Linear Technology – Dual Channel 3A, 20V Monolithic Synchronous Step-Down Regulator

LTC3633A-2/LTC3633A-3
APPLICATIONS INFORMATION
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). Note that the largest ripple
current occurs at the highest PV_IN. Exceeding 60% of
I_OUT(MAX) is not recommended. To guarantee that ripple
current does not exceed a specified maximum, the induc-
tance should be chosen according to:
L
=


f
•
V_OUT
∆I_{L(M AX )}

 ^1–
V_OUT
V_IN(MAX
)


The inductor ripple current also must not be so large that
its valley current level exceeds the negative current limit,
which can be as small as –1.2A. If the negative current
limit is exceeded while the part is in the forced continu-
ous mode of operation, V_OUT can get charged up to above
its regulation level – until the inductor current no longer
exceeds the negative current limit. In such instances,
choose a larger inductor value to reduce the inductor
ripple current. The alternative is to reduce the inductor
ripple current by decreasing the R_T resistor value which
will increase the switching frequency.
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
losses decrease. Unfortunately, increased inductance
requires more turns of wire, leading to increased DCR
and copper loss.
Ferrite designs exhibit very low core loss and are pre-
ferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. 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, so it is important to ensure that
the core will not saturate.
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. Table 1 gives a
sampling of available surface mount inductors.
Table 1. Inductor Selection Table
INDUCTANCE DCR
(µH)
(mΩ)
MAX
CURRENT
(A)
DIMENSIONS
(mm)
Würth Electronik WE-HC 744312 Series
0.25
2.5
18
0.47
3.4
16
0.72
7.5
12
7 × 7.7
1.0
9.5
11
1.5
10.5
9
Vishay IHLP-2020BZ-01 Series
0.22
5.2
15
0.33
8.2
12
0.47
8.8
11.5
0.68
12.4
10
1
20
7
5.2 × 5.5
Toko FDV0620 Series
0.20
4.5
12.4
0.47
8.3
9.0
7 × 7.7
1.0
18.3
5.7
Coilcraft D01813H Series
0.33
4
10
0.56
10
7.7
1.2
17
5.3
6 × 8.9
TDK RLF7030 Series
1.0
8.8
6.4
1.5
9.6
6.1
6.9 × 7.3
HEIGHT
(mm)
3.8
2
2.0
5.0
3.2
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
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
from capacitor manufacturers are often based on only
2000 hours of life which makes it advisable to further de-
rate the capacitor, or choose a capacitor rated at a higher
temperature than required.
For more information www.linear.com/LTC3633A-2
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