Datasheet
LTC3621/LTC3621-2
12
3621fb
For more information www.linear.com/LTC3621
applicaTions inForMaTion
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can
be expressed as:
% Efficiency = 100% – (Loss1 + Loss2 + …)
where Loss1, Loss2, etc. are the individual losses as a
percentage of input power. Although all dissipative elements
in the circuit produce losses, three main sources usually
account for most of the losses in LTC3621 circuits: 1) I
2
R
losses, 2) switching and biasing losses, 3) other losses.
1. I
2
R losses are calculated from the DC resistances of
the internal switches, R
SW
, and external inductor, R
L
.
In continuous mode, the average output current flows
through inductor L but is “chopped” between the
internal top and bottom power MOSFETs. Thus, the
series resistance looking into the SW pin is a function
of both top and bottom MOSFET R
DS(ON)
and the duty
cycle (DC) as follows:
R
SW
= (R
DS(ON)TOP
)(DC) + (R
DS(ON)BOT
)(1 – DC)
The R
DS(ON)
for both the top and bottom MOSFETs can be
obtained from the Typical Performance Characteristics
curves. Thus to obtain I
2
R losses:
I
2
R losses = I
OUT
2
(R
SW
+ R
L
)
Table 1. Inductor Selection Table
INDUCTOR
INDUCTANCE
(µH)
DCR
(mΩ)
MAX CURRENT
(A)
DIMENSIONS
(mm)
HEIGHT
(mm) MANUFACTURER
IHLP-1616BZ-11 Series 1.0
2.2
4.7
24
61
95
4.5
3.25
1.7
4.3 × 4.7
4.3 × 4.7
4.3 × 4.7
2
2
2
Vishay
www.vishay.com
IHLP-2020BZ-01 Series 1
2.2
3.3
4.7
5.6
6.8
18.9
45.6
79.2
108
113
139
7
4.2
3.3
2.8
2.5
2.4
5.4 × 5.7
5.4 × 5.7
5.4 × 5.7
5.4 × 5.7
5.4 × 5.7
5.4 × 5.7
2
2
2
2
2
2
FDV0620 Series 1
2.2
3.3
4.7
18
37
51
68
5.7
4
3.2
2.8
6.7 × 7.4
6.7 × 7.4
6.7 × 7.4
6.7 × 7.4
2
2
2
2
Toko
www.toko.com
MPLC0525L Series 1
1.5
2.2
16
24
40
6.4
5.2
4.1
6.2 × 5.4
6.2 × 5.4
6.2 × 5.4
2.5
2.5
2.5
NEC/Tokin
www.nec-tokin.com
XFL4020 Series 1.0
1.5
2.2
3.3
4.7
10.8
14.4
21.3
34.8
52.2
5.1
4.4
3.5
2.5
2.5
4 × 4
4 × 4
4 × 4
4 × 4
4 × 4
2.1
2.1
2.1
2.1
2.1
Coilcraft
www.coilcraft.com
RLF7030 Series 1
1.5
2.2
3.3
4.7
6.8
8.8
9.6
12
20
31
45
6.4
6.1
5.4
4.1
3.4
2.8
6.9 × 7.3
6.9 × 7.3
6.9 × 7.3
6.9 × 7.3
6.9 × 7.3
6.9 × 7.3
3.2
3.2
3.2
3.2
3.2
3.2
TDK
www.tdk.com
WE-TPC 4828 Series 1.2
1.8
2.2
2.7
3.3
3.9
4.7
17
20
23
27
30
47
52
3.1
2.7
2.5
2.35
2.15
1.72
1.55
4.8 × 4.8
4.8 × 4.8
4.8 × 4.8
4.8 × 4.8
4.8 × 4.8
4.8 × 4.8
4.8 × 4.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
Würth Elektronik
www.we-online.com