LTC3122_15 Datasheet, PDF (12/26 Pages) Linear Technology – 15V, 2.5A Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3122
Applications Information
Pcb Layout Guidelines
The high switching frequency of the LTC3122 demands
careful attention to board layout. A careless layout will
result in reduced performance. Maximizing the copper
area for ground will help to minimize die temperature rise.
A multilayer board with a separate ground plane is ideal,
but not absolutely necessary. See Figure 3 for an example
of a two-layer board layout.
PGND
V_IN
SW 1
2
3
4
V_CC 5
6
RT
13
PGND
CAP
12
11
10 SGND
9
8 FB
7
V_C
PGND
V_OUT
rent capability by reducing the inductor ripple current. The
minimum inductance value, L, is inversely proportional to
operating frequency and is given by the following equation:
( ) _{L >}
V_IN • V_OUT − V_IN
ƒ • Ripple • V_OUT
µH and L >
3
ƒ
where:
Ripple = Allowable inductor current ripple (amps
peak-to-peak)
ƒ = Switching Frequency in MHz
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High frequency
ferrite core inductor materials reduce frequency depen-
dent power losses compared to cheaper powdered iron
types, improving efficiency. The inductor should have
low ESR (series resistance of the windings) to reduce the
I²R power losses, and must be able to support the peak
inductor current without saturating. Molded chokes and
some chip inductors usually do not have enough core
area to support the peak inductor currents of 3A to 4A
seen on the LTC3122. To minimize radiated noise, use a
shielded inductor.
See Table 1 for suggested components and suppliers.
3122 F02
Figure 3. Traces Carrying High Current Are Direct (PGND, SW, V_IN
and V_OUT). Trace Area at FB and V_C Are Kept Low. Trace Length to
Input Supply Should Be Kept Short. V_IN and V_OUT Ceramic Capacitors
Should Be Placed as Close to the LTC3122 Pins as Possible
Schottky Diode
Although it is not required, adding a Schottky diode from
SW to V_OUT can improve the converter efficiency by about
4%. Note that this defeats the output disconnect and short-
circuit protection features of the LTC3122.
Component Selection
Inductor Selection
The LTC3122 can utilize small surface mount inductors
due to its high switching frequency (up to 3MHz). Larger
values of inductance will allow slightly greater output cur-
Table 1. Recommended Inductors
PART NUMBER
MAX DC
VALUE DCR CURRENT
(µH) (mΩ) (A)
SIZE (mm)
W×L×H
Coilcraft LPS4018
Coilcraft MSS7341
Coilcraft MSS1260T
1 42
3.3 20
33 54.9
3.8
4 × 4 × 1.8
3.72 7.3 × 7.3 × 4.1
4.34 12.3 × 12.3 × 6.2
Coiltronics DRQ73
Coiltronics SD7030
Coiltronics DR125
0.992 24
3.3 24
33 59
3.99 7.6 × 7.6 × 3.55
3
7×7×3
3.84 12.5 × 12.5 × 6
Murata LQH6PP
Murata LQH6PP
1
9
4.3
3.3 16
3.8
6 × 6 × 4.3
6 × 6 × 4.3
Sumida CDRH50D28RNP 1.2 13
4.8
5 × 5 × 2.8
Sumida CDRH8D28NP 3.3 18
4
8×8×3
Sumida CDRH129HF
33 53
4.25
12 × 12 × 10
Taiyo-Yuden NR6045
3 31
3.2
6 × 6 × 4.5
TDK LTF5022T
TDK SPM6530T
TDK VLF12060T
1.2 25
4.2
5 × 5.2 × 2.2
3.3 20
4.1
7 × 7 × 3.2
33 53
3.4
11.7 × 12 × 6
Würth WE-PD
3.3 32.5 3.1
7.3 × 7.3 × 2
3122fa
12
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