LTC3405A-1.8_15 Datasheet, PDF (11/16 Pages) Linear Technology – 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT

LTC3405A-1.5/LTC3405A-1.8
APPLICATIO S I FOR ATIO
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3405A series parts. These items are also illustrated
graphically in Figures 4 and 5. Check the following in your
layout:
1. The power traces, consisting of the GND trace, the SW
trace and the V_IN trace should be kept short, direct and
wide.
2. Does the (+) plate of C_IN connect to V_IN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
3. Keep the (–) plates of C_IN and C_OUT as close as possible.
Design Example
As a design example, assume the LTC3405A-1.8 is used
in a single lithium-ion battery-powered cellular phone
application. The V_IN will be operating from a maximum of
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.25A but most of the time it will be in
standby mode, requiring only 2mA. Efficiency at both low
and high load currents is important. Output voltage is
1.8V. With this information we can calculate L using
equation (1),
L
=
1
(^f)(^∆I_L )
V_OUT ^{⎛⎝⎜} 1−
V_OUT
V_IN
⎞
⎠⎟
(3)
Substituting V_OUT = 1.8V, V_IN = 4.2V, ∆I_L = 100mA and
f = 1.5MHz in equation (3) gives:
L
=
1.8V
1.5MHz(100mA)
⎛⎝⎜_1−
1.8V
4.2V
⎞⎠⎟
≅
6.8µH
For best efficiency choose a 300mA or greater inductor
with less than 0.3Ω series resistance.
C_IN will require an RMS current rating of at least 0.125A ≅
I_LOAD(MAX)/2 at temperature and C_OUT will require an ESR
of less than 0.5Ω. In most cases, a tantalum capacitor will
satisfy this requirement.
Figure 6 shows the complete circuit along with its effi-
ciency curve.
1
6
RUN MODE
–
V_OUT
+
C_OUT
L1
LTC3405A-1.8
2
5
GND V_OUT
3 _SW
4
V_IN
C_IN
V_IN
3405A1518 F04
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 4. LTC3405A-1.8 Layout Diagram
V_OUT
VIA TO V_IN
V_IN
PIN 1
LTC3405A-1.8
L1 ^SW
C_OUT
C_IN
GND
3405A1518 F05
Figure 5. LTC3405A-1.8 Suggested Layout
3405a1518fa
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