LTC3543 Datasheet, PDF (13/16 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

LTC3543
APPLICATIO S I FOR ATIO
problem. For a detailed explanation of the switching control
loop theory, see Application Note 76.
A second, more severe transient is caused by switching
in loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in paral-
lel with C_OUT, causing a rapid drop in V_OUT. No regulator
can deliver enough current to prevent this problem if the
load switch resistance is low and it is driven quickly. The
only solution is to limit the rise time of the switch drive
so that the load rise time is limited to approximately
25 • C_LOAD. Thus, a 10µF capacitor charging to 3.3V would
require a 250µs rise time, limiting the charging current
to about 130mA.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3543. These items are also illustrated graphically
in Figures 5 and 6. 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 V_FB pin connect directly to the feedback voltage
reference? Ensure that there is no load current running
from the feedback reference voltage and the V_FB pin.
3. 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.
4. Keep the switching node, SW, away from the sensitive
V_FB node.
V_OUT
GND
C_OUT
+–
5. Keep the (–) plates of C_IN and C_OUT as close as pos-
sible.
Design Example
As a design example, assume the LTC3543 is used in
a single lithium-ion battery-powered cellular phone ap-
plication. 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 600mA, 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 1.5V. With this information we can calculate L using
Equation 1,
L
=
V_OUT
f • ∆I_L
•
⎛
⎝⎜
1−
V_OUT ⎞
V_IN ⎠⎟
A 3.3µH inductor works well for this application, yielding
a ripple current of only 130mA. For best efficiency choose
a 665mA or greater inductor with less than 0.05Ω series
resistance.
In Equation 2, C_IN will require an RMS current rating of at
least 0.3A at I_LOAD(MAX)/2 at temperature.
Using Equation 3, Selecting a 4.7µF capacitor with an
ESR of 0.05Ω for C_OUT yields an 8mV voltage ripple on
the regulated output voltage.
For feedback resistors, choose R1 = 402k. R2 can then
be calculated using Equation 4.
R2
=
⎛
⎝⎜
V_OUT
0.6V
−
1^{⎞⎠⎟}
• R1=
604k
Figure 6 shows the complete circuit along with its ef-
ficiency curve.
C_IN
L
SW
V_IN
V_IN
CAP
RUN
+
– ^C1
MODE
V_FB
–
R1 R2
C_FWD
+
3543 F07
Figure 5. LTC3543 Layout Diagram
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