LTC3543
16
3543fa
V
OUT
4
5
6
3
2
1
R1 R2
SW
V
IN
VFB
MODE
RUN
C
FWD
C
IN
L
GND
GND
V
IN
CAP
C
OUT
3543 F06
C1
VIA TO GND VIA TO V
OUT
VIA TO R2
VIA TO R1
Figure 6. LTC3543 Suggested Layout
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. Effi ciency 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
fI
V
V
OUT
L
OUT
IN
=−
⎛
⎝
⎜
⎞
⎠
⎟
•
•
Δ
1
A 3.3μH inductor works well for this application, yielding
a ripple current of only 130mA. For best effi ciency 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.
R
V
V
Rk
OUT
2
06
1 1 604=−
⎛
⎝
⎜
⎞
⎠
⎟
=
.
•
Figure 6 shows the complete circuit along with its ef-
fi ciency curve.
APPLICATIONS INFORMATION
