LTC3543_15 Datasheet, PDF (16/20 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

LTC3543
APPLICATIONS INFORMATION
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 • ΔI_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.
V_OUT
VIA TO R2
C_OUT
GND
VIA TO R1
V_IN
C_IN
L
C1
SW
V_IN
CAP
GND
RUN
MODE
VFB
R1
R2
C_FWD
VIA TO GND
Figure 6. LTC3543 Suggested Layout
VIA TO V_OUT
3543 F06
3543fa
16