LTC3550-1_15 Datasheet, PDF (21/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550-1
APPLICATIO S I FOR ATIO
and high load currents is important. With this information
we can calculate L using Equation (1),
∆I_L
=
V_OUT
f_O • L
•
⎛
⎝⎜
1−
V_OUT
V_CC
⎞
⎠⎟
Substituting V_OUT = 1.875V, V_CC = 4.2V, ΔI_L = 240mA and
f_O = 1.5MHz in Equation (3) gives:
L
=
1.875V
1.5MHz • (240mA)
•
⎛⎝⎜_1−
1.875V ⎞
4.2V ⎠⎟
=
2.88µH
A 2.2µH inductor works well for this application. For best
efficiency choose a 720mA or greater inductor with less
than 0.2Ω series resistance. C_IN will require an RMS cur-
rent rating of at least 0.3A = I_LOAD(MAX)/2 at temperature
and C_OUT will require an ESR of less than 0.25Ω. In most
cases, a ceramic capacitor will satisfy this requirement.
Figure 7 shows the complete circuit along with its ef-
ficiency curve.
LTC3550-1
6
V_OUT
BOLD LINES INDICATE
HIGH CURRENT PATHS
V_CC
+
7
V_CC
10
SW
–
C_IN
8 GND
GND 9
L1
–
17
V_OUT
C_{OUT +}
3550-1 F05
Figure 5. DC-DC Converter Layout Diagram
VIA TO V_OUT
VIA TO V_CC
SW
C_IN
L1
V_CC
GND
C_OUT
V_OUT
3550-1 F06
Figure 6. DC-DC Converter Suggested Layout
35501f
21