LTC3550_15 Datasheet, PDF (22/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
APPLICATIO S I FOR ATIO
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. 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 = 1.8V, V_CC = 4.2V, ΔI_L = 240mA and
f_O = 1.5MHz in Equation (3) gives:
L
=
1.8V
1.5MHz • (240mA)
•
⎛⎝⎜_1−
1.8V
4.2V
⎞
⎠⎟
=
2.86µH
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.
For the feedback resistors, choose R1 = 301k. R2 can then
be calculated from equation (4) to be:
R2 = R1^{⎛⎝⎜}
V_OUT
0.6
– 1^{⎞⎠⎟}
=
604k
Figure 8 shows the complete circuit along with its ef-
ficiency curve.
WALL
ADAPTER
USB
PORT
SW
1µF
1µF
LTC3550
DCIN
V_FB
EN
RUN
USBIN V_CC
2.1k
1%
1.24k
1%
IUSB BAT
IDC ITERM
GND
* MURATA LQH32CN2R2M33
** TAIYO YUDEN JMK316BJ106ML
† _{TAIYO YUDEN LMK212BJ475MG}
2.2µH*
22pF 604k
V_OUT
1.8V
10µF** _600mA
CER
301k
800mA (WALL)
475mA (USB)
1.24k
1%
4.7µF^†
+
4.2V
SINGLE-CELL
Li-Ion BATTERY
3550 F08a
95
V_OUT = 1.8V
90
V_CC = 2.7V
85
V_CC = 4.2V
80
V_CC = 3.6V
75
70
65
60
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
3550 G24
Figure 8a. Design Example Circuit
Figure 8b. Buck Regulator Efficiency vs Output Current
TYPICAL APPLICATIO S
Full Featured Dual Input Charger Plus Step-Down Converter
WALL
ADAPTER
USB
POWER
1mF
1mF
2.1k 1.24k 1k
1% 1% 1%
DCIN
BAT
LTC3550
USBIN PWR
CHRG
EN
RUN
IUSB
V_CC
IDC
SW
ITERM
V_FB
GND
1k
1k
2.2mH
22mF
800mA (WALL)
475mA (USB)
4.7mF
+
4.2V
SINGLE-CELL
Li-Ion BATTERY
10mF
CER
V_OUT
1.8V
600mA
604k
301k
3550 TA03
22
3550fa