LTC3550 Linear Technology, LTC3550 Datasheet - Page 22

• Manufacturer Part Number: LTC3550
• Description: Dual Input USB/AC Adapter Li-Ion Battery Charger
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
ADAPTER
LTC3550
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. With this information
we can calculate L using Equation (1),
Substituting V
f
TYPICAL APPLICATIO S
22
O
WALL
PORT
= 1.5MHz in Equation (3) gives:
L
∆ =
USB
=
I
L
1 5
**
.
*
1µF
†
V
f
MURATA LQH32CN2R2M33
TAIYO YUDEN JMK316BJ106ML
TAIYO YUDEN LMK212BJ475MG
O
MHz
1µF
OUT
•
L
1 8
OUT
.
• 1
• (
⎛
⎝ ⎜
2.1k
1%
V
240
= 1.8V, V
U
−
ADAPTER
Figure 8a. Design Example Circuit
1.24k
1%
POWER
mA
V
WALL
V
OUT
USB
CC
U
DCIN
EN
USBIN
IUSB
IDC
)
•
CC
LTC3550
⎞
⎠ ⎟
⎛
⎝ ⎜
GND
1
2.1k
1%
= 4.2V, ΔI
ITERM
−
RUN
Full Featured Dual Input Charger Plus Step-Down Converter
BAT
V
SW
V
1 F
1 8
4 2
CC
U
FB
.
W
.
1.24k
1%
V
V
800mA (WALL)
475mA (USB)
⎞
⎠ ⎟
1.24k
1%
1 F
L
=
1k
1%
= 240mA and
2 86
2.2µH*
.
DCIN
USBIN
EN
IUSB
IDC
ITERM
4.7µF
U
22pF
µ
LTC3550
+
GND
†
H H
604k
301k
3550 F08a
CHRG
4.2V
SINGLE-CELL
Li-Ion BATTERY
PWR
RUN
BAT
V
V
SW
CC
FB
10µF**
CER
1k
A 2.2µH inductor works well for this application. For best
effi ciency choose a 720mA or greater inductor with less
than 0.2Ω series resistance. C
rent rating of at least 0.3A = I
and C
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:
Figure 8 shows the complete circuit along with its ef-
fi ciency curve.
22 F
2.2 H
R
1k
2
V
1.8V
600mA
OUT
OUT
=
R
Figure 8b. Buck Regulator Effi ciency vs Output Current
1
will require an ESR of less than 0.25Ω. In most
604k
301k
3550 TA03
⎛
⎝ ⎜
V
0 6
OUT
.
95
90
85
80
75
70
65
60
800mA (WALL)
475mA (USB)
0.1
–
V
OUT
4.7 F
10 F
CER
1
⎞
⎠ ⎟
= 1.8V
=
+
604
1
OUTPUT CURRENT (mA)
V
1.8V
600mA
OUT
V
V
4.2V
SINGLE-CELL
Li-Ion BATTERY
V
CC
CC
k
CC
LOAD(MAX)
= 2.7V
= 3.6V
IN
= 4.2V
10
will require an RMS cur-
/2 at temperature
100
3550 G24
1000
3550fa