LTC3550-1 Linear Technology, LTC3550-1 Datasheet - Page 15

• Manufacturer Part Number: LTC3550-1
• Description: Dual Input USB/AC Adapter Li-Ion Battery Charger
• Manufacturer: Linear Technology

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Figure 2 shows the basic LTC3550-1 application circuit.
External component selection is driven by the charging
requirements and the buck regulator load requirements.
Programming and Monitoring Charge Current
The charge current delivered to the battery from the wall
adapter supply is programmed using a single resistor
from the IDC pin to ground. Likewise, the charge current
from the USB supply is programmed using a single resis-
tor from the IUSB pin to ground. The program resistor
and the charge current (I
following equations:
Charge current out of the BAT pin can be determined at
any time by monitoring the IDC or IUSB pin voltage and
using the following equations:
APPLICATIO S I FOR ATIO
ADAPTER
POWER
WALL
R
R
I
I I
USB
BAT
BAT
IDC
IUS
B B
=
=
=
=
R
R
V
V
I
C1
CHRG DC
IDC
IDC
IUSB
IUSB
1000
I
CHRG USB
Figure 2. LTC3550-1 Basic Circuit
1000
C2
•
(
R
•
1000
(
V
IUSB
1000
U
V
)
R
,
IDC
I
)
CHRG DC
RUN
DCIN
USBIN
IUSB
IDC
,
( arg
LTC3550-1
I
ch
CHRG USB
( arg
CHRG
U
ch
GND
(
ITERM
V
BAT
V
OUT
SW
) are calculated using the
(
)
CC
ing from wall adapter
=
ing from USB
1000
)
W
R
R
=
L1
ITERM
C
IDC
IN
1000
R
V
IUSB
+
V
C
U
3550-1 F02
OUT
4.2V
SINGLE
CELL Li-Ion
BATTERY
sup
V
1.875V
600mA
OUT
ply
)
) )
Programming Charge Termination
The charge cycle terminates when the charge current falls
below the programmed termination threshold during con-
stant-voltage mode. This threshold is set by connecting an
external resistor, R
The charge termination current threshold (I
set by the following equation:
The termination condition is detected by using an internal
fi ltered comparator to monitor the ITERM pin. When the
ITERM pin voltage drops below 100mV* for longer than
t
charge current is latched off and the LTC3550-1 enters
standby mode.
When charging, transient loads on the BAT pin can cause
the ITERM pin to fall below 100mV for short periods of
time before the DC charge current has dropped below the
programmed termination current. The 1.5ms fi lter time
(t
transient loads of this nature do not result in premature
charge cycle termination. Once the average charge current
drops below the programmed termination threshold, the
LTC3550-1 terminates the charge cycle and stops providing
any current out of the BAT pin. In this state, any load on
the BAT pin must be supplied by the battery.
Buck Regulator Inductor Selection
For most applications, the value of the inductor will fall in
the range of 1µH to 4.7µH. Its value is chosen based on
the desired inductor ripple current. Large value inductors
lower ripple current and small value inductors result in
higher ripple currents. Higher V
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI
(40% of 600mA).
* Any external sources that hold the ITERM pin above 100mV will prevent the LTC3550-1
TERMINATE
TERMINATE
from terminating a charge cycle.
∆ =
R
I
ITERM
L
V
f
O
(typically 1.5ms), charging is terminated. The
) on the termination comparator ensures that
OUT
=
•
L
I
TERMINATE
• 1
100
⎛
⎝ ⎜
ITERM
−
V
V
V
OUT
, from the ITERM pin to ground.
CC
,
I
TERMINATE
⎞
⎠ ⎟
CC
or V
LTC3550-1
OUT
=
R
also increases
100
TERMINATE
ITERM
L
= 240mA
V
15
35501f
) is
(1)