LTC3550-1_15 Datasheet, PDF (15/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550-1
APPLICATIO S I FOR ATIO
Figure 2 shows the basic LTC3550-1 application circuit.
External component selection is driven by the charging
requirements and the buck regulator load requirements.
WALL
ADAPTER
USB
POWER
LTC3550-1
RUN
SW
DCIN V_OUT
C2
R_IUSB
USBIN V_CC
IUSB BAT
IDC ITERM
C1
GND
R_IDC
L1
C_IN
R_ITERM
V_OUT
1.875V
C_OUT ^600mA
+ ^4.2V
SINGLE
CELL Li-Ion
BATTERY
Figure 2. LTC3550-1 Basic Circuit
3550-1 F02
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_CHRG) are calculated using the
following equations:
R_IDC
=
1000V
I_CHRG(DC)
, I_CHRG(DC)
=
1000V
R_IDC
R_IUSB
=
1000V
I_CHRG(USB)
,
I_CHRG(USB)
=
1000V
R_IUSB
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:
I_BAT
=
V_IDC
R_IDC
• 1000
(ch arg in g
from wall adapter)
I_BAT
=
V_IUSB
R_IUSB
• 1000
(ch arg in g
from USB supply)
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_ITERM, from the ITERM pin to ground.
The charge termination current threshold (I_TERMINATE) is
set by the following equation:
R_ITERM
=
100V
I_TERMINATE
, I_TERMINATE
=
100V
R_ITERM
The termination condition is detected by using an internal
filtered comparator to monitor the ITERM pin. When the
ITERM pin voltage drops below 100mV* for longer than
t_TERMINATE (typically 1.5ms), charging is terminated. The
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 filter time
(t_TERMINATE) on the termination comparator ensures that
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_CC or V_OUT also increases
the ripple current as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI_L = 240mA
(40% of 600mA).
∆I_L
=
V_OUT
f_O • L
•
⎛
⎝⎜ ^1−
V_OUT
V_CC
⎞
⎠⎟
(1)
* Any external sources that hold the ITERM pin above 100mV will prevent the LTC3550-1
from terminating a charge cycle.
35501f
15