LTC3550_15 Datasheet, PDF (15/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
APPLICATIO S I FOR ATIO
Figure 2 shows the basic LTC3550 application circuit.
External component selection is driven by the charging
requirements and the buck regulator load requirements.
WALL
ADAPTER
USB
PORT
C2
C1
L1
SW
LTC3550
C_F
R2
DCIN
V_FB
RUN
R1
USBIN V_CC
R_IUSB
R_IDC
IUSB BAT
IDC ITERM
GND
C_IN
+
R_ITERM
V_OUT
1.2V
600mA
C_OUT
4.2V
SINGLE
CELL Li-Ion
BATTERY
3550 F02
Figure 2. LTC3550 Basic Circuit
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.
R_IDC
=
1000V
I_CHRG(DC)
,
I_CHRG(DC)
=
1000V
R_IDC
Similarly, the charge current from the USB supply is
programmed using a single resistor from the IUSB pin
to ground. Setting HPWR pin to its high state will select
100% of the programmed charge current, while setting
HPWR to its low state will select 20% of the programmed
charge current.
R_IUSB
= ^1000V
I_CHRG(USB)
(HPWR = HIGH)
I_CHRG(USB)
=
1000V
R_IUSB
(HPWR = HIGH)
I_CHRG(USB)
=
200V
R_IUSB
(HPWR = LOW)
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, (charging
from
wall
adapter)
I_BAT
=
V_IUSB
R_IUSB
•
1000, (charging from USBsupply,
HPWR = HIGH)
I_BAT
=
V_IUSB
R_IUSB
•
200, (charging from USBsupply,
HPWR = LOW)
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:
100V
100V
R_ITERM = _ITERMINATE ,I_TERMINATE = _RITERM
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 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 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.
*Any external sources that hold the ITERM pin above 100mV will prevent the LTC3550 from
terminating a charged cycle.
3550fa
15