LTC3550 Datasheet, PDF (12/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550
U
OPERATIO
The LTC3550 consists of two main blocks: a lithium-ion
battery charger and a high-efficiency buck converter that
can be powered from the battery. The charger is designed
to efficiently manage charging of a single-cell lithium-ion
battery from two separate power sources: a wall adapter
and USB power bus. The internal P-channel MOSFETs
can supply up to 950mA from the wall adapter source
and 500mA from the USB power source. The final float
voltage accuracy is ±0.6%.
The buck converter uses a constant frequency, current
mode step-down architecture. Both the main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
for the buck converter are internal. The LTC3550 requires
no external diodes or sense resistors.
Lithium-Ion Battery Charger
A charge cycle begins when the voltage at either the DCIN pin
or USBIN pin rises above the UVLO threshold level and the
charger is enabled through the EN pin. When either input is
supplying power, logic low enables the charger and logic high
disables it (a 2MΩ pull-down defaults the charger to the
charging state). The DCIN input draws 20µA when the
charger is in shutdown. The USBIN input draws 18µA
during shutdown if no power is applied to DCIN, but draws
only 10µA when V_DCIN > V_USBIN.
Once the charger is enabled, it enters constant-current
mode, where the programmed charge current is supplied
to the battery. When the BAT pin approaches the final
float voltage (4.2V), the charger enters constant-voltage
STARTUP
BAT < 2.9V
DCIN POWER APPLIED
ONLY USB POWER APPLIED
POWER SELECTION
TRICKLE CHARGE
MODE
1/10th FULL CURRENT
CHRG STATE: PULLDOWN
DCIN POWER
REMOVED
USBIN POWER
REMOVED OR
DCIN POWER
APPLIED
TRICKLE CHARGE
MODE
1/10th FULL CURRENT
CHRG STATE: PULLDOWN
BAT < 2.9V
BAT > 2.9V
2.9V < BAT
CHARGE
MODE
FULL CURRENT
CHRG STATE: PULLDOWN
I_BAT < I_TERMINATE
IN VOLTAGE MODE
BAT > 2.9V
CHARGE
MODE
FULL CURRENTÞHPWR = HIGH
1/5 FULL CURRENTÞHPWR = LOW
2.9V < BAT
CHRG STATE: PULLDOWN
I_BAT < I_TERMINATE
IN VOLTAGE MODE
BAT < 4.1V
STANDBY
MODE
NO CHARGE CURRENT
CHRG STATE: Hi-Z
STANDBY
MODE
NO CHARGE CURRENT
CHRG STATE: Hi-Z
BAT < 4.1V
EN
DRIVEN LOW
12
SHUTDOWN
MODE
I_DCIN DROPS TO 20mA
CHRG STATE: Hi-Z
EN
DRIVEN HIGH
DCIN POWER
REMOVED
EN
DRIVEN HIGH
USBIN POWER
REMOVED OR
DCIN POWER
APPLIED
SHUTDOWN
MODE
I_USBIN DROPS TO 18mA
CHRG STATE: Hi-Z
EN
DRIVEN LOW
3550 F01
Figure 1. LTC3550 State Diagram of a Charge Cycle
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