LTC3550 Linear Technology, LTC3550 Datasheet - Page 12

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

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OPERATIO
LTC3550
The LTC3550 consists of two main blocks: a lithium-ion
battery charger and a high-effi ciency buck converter that
can be powered from the battery. The charger is designed
to effi ciently 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 fi nal fl oat
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.
12
EN
DRIVEN LOW
BAT < 2.9V
2.9V < BAT
BAT < 4.1V
IN VOLTAGE MODE
U
I
BAT
< I
CHRG STATE: PULLDOWN
CHRG STATE: PULLDOWN
BAT > 2.9V
TERMINATE
NO CHARGE CURRENT
1/10th FULL CURRENT
I
DCIN
CHRG STATE: Hi-Z
CHRG STATE: Hi-Z
TRICKLE CHARGE
FULL CURRENT
SHUTDOWN
DROPS TO 20 A
STANDBY
CHARGE
MODE
MODE
MODE
MODE
DCIN POWER APPLIED
Figure 1. LTC3550 State Diagram of a Charge Cycle
DCIN POWER
REMOVED
EN
DRIVEN HIGH
DCIN POWER
REMOVED
POWER SELECTION
STARTUP
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
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 fi nal
fl oat voltage (4.2V), the charger enters constant-voltage
EN
DRIVEN HIGH
USBIN POWER
REMOVED OR
DCIN POWER
APPLIED
USBIN POWER
REMOVED OR
DCIN POWER
ONLY USB POWER APPLIED
APPLIED
1/5 FULL CURRENT
FULL CURRENT
CHRG STATE: PULLDOWN
CHRG STATE: PULLDOWN
I
1/10th FULL CURRENT
NO CHARGE CURRENT
USBIN
DCIN
TRICKLE CHARGE
CHRG STATE: Hi-Z
CHRG STATE: Hi-Z
SHUTDOWN
DROPS TO 18 A
STANDBY
CHARGE
MODE
MODE
MODE
MODE
> V
Þ
Þ
I
IN VOLTAGE MODE
BAT > 2.9V
HPWR = HIGH
BAT
HPWR = LOW
USBIN
< I
TERMINATE
3550 F01
.
EN
DRIVEN LOW
BAT < 4.1V
BAT < 2.9V
2.9V < BAT
3550fa