LTC3550-1_15 Datasheet, PDF (12/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter

LTC3550-1
U
OPERATIO
The LTC3550-1 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 LTC3550-1 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 ENABLE pin. The “on”
state of this pin depends on which source is supplying
power. When the wall adapter input (DCIN) is supply-
ing power, logic low enables the charger and logic high
disables it (a 2MΩ pulldown defaults the charger to
the charging state). The opposite is true when the USB
input (USBIN) is supplying power; logic low disables the
charger and logic high enables it (the default is the
shutdown state). The DCIN input draws 20µA when the
charger is in shutdown. The USBIN input draws 18µA dur-
ing 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
mode and the charge current begins to decrease. Once
the charge current drops below the programmed termina-
tion threshold (set by the external resistor R_ITERM), the
internal P-channel MOSFET is shut off and the charger
enters standby mode.
In standby mode, the charger sits idle and monitors the
battery voltage using a comparator with a 6ms filter time
(t_RECHRG). A charge cycle automatically restarts when the
battery voltage falls below 4.1V (which corresponds to ap-
proximately 80% to 90% battery capacity). This ensures
that the battery is kept near a fully charged condition and
eliminates the need for periodic charge cycle initiations.
Figure 1 uses a state diagram to describe the behavior of
the LTC3550-1 battery charger.
STARTUP
DCIN POWER APPLIED
ONLY USB POWER APPLIED
POWER SELECTION
CHARGE
MODE
FULL CURRENT
DCIN POWER
REMOVED
USBIN POWER
REMOVED OR
DCIN POWER
APPLIED
CHARGE
MODE
FULL CURRENT
CHRG STATE: PULLDOWN
CHRG STATE: PULLDOWN
I_BAT < I_TERMINATE
IN VOLTAGE MODE
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
ENABLE
DRIVEN LOW
SHUTDOWN
MODE
I_DCIN DROPS TO 20µA
CHRG STATE: Hi-Z
ENABLE
DRIVEN HIGH
DCIN POWER
REMOVED
ENABLE
DRIVEN LOW
USBIN POWER
REMOVED OR DCIN
POWER APPLIED
SHUTDOWN
MODE
I_USBIN DROPS TO 18µA
CHRG STATE: Hi-Z
ENABLE
DRIVEN HIGH
3550-1 F01
Figure 1. LTC3550-1 State Diagram of a Charge Cycle
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