LTC3552 Datasheet, PDF (10/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

LTC3552
U
OPERATIO
The LTC3552 is made up of two parts: a standalone
constant-current/constant-voltage linear charger for a
single-cell lithium-ion battery and a high efficiency dual
DC/DC switching regulator. The charger can deliver up to
950mA of charge current (using a good thermal PCB lay-
out) with a final float voltage accuracy of ±1%. An internal
P-channel power MOSFET and thermal regulation circuitry
are included. No blocking diode or external current sense
resistor is required; furthermore, the charger is capable
of operating from a USB power source.
The switching regulators use a constant frequency, cur-
rent mode step-down architecture. Both main (P-channel
MOSFET) and synchronous (N-channel MOSFET) switches
are internal.
LITHIUM-ION BATTERY CHARGER
Normal Charge Cycle
A charge cycle begins when the voltage at the V_IN pin
rises above the UVLO threshold level and a 1% program
resistor is connected from the PROG pin to ground. If the
BAT pin is less than 2.9V, the charger enters trickle charge
mode. In this mode, the charger supplies approximately
one-tenth the programmed charge current to bring the bat-
tery voltage up to a safe level for full current charging.
When the BAT pin voltage rises above 2.9V, the charger
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. When the charge current drops to the
programmed termination threshold (set by the external
resistor R_TERM), the charge cycle ends. Figure 1 shows
the state diagram of a typical charge cycle.
Charge Status Indicator (⎯C⎯H⎯R⎯G)
The open drain charge status output has two states: pull-
down and high impedance. The pull-down state indicates
that the charger is in a charge cycle. Once the charge
cycle has terminated or the charger is disabled, the pin
becomes high impedance.
10
Automatic Recharge
Once the charge cycle terminates, the charger continuously
monitors the voltage on the BAT pin using a comparator
with a 2ms filter time (t_RECHARGE). A charge cycle restarts
when the battery voltage falls below 4.10V (which corre-
sponds to approximately 80% to 90% battery capacity).
This ensures that the battery is kept at, or near, a fully
charged condition and eliminates the need for periodic
charge cycle initiations. The ^⎯C^⎯H^⎯R^⎯G output enters a pull-
down state during recharge cycles. If the battery is removed
from the charger, a sawtooth waveform of approximately
100mV appears at the charger output. This is caused by
the repeated cycling between termination and recharge
events. This cycling results in pulsing at the ^⎯C^⎯H^⎯R^⎯G output;
an LED connected to this pin will exhibit a pulsing pattern,
indicating to the user that a battery is not present. The
frequency of the sawtooth is dependent on the amount
of output capacitance.
Power Supply Status Indicator (^⎯P^⎯W^⎯R)
The power supply status output has two states: pull-down
and high impedance. The pull-down state indicates that
V_IN is above the UVLO threshold (3.8V) and is also 100mV
above the battery voltage. If these conditions are not met,
the ^⎯P^⎯W^⎯R pin is high impedance indicating that the charger
is unable to charge the battery.
POWER ON
BAT < 2.9V
EN DRIVEN LOW
OR
UVLO CONDITION
STOPS
SHUTDOWN MODE
I_IN DROPS TO <25µA
CHRG: Hi-Z
TRICKLE CHARGE
MODE
1/10TH FULL CURRENT
CHRG: STRONG
PULL-DOWN
BAT > 2.9V
CHARGE MODE
BAT > 2.9V
FULL CURRENT
CHRG: STRONG
PULL-DOWN
ITERM < 100mV
EN DRIVEN HIGH
OR
UVLO CONDITION
STANDBY MODE
NO CHARGE CURRENT
CHRG: Hi-Z
3552 F01
2.9V < BAT < 4.1V
Figure 1. State Diagram of a Typical Charge Cycle
3552f