LTC3552-1 Linear Technology, LTC3552-1 Datasheet - Page 10

• Manufacturer Part Number: LTC3552-1
• Description: Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter
• Manufacturer: Linear Technology

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OPERATIO
LTC3552-1
The LTC3552-1 is made up of two circuit blocks: a stand-
alone constant-current/constant-voltage linear charger for
a single-cell lithium-ion battery and a high effi ciency dual
DC/DC switching regulator. The charger can deliver up to
950mA of charge current (using a good thermal PCB lay-
out) with a fi nal fl oat 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-chan-
nel MOSFET) and synchronous (N-channel MOSFET)
switches are internal. The LTC3552-1 requires no exter-
nal diodes or sense resistors.
LITHIUM-ION BATTERY CHARGER
Normal Charge Cycle
A charge cycle begins when the voltage at the V
rises above the UVLO threshold level and a 1% program
resistor is connected from the PROG pin to ground. The
charger enters constant-current mode where the pro-
grammed 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 mode and the charge cur-
rent begins to decrease. When the charge current drops to
the programmed termination threshold (set by the external
resistor R
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.
Automatic Recharge
Once the charge cycle terminates, the charger continuously
monitors the voltage on the BAT pin using a comparator
with a 2ms fi lter time (t
when the battery voltage falls below 4.10V (which corre-
10
TERM
), the charge cycle ends. Figure 1 shows
U
RECHARGE
). A charge cycle restarts
IN
pin
Charge Current Soft-Start
The charger includes a soft-start circuit to minimize the
inrush current at the start of a charge cycle. When a charge
cycle is initiated, the charge current ramps from zero to
full-scale current over a period of approximately 100µs.
This has the effect of minimizing the transient current load
on the power supply during start-up.
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
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.
IN
POWER ON
UVLO CONDITION ENDS
is above the UVLO threshold (3.8V) and is also 100mV
EN DRIVEN LOW OR
Figure 1. State Diagram of a Typical Charge Cycle
I
CC
SHUTDOWN MODE
DROPS TO <25µA
CHRG: Hi-Z
UVLO CONDITION
EN DRIVEN HIGH
OR
NO CHARGE CURRENT
STANDBY MODE
FULL CURRENT
CHRG: STRONG
CHRG: Hi-Z
CHARGE MODE
PULL-DOWN
I
TERM
2.9V < BAT < 4.1V
< 100mV
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