lt3651-4.2 Linear Technology Corporation, lt3651-4.2 Datasheet - Page 9

• Manufacturer Part Number: lt3651-4.2
• Description: Monolithic 4a High Voltage Li-ion Battery Charger Features
• Manufacturer: Linear Technology Corporation

operaTion
Overview
The LT3651 is a complete Li-Ion battery charger, addressing
wide input voltage (4.8V to 32V) and high currents (up to
4A). High charging efficiency is produced with a constant
frequency, average current mode synchronous step-down
switcher architecture.
The charger includes the necessary circuitry to allow for
programming and control of constant current, constant
voltage (CC/CV) charging with both current only and timer
termination. High charging efficiency is achieved by the
switcher by using a bootstrapped supply for low switch
drop for the high side driver and a MOSFET for the low
side (synchronous) switch.
Maximum charge current is set with an external sense resis-
tor in series with the inductor and is adjustable through the
RNG/SS pin. Total system input current is monitored with
an input sense resistor and is used to maintain constant
input current by regulating battery charge current. It is
adjustable through the I
If the battery voltage is low, charge current is automatically
reduced to 15% of the programmed current to provide
safe battery preconditioning. Once the battery voltage
climbs above the battery precondition threshold, the IC
automatically increases the maximum charge current to
the full programmed value.
Charge termination can occur when charge current de-
creases to one-tenth the programmed maximum charge
current (C/10 termination). Alternately, termination can
be time based through the use of an internal program-
mable charge cycle control timer. When using the timer
termination, charging can continue beyond the C/10 level
to “top-off” a battery. Charging typically terminates three
hours after initiation. When the timer-based scheme is
used, bad battery detection is also supported. A system
fault is triggered if a battery stays in precondition mode
for more than one-eighth of the total charge cycle time.
Once charging is terminated and the LT3651 is not actively
charging, the IC automatically enters a low current standby
mode in which supply bias currents are reduced to <85µA.
If the battery voltage drops 2.5% from the full charge float
LIM
pin.
voltage, the LT3651 engages an automatic charge cycle
restart. The IC also automatically restarts a new charge
cycle after a bad battery fault once the failed battery is
removed and replaced with another battery.
After charging is completed the input bias currents on the
pins connecting to the battery are reduced to minimize
battery discharge.
The LT3651 contains provisions for a battery temperature
monitoring circuit. Battery temperature is monitored by
using a NTC thermistor located with the battery. If the
battery temperature moves outside a safe charging range
of 0°C to 40°C the charging cycle suspends and signals
a fault condition.
The LT3651 contains two digital open-collector outputs,
which provide charger status and signal fault conditions.
These binary coded pins signal battery charging, standby
or shutdown modes, battery temperature faults and bad
battery faults.
A precision undervoltage lockout is possible by using a
resistor divider on the shutdown pin (SHDN). The input
supply current is 17µA when the IC is in shutdown.
General Operation (See Block Diagram)
The LT3651 uses an average current mode control loop
architecture to control average charge current. The LT3651
senses charger output voltage via the BAT pin. The dif-
ference between this voltage and the internal float volt-
age reference is integrated by the voltage error amplifier
(V-EA). The amplifier output voltage (I
to the desired average voltage across the inductor sense
resistor, R
pins. The I
and provides a voltage offset on the input of the current
error amplifier (C-EA). The difference between this im-
posed voltage and the current sense resistor voltage is
integrated by C-EA. The resulting voltage (V
voltage that is compared against an internally generated
ramp and generates the switch duty cycle that controls
the charger’s switches.
SENSE
TH
voltage is divided down by a factor of 10,
, connected between the SENSE and BAT
LT3651-4.2
TH
) corresponds
C
) provides a
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