lt3650-4.1 Linear Technology Corporation, lt3650-4.1 Datasheet - Page 9

• Manufacturer Part Number: lt3650-4.1
• Description: High Voltage 2 Amp Monolithic Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

OPERATION
OVERVIEW
LT3650 is a complete monolithic, mid-power, Li-Ion battery
charger, addressing high input voltage applications with
solutions that use a minimum of external components.
The IC uses a 1MHz constant-frequency, average current
mode step-down architecture. Internal reverse-voltage
protection allows direct connection to the input supply
without a blocking diode.
The LT3650 incorporates a 2A switch that is driven by
a bootstrapped supply to maximize effi ciency during
charging cycles. A wide input range allows the operation
to full charge from 5V ±5% to 32V. A precision threshold
shutdown pin allows incorporation of UVLO functionality
using a simple resistor divider. The IC can also be put into
a low current shutdown mode, in which the input supply
bias is reduced to only 15μA.
The LT3650 incorporates several degrees of charge current
control freedom. The overall maximum charge current
is set using an external inductor current sense resistor.
A maximum charge current programming pin allows
dynamic manipulation of the battery charge current. The
LT3650 also incorporates a system input supply current
limit control feature that servos the battery charge current
to accommodate overall system load requirements.
The LT3650 automatically enters a battery precondi-
tion mode if the sensed battery voltage is very low. In
this mode, the charge current is reduced to 15% of the
programmed maximum, as set by the inductor sense
resistor, R
the internally set precondition threshold of 2.9V, the IC
automatically increases the maximum charge current to
the full programmed value.
The LT3650 can use a charge current based C/10 ter-
mination scheme, which ends a charge cycle when the
battery charge current falls to one-tenth the programmed
maximum charge current. The LT3650 also contains an
internal charge cycle control timer, for timer-based termina-
tion. When using the internal timer, the IC combines C/10
detection with a programmable time constraint, during
which the charging cycle can continue beyond the C/10
level to top-off a battery. The charge cycle terminates when
a specifi c time elapses, typically three hours. When the
SENSE
. Once the battery voltage climbs above
timer-based scheme is used, the IC also supports bad-
battery detection, which triggers a system fault if a battery
stays in precondition mode for more than one-eighth of
the total programmed charge cycle time.
Once charging is terminated and the LT3650 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 fl oat voltage, the LT3650 engages
an automatic charge cycle restart. The IC also automati-
cally restarts a new charge cycle after a bad-battery fault
once the failed battery is removed and replaced with
another battery.
The LT3650 contains provisions for a battery temperature
monitoring circuit. This feature monitors battery tempera-
ture by using a thermistor during the charging cycle, sus-
pends charging, and signals a fault condition if the battery
temperature moves outside a safe charging range.
The LT3650 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.
General Operation (See Block Diagram)
The LT3650 uses average current mode control architec-
ture, such that the IC servos directly to average charge
current. The LT3650 senses charger output voltage via
the BAT pin. The difference between the voltage on this
pin and an internal fl oat voltage reference is integrated by
the voltage error amplifi er (V-EA). This amplifi er generates
an error voltage on its output (I
to the average current sensed across the inductor cur-
rent sense resistor, R
the SENSE and BAT pins. The I
down by a factor of 10, and imposed on the input of the
current error amplifi er (C-EA). The difference between this
imposed voltage and the current sense resistor voltage
is integrated, with the resulting voltage (V
threshold that is compared against an internally gener-
ated ramp. The output of this comparison controls the
charger’s switch.
LT3650-4.1/LT3650-4.2
SENSE
, which is connected between
TH
TH
voltage is then divided
), which corresponds
C
) used as a
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