ltc1734es6-4.2-trmpbf Linear Technology Corporation, ltc1734es6-4.2-trmpbf Datasheet - Page 6

• Manufacturer Part Number: ltc1734es6-4.2-trmpbf
• Description: Lithium-ion Linear Battery Charger In Thinsot
• Manufacturer: Linear Technology Corporation

OPERATIO
APPLICATIO S I FOR ATIO
LTC1734L
The LTC1734L is a linear battery charger controller.
Operation can best be understood by referring to the
Block Diagram. Charging begins when V
the UVLO (Undervoltage Lockout) threshold V
an external program resistor is connected between the
PROG pin and ground. When charging, the collector of the
external PNP provides the charge current. The PNP’s
emitter current flows through the I
the internal 0.24 current sense resistor. This current is
close in magnitude, but slightly more than the collector
current since it includes the base current. Amplifier A3,
along with the P-channel FET, will force the same voltage
that appears across the 0.24 resistor to appear across
the internal 60
resistor values will cause the FET’s drain current to be 1/
250 of the charge current and it is this current that flows
through the PROG pin. In the constant current mode,
amplifier A2 is used to limit the charge current to the value
that is programmed by R
The PROG pin current, which is 1/250 of the charge
current, develops a voltage across the program resistor.
When this voltage reaches 1.5V, amplifier A2 begins
diverting current away from the output driver, thus limit-
ing the charge current. This is the constant current mode.
The constant charge current is 250 • (1.5V/R
As the battery accepts charge, its voltage rises. When it
reaches the preset float voltage of 4.2V, a precisely divided
down version of this voltage (2.5V) is compared to the
2.5V internal reference voltage by amplifier A1. If the
battery voltage attempts to exceed 4.2V (2.5V at amplifier
Charger Operation
Charging begins when an input voltage is present that
exceeds the undervoltage lockout threshold (V
Li-Ion battery is connected to the charger output and a
program resistor is connected from the PROG pin to
ground. During the first portion of the charge cycle, when
the battery voltage is below the preset float voltage, the
charger is in the constant current mode. As the battery
voltage rises and reaches the preset float voltage, the
6
U
resistor. The scale factor of 250:1 in
U
PROG
U
.
SENSE
W
pin and through
CC
rises above
PROG
U
UVLOI
UVLOI
).
and
), a
charge current begins to decrease and the constant
voltage portion of the charge cycle begins. The charge
current will continue to decrease exponentially as the
battery approaches a fully charged condition.
Should the battery be removed during charging, a fast
built-in protection circuit will prevent the BAT pin from ris-
ing above 5V, allowing the precision constant voltage
circuit time to respond.
A1’s input) the amplifier will divert current away from the
output driver thus maintaining 4.2V on the battery. This is
the constant voltage mode.
When in the constant voltage mode, the 250:1 current ratio
is still valid and the voltage on the PROG pin will indicate
the charge current as a proportion of the maximum cur-
rent set by the current programming resistor. The battery
charge current is 250 • (V
allows a microcontroller with an ADC to easily monitor
charge current and if desired, manually shut down the
charger at the appropriate time.
When V
down by floating the otherwise grounded end of R
An internal 3 A current source pulls the PROG pin above
the 2.15V threshold of voltage comparator C1 initiating
shutdown.
For charging NiMH or NiCd batteries, the LTC1734L can
function as a constant current source by grounding the
BAT pin. This will prevent amplifier A1 from trying to limit
charge current and only A2 will control the current.
Fault conditions such as overheating of the die or exces-
sive DRIVE pin current are monitored and limited.
When input power is removed or manual shutdown is
entered, the charger will drain only tiny leakage currents
from the battery, thus maximizing battery standby time.
With V
the battery by the charger. In manual shutdown the base
is connected to V
CC
CC
removed the external PNP’s base is connected to
is applied, the charger can be manually shut
CC
by the charger.
PROG
/R
PROG
) amps. This feature
PROG
1734lf
.