LTC1734L Datasheet, PDF (7/12 Pages) Linear Technology – Lithium-Ion Linear Battery Charger in ThinSOT

LTC1734L
APPLICATIONS INFORMATION
Manual Shutdown
Floating the program resistor allows an internal 3µA
current source (I_PROGPU) to pull the PROG pin above the
2.15V shutdown threshold (V_MSDT), thus shutting down
the charger. In this mode, the LTC1734L continues to
draw quiescent current from the supply (I_SHDN), but only
a negligible leakage current is delivered to the battery
(I_BMS).
Shutdown can also be accomplished by pulling the other-
wise grounded end of the program resistor to a voltage
greater than 2.25V (V_MSDTMax). Charging will cease above
1.5V, but the internal battery voltage resistor divider will
draw about 34µA from the battery until shutdown is
entered. Figure 1 illustrates a microcontroller configura-
tion that can either float the resistor or force it to a voltage.
The voltage should be no more than 8V when high and
have an impedance to ground of less than 10% of the
program resistor value when low to prevent excessive
charge current errors. To reduce errors the program
resistor value may be adjusted to account for the imped-
ance to ground. The programming resistor will prevent
potentially damaging currents if the PROG pin is forced
above V_CC. Under this condition V_CC may float, be loaded
down by other circuitry or be shorted to ground. If V_CC is
not shorted to ground, the current through the resistor will
pull V_CC up slightly.
Another method is to directly switch the PROG pin to a
voltage source when shutdown is desired (Caution: pull-
ing the PROG below 1.5V with V_CC applied will cause
excessive and uncontrolled charge currents). The volt-
age source must be capable of sourcing the resulting
current through the program resistor. This has the ad-
vantage of not adding any error to the program resistor
during normal operation. The voltage on the PROG pin
must be greater than 2.25V (V_MSDT(MAX)) to ensure
entering shutdown, but no more than 0.3V above V_CC to
prevent damaging the LTC1734L from excessive PROG
pin current. An exception is if V_CC is allowed to float with
no other circuitry loading V_CC down. Then, because the
current will be low, it is allowable to have the PROG pin
shutdown voltage applied. A three-state logic driver with
sufficient pull-up current can be used to perform this
function by enabling the high impedance state to charge
or enabling the pull-up device to enter shutdown.
An NPN transistor or a diode can also be utilized to
implement shutdown from a voltage source. These have
the advantage of blocking current when the voltage source
goes low, thus automatically disconnecting the voltage
source for normal charging operation. Using an NPN
allows the use of a weak voltage source due to the current
gain of the transistor. For an NPN, connect the collector to
V_CC, the base to the voltage source and the emitter to the
PROG pin. For a diode, connect the anode to the voltage
source and cathode to the PROG pin. An input high level
ranging from 3.3V to V_CC should be adequate to enter
shutdown while a low level of 0.5V or less should allow for
normal charging operation. Use of inexpensive small
signal devices such as the 2N3904 or 1N914 is recom-
mended to prevent excessive capacitive loading on the
PROG pin (see Stability section).
Sleep Mode
When the input supply is disconnected, the IC enters the
sleep mode. In this mode, the battery drain current (I_BSL)
is a negligible leakage current, allowing the battery to re-
main connected to the charger for an extended period of
time without discharging the battery. The leakage current
is due to the reverse-biased B-E junction of the external
PNP transistor.
OPEN DRAIN
OR TOTEM
POLE OUTPUT
µC
ADC INPUT
R_PROG
PROG
LTC1734L
1734 F01
Figure 1. Interfacing with a Microcontroller
Undervoltage Lockout
Undervoltage lockout (UVLO) keeps the charger off until
the input voltage exceeds a predetermined threshold level
(V_UVLOI) that is typically 4.56V. Approximately 150mV of
hysteresis is built in to prevent oscillation around the
threshold level. In undervoltage lockout, battery drain
current is very low (< 1µA).
1734lf
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