7

LTC1734

Manual Shutdown

Floating the program resistor allows an internal 3

µA

the charger. In this mode, the LTC1734 continues to draw

Shutdown can also be accomplished by pulling the other-

wise grounded end of the program resistor to a voltage

1.5V, but the internal battery voltage resistor divider will

draw about 34

µA 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

not shorted to ground the current through the resistor will

Another method is to directly switch the PROG pin to a

voltage source when shutdown is desired (Caution: pull-

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

prevent damaging the LTC1734 from excessive PROG

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. The use of an NPN

allows for use of a weak voltage source due to the current

gain of the transistor. For an NPN connect the collector to

PROG pin. For a diode, connect the anode to the voltage

source and cathode to the PROG pin. An input high level

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

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.

Undervoltage Lockout

Undervoltage lockout (UVLO) keeps the charger off until

the input voltage exceeds a predetermined threshold level

hysteresis is built in to prevent oscillation around the

threshold level. In undervoltage lockout, battery drain

current is very low (< 1

µA).

Figure 1. Interfacing with a Microcontroller

LTC1734

PROG

µC

ADC INPUT

1734 F01

OPEN DRAIN

OR TOTEM

POLE OUTPUT