LTC1733

10

sn1733 1733fs

Open-Drain Status Outputs

The LTC1733 has three open-drain status outputs: ACPR,

CHRG and FAULT. The ACPR pin pulls low when an input

voltage greater than the undervoltage lockout threshold is

is removed. CHRG and FAULT work together to indicate

the status of the charge cycle. Table 1 describes the status

of the charge cycle based on the CHRG and FAULT

outputs.

Table 1.

FAULT

CHRG

Description

High

Low

Charge cycle has started, C/10 has not been

reached and charging is proceeding normally.

Low

Low

Charge cycle has started, C/10 has not been

reached, but the charge current and timer

have been paused due to an NTC out-of-

temperature condition.

High

25

µA

C/10 has been reached and charging is

pulldown

proceeding normally.

Low

25

µA

C/10 has been reached but the charge current

pulldown

and timer have paused due to an

NTC out-of-temperature condition.

High

High

Normal timeout (charging has terminated).

Low

High

If FAULT goes low and CHRG goes high

impedance simultaneously, then the LTC1733

after one-quarter the programmed charge time).

If CHRG goes high impedance first, then

the LTC1733 has timed out normally (charging

has terminated), but NTC is indicating an out-

of-temperature condition.

CHRG Status Output Pin

When the charge cycle starts, the CHRG pin is pulled to

ground by an internal N-channel MOSFET capable of

driving an LED. When the charge current drops to 10% of

the full-scale current (C/10), the N-channel MOSFET is

latched off and a weak 25

µA current source to ground is

connected to the CHRG pin. After a time-out occurs, the

pin assumes a high impedance state. By using two differ-

ent value pull-up resistors a microprocessor can detect

three states from this pin (charging, C/10, and time-out).

See Figure 2.

APPLICATIO S I FOR ATIO

When the LTC1733 is in charge mode, the CHRG pin is

pulled low by the internal N-channel MOSFET. To detect

this mode, force the digital output pin, OUT, high and

measure the voltage at the CHRG pin. The N-channel

MOSFET will pull the pin low even with the 2k pull-up

resistor. Once the charge current drops to 10% of the full-

scale current (C/10), the N-channel MOSFET is turned off

and a 25

µA current source is connected to the CHRG pin.

The IN pin will then be pulled high by the 2k pull-up. By

forcing the OUT pin to a high impedance state, the current

source will pull the pin low through the 400k resistor.

When the internal timer has expired, the CHRG pin will

assume a high impedance state and the 400k resistor will

then pull the pin high to indicate that charging has termi-

nated.

NTC Thermistor

The battery temperature is measured by placing a negative

temperature coefficient (NTC) thermistor close to the

battery pack. The NTC circuitry is shown in Figure 3. To use

this feature, connect a 10k NTC thermistor between the

the value of the chosen NTC thermistor at 50

°C (this value

is 4.1k for a Vishay NTHS0603N02N1002J thermistor).

The LTC1733 goes into hold mode when the resistance of

the NTC thermistor drops below 4.1k which should be

at 50

°C. The hold mode freezes the timer and stops

the charge cycle until the thermistor indicates a return

to a valid temperature. As the temperature drops, the

Figure 2. Microprocessor Interface

8

3

400k

2k

LTC1733

1733 F02

CHRG

µPROCESSOR

OUT

IN