LTC4067 datasheet(13/20 Pages) LINER

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LTC4067

4067f

OPERATIO

charge current begins to decrease as the LTC4067 enters

constant-voltage mode. Once the battery charger detects

that it has entered constant-voltage mode, the two hour

safety timer is started. After the safety timer expires,

charging of the battery will discontinue and no more cur-

rent will be delivered.

Automatic Recharge

After the battery charger terminates, it will remain off

drawing only microamperes of current from the battery.

If the portable product remains in this state long enough,

the battery will eventually self discharge. To ensure that the

battery is always topped off, a charge cycle will automati-

cally begin when the battery voltage falls below VRECHRG

(typically 4.1V). In the event that the safety timer is run-

ning when the battery voltage falls below VRECHRG, the

timer will reset back to zero. To prevent brief excursions

below VRECHRG from resetting the safety timer, the battery

voltage must be below VRECHRG for more than 1.3ms. The

charge cycle and safety timer will also restart if the IN

UVLO cycles low and then high (e.g. IN is removed and

then replaced).

Charge Current

The charge current is programmed using a single resistor

from PROG to ground. 1/1000th of the battery charge cur-

rent is delivered to PROG which will attempt to servo to

1.000V. Thus, the battery charge current will try to reach

1000 times the current in the PROG pin. The program

resistor and the charge current are calculated using the

following equations:

PROG

CC CHG

PROG

−

1000

In either the constant-current or constant-voltage charging

modes, the PROG pin voltage will be proportional to the

actual charge current delivered to the battery. Therefore,

the actual charge current can be determined at any time

by monitoring the PROG pin voltage and using the fol-

lowing equation:

BAT

PROG

• 1000

In many cases, the actual battery charge current, IBAT, will

be lower than ICC-CHG due to limited input current available

and prioritization with the system load drawn from OUT.

Thermal Regulation

To prevent thermal damage to the IC or surrounding

components, an internal thermal feedback loop will

automatically decrease the programmed charge current

if the die temperature rises to approximately 105°C.

Thermal regulation protects the LTC4067 from excessive

temperature due to high power operation or high ambient

thermal conditions and allows the user to push the limits

of the power handling capability with a given circuit board

design without risk of damaging the LTC4067 or external

components. The beneﬁt of the LTC4067 thermal regula-

tion loop is that charge current can be set according to

actual conditions rather than worst-case conditions with

the assurance that the battery charger will automatically

reduce the current in worst-case conditions.

Low Power Shutdown

The LTC4067 enters a low power shutdown mode if the

PROG pin is pulled above the shutdown threshold, VSD. In

shutdown, the BAT pin current is reduced to 5μA, the IN

pin current is reduced to 10μA, the internal battery charge

timer and end-of-charge comparator output are both reset.

All power paths are put in a high impedance state.

A weak (2.5μA) pull up at the PROG pin causes the LTC4067

to enter low power shutdown if the PROG pin is ﬂoated. An

external N-channel MOSFET transistor with its drain tied

in series with the PROG pin, or an open drain driver may

thereby serve as an enable input for the LTC4067.

NTC

Under normal operation, tie a thermistor from the NTC

pin to GND and a resistor of equal value from NTC to IN.

When the voltage on this pin is above 0.74 • VIN (cold,

0°C) or below 0.29 • VIN (hot, 50°C) the charge timer is

suspended, but not cleared, the charging is disabled and

the ⎯C⎯H⎯R⎯G pin ﬂashes from Hi-Z to active pull-down with

a serrated pulse. When the voltage on NTC comes back

to between 0.29 • VIN and 0.74 • VIN, the charger timer

continues from where it left off, the charging is re-enabled

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