LTC4080X datasheet(12/20 Pages) LINER

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LTC4080X

4080Xf

If the die temperature starts to creep up above 115°C

due to internal power dissipation, the transconductance

ampliﬁer, TA, limits the die temperature to approximately

115°C by reducing the charge current. Diode D3 ensures

that TA does not affect the charge current when the die

temperature is below 115°C. In thermal regulation, the

PROG pin voltage continues to give an indication of the

charge current.

In typical operation, the charge cycle begins in constant-

current mode with the current delivered to the battery equal

to 400V/RPROG. If the power dissipation of the LTC4080X

results in the junction temperature approaching 115°C, the

ampliﬁer (TA) will begin decreasing the charge current to

limit the die temperature to approximately 115°C. As the

battery voltage rises, the LTC4080X either returns to full

constant-current mode or enters constant-voltage mode

straight from constant-temperature mode.

Battery Charger Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the VCC

input voltage and keeps the battery charger off until VCC

rises above 3.6V and approximately 82mV above the BAT

pin voltage. The 3.6V UVLO circuit has a built-in hysteresis

of approximately 0.6V, and the 82mV automatic shutdown

threshold has a built-in hysteresis of approximately 50mV.

During undervoltage lockout conditions, maximum battery

drain current is 5µA and maximum supply current is 10µA.

Undervoltage Charge Current Limiting (UVCL)

The battery charger in the LTC4080X includes undervoltage

charge current limiting that prevents full charge current

until the input supply voltage reaches approximately 300mV

above the battery voltage (ΔVUVCL1).Thisfeatureisparticu-

larly useful if the LTC4080X is powered from a supply with

long leads (or any relatively high output impedance). See

Applications Information section for further details.

Defective Battery Detection

At the beginning of a charge cycle, if the battery voltage

is below 2.9V for one quarter of the total charge time

(1.125 hr), the battery is assumed to be defective, the

charge cycle terminates and the

⎯C⎯H⎯R⎯G output pulses at a

frequency of 2Hz with a 75% duty cycle. If, for any reason,

the battery voltage rises above 2.9V, the charge cycle will

be restarted. To restart the charge cycle (i.e., when the

dead battery is replaced with a discharged battery less

than 2.9V), the charger must be reset by removing the

input voltage and reapplying it or temporarily pulling the

⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin above the shutdown threshold.

Battery Charger Shutdown Mode

The LTC4080X’s battery charger can be disabled by pulling

the

⎯E⎯N⎯_⎯C⎯H⎯R⎯G pin above the shutdown threshold (VIH).

In shutdown mode, the battery drain current is reduced

to about 2µA and the VCC supply current to about 5µA

provided the regulator is off. When the input voltage is

not present, the battery charger is in shutdown and the

battery drain current is less than 5µA.

Power Supply Status Indicator (

⎯A⎯C⎯P⎯R)

The power supply status output has two states: pulldown

and high impedance. The pulldown state indicates that VCC

is above the undervoltage lockout threshold and at least

82mV above the BAT voltage (see Undervoltage Lockout).

When these conditions are not met, the

⎯A⎯C⎯P⎯R pin is high

impedance indicating that the LTC4080X is unable to

charge the battery.

⎯C⎯H⎯R⎯G Status Output Pin

The charge status indicator pin has three states: pulldown,

pulsing at 2Hz (see Defective Battery Detection) and high

impedance. The pulldown state indicates that the battery

charger is in a charge cycle. A high impedance state indi-

cates that the charge current has dropped below 10% of

the full-scale current or the battery charger is disabled.

When the timer runs out (4.5 hrs), the

⎯C⎯H⎯R⎯G pin is also

forced to the high impedance state. If the battery charger

is not in constant-voltage mode when the charge current

is forced to drop below 10% of the full-scale current by

UVCL,

⎯C⎯H⎯R⎯G will stay in the strong pulldown state.

Charge Current Soft-Start

The LTC4080X’s battery charger includes a soft-start circuit

to minimize the inrush current at the start of a charge cycle.

When a charge cycle is initiated, the charge current ramps

from zero to full-scale current over a period of approximate-

ly 180µs. This has the effect of minimizing the transient

current load on the power supply during start-up.

OPERATIO

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