LTC4077

10

4077f

*Any external sources that hold the pin above 100mV will prevent the LTC4077 from terminating

a charge cycle.

Programming Charge Termination

The charge cycle terminates when the voltage at the IUSB,

IUSBL and IDC pins falls below 100mV. For wall adapter

or high power USB use the charge termination is set to

1/10th of the programmed charge current. For low power

USB use (HPWR = 0V) the charge termination is set to

1/2 of the programmed charge current.

The termination condition is detected by using an internal

filtered comparator to monitor the IDC, IUSB and IUSBL

pin voltages. When one of these voltages drops below

charge cycle terminates, charge current latches off and

the LTC4077 enters standby mode.

When charging, transient loads on the BAT pin can cause

the active charge current program pin to fall below 100mV

for short periods of time before the steady-state charge

current has dropped below the programmed termination

tion comparator ensures that transient loads of this nature

do not result in premature charge cycle termination. Once

the average charge current drops below the termination

threshold, the LTC4077 terminates the charge cycle and

ceases to provide any current out of the BAT pin. In this

state, any load on the BAT pin must be supplied by the

battery.

Low-Battery Charge Conditioning (Trickle Charge)

This feature ensures that deeply discharged batteries

are gradually charged before reapplying full charge cur-

rent. If the BAT pin voltage is below 2.9V, the LTC4077

supplies 1/10th of the full charge current to the battery,

when HPWR = high or 1/2 of the full charge current

low, until the BAT pin rises above 2.9V. For example, if

the charger is programmed to charge at 800mA from the

wall adapter input and 500mA from the USB input when

HPWR is high or 50mA if HPWR is low, the charge current

during trickle charge mode would be 80mA, 50mA and

25mA respectively.

Automatic Recharge

In standby mode, the charger sits idle and monitors the

battery voltage using a comparator with a 6ms filter time

battery voltage falls below 4.1V (which corresponds to

approximately 80%-90% battery capacity). This ensures

that the battery is kept at, or near, a fully charged condi-

tion and eliminates the need for periodic charge cycle

initiations.

If the battery is removed from the charger, a sawtooth

waveform of approximately 100mV appears at the battery

output. This is caused by the repeated cycling between

termination and recharge events. This cycling results in

pulsing at the ⎯C⎯H⎯R⎯G output; an LED connected to this pin

will exhibit a blinking pattern, indicating to the user that

a battery is not present. The frequency of the sawtooth is

dependent on the amount of output capacitance.

Manual Shutdown

The ⎯E⎯N pin has a 2MΩ pulldown resistor to GND. A logic

low enables the charger and logic high disables it (the

pulldown defaults the charger to the charging state).

The DCIN input draws 20µA when the charger is in shut-

down. The USBIN input draws 18µA during shutdown if

no power is applied to DCIN, but draws only 10µA when

Charge Current Soft-Start and Soft-Stop

The LTC4077 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 250µs. Likewise,

internal circuitry slowly ramps the charge current from

full-scale to zero in a period of approximately 30µs when

the charger shuts down or self terminates. This minimizes

the transient current load on the power supply during

start-up and shut-off.

OPERATIO