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LTC4068-4.2/LTC4068X-4.2

406842fa

OPERATIO

The LTC4068 is a single cell lithium-ion battery charger

using a constant-current/constant-voltage algorithm. It

can deliver up to 950mA of charge current (using a good

thermal PCB layout) with a final float voltage accuracy of

±1%. The LTC4068 includes an internal P-channel power

MOSFET and thermal regulation circuitry. No blocking

diode or external current sense resistor is required; thus,

the basic charger circuit requires only two external com-

ponents. Furthermore, the LTC4068 is capable of operat-

ing from a USB power source.

Normal Charge Cycle

above the UVLO threshold level and a 1% program resistor

is connected from the PROG pin to ground. If the BAT pin

is less than 2.9V, the charger enters trickle charge mode.

In this mode, the LTC4068 supplies approximately 1/10th

the programmed charge current to bring the battery volt-

age up to a safe level for full current charging. (Note: The

LTC4068X does not include this trickle charge feature.)

When the BAT pin voltage rises above 2.9V, the charger

enters constant-current mode where the programmed

charge current is supplied to the battery. When the BAT pin

approaches the final float voltage (4.2V), the LTC4068

enters constant-voltage mode and the charge current

begins to decrease. When the charge current drops to the

programmed termination threshold (set by the external

Programming Charge Current

The charge current is programmed using a single resistor

from the PROG pin to ground. The charge current out of

the BAT pin is 1000 times the current out of the PROG pin.

The program resistor and the charge current are calcu-

lated using the following equations:

R

V

I

I

V

R

PROG

CHG

CHG

PROG

==

1000

1000

,

Charge current out of the BAT pin can be determined at any

time by monitoring the PROG pin voltage and using the

following equation:

I

V

R

BAT

PROG

PROG

=

• 1000

Programming Charge Termination

The charge cycle terminates when the charge current

falls below the programmed termination threshold. This

from the ITERM pin to ground. The charge termination

I

V

R

IR

R

R

V

I

TERM

TERM

CHG

PROG

TERM

TERM

TERM

==

=

100

10

100

•,

The termination condition is detected by using an internal

filtered comparator to monitor the ITERM pin. When the

current is latched off and the LTC4068 enters standby

mode where the input supply current drops to 200µA.

(Note: Termination is disabled in trickle charging and

thermal limiting modes.)

pin to the PROG pin, thus eliminating the need for external

is set by the equation:

I

V

R

R

V

I

CHG

PROG

PROG

CHG

==

500

500

,

**

When charging, transient loads on the BAT pin can cause

the ITERM pin to fall below 100mV for short periods of

time before the DC charge current has dropped to 10% of

termination comparator ensures that transient loads of

this nature do not result in premature charge cycle termi-

nation. Once the

average charge current drops below the

programmed termination threshold, the LTC4068 termi-

nates 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.

The LTC4068 constantly monitors the BAT pin voltage in

standby mode. If this voltage drops below the 4.1V recharge

Any external sources that hold the ITERM pin above 100mV will prevent the LTC4068 from

terminating a charge cycle.

These equations apply only when the ITERM pin is shorted to the PROG pin.

*

**