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LTC1733

Monolithic Linear Lithium-Ion Battery Charger with Thermal Regulation

Linear Technology 

LTC1733

APPLICATIO S I FOR ATIO

Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the input

voltage and keeps the charger in shutdown mode until VCC

rises above the undervoltage lockout threshold. The UVLO

circuit has a built-in hysteresis of 150mV. Furthermore, to

protect against reverse current in the power MOSFET, the

UVLO circuit keeps the charger in shutdown mode if VCC

falls to within 30mV of the battery voltage. If the UVLO

comparator is tripped, the charger will not come out of

shutdown until VCC rises 60mV above the battery voltage.

Trickle Charge and Defective Battery Detection

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

low (below 2.48V) the charger goes into trickle charge

reducing the charge current to 10% of the full-scale

current. If the low battery voltage persists for one quarter

of the total charge time, the battery is assumed to be

defective, the charge cycle is terminated, the CHRG pin

output assumes a high impedance state, and the FAULT

pin latches low. The fault can be cleared by toggling VCC,

temporarily forcing the PROG pin above 2.15V, or tempo-

rarily forcing the BAT pin voltage above 2.48V.

Shutdown

The LTC1733 can be shutdown (ICC = 0.9mA) by pulling

the PROG pin above the 2.15V shutdown threshold volt-

age. In shutdown the internal linear regulator is turned off,

and the internal timer is reset.

Recharge

The LTC1733 has the ability to recharge a battery

assuming that the battery voltage has been charged above

4.05V (SEL = 5V) or 3.95V (SEL = 0V). Once above these

thresholds, a new charge cycle will begin if the battery

voltage drops below 4V (SEL = 5V) or 3.9V (SEL = 0V) due

to either a load on the battery or self-discharge. The

recharge circuit integrates the BAT pin voltage for a few

milliseconds to prevent a transient from restarting the

charge cycle.

If the battery voltage remains below 2.48V during trickle

charge for 1/4 of the programmed time, the battery may be

defective and the charge cycle will end. In addition, the

recharge comparator is disabled and a new charge cycle

will not begin unless the input voltage is toggled, the PROG

pin is pulled above the 2.15V shutdown threshold, or the

BAT pin is pulled above the 2.48V trickle charge threshold.

Programming Charge Current

The formula for the battery charge current (see Figure 1)

is:

ICHG = (IPROG) • 1000

= (1.5V / RPROG) • 1000 or

RPROG = 1500/ICHG

where RPROG is the total resistance from the PROG pin to

ground. Under trickle charge conditions, this current is

reduced to 10% of the full-scale value.

For example, if 500mA charge current is required,

calculate:

RPROG = 1500/0.5A = 3kΩ

For best stability over temperature and time, 1% metal-

film resistors are recommended.

If the charger is in constant-temperature or constant-

voltage mode, the battery current can be monitored by

measuring the PROG pin voltage as follows:

ICHG = (VPROG / RPROG) • 1000

Programming the Timer

The programmable timer is used to terminate the charge

cycle. The timer duration is programmed by an external

capacitor at the TIMER pin. The total charge time is:

Time (Hours) = (3 Hours) • (CTIMER / 0.1µF) or

CTIMER = 0.1µF • Time (Hours)/3 (Hours)

The timer starts when an input voltage greater than the

undervoltage lockout threshold level is applied and the

program resistor is connected to ground. After a time-out

occurs, the charge current stops, and the CHRG output

assumes a high impedance state to indicate that the

charging has stopped. Connecting the TIMER pin to ground

disables the timer function.

sn1733 1733fs

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