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LTC1733

Monolithic Linear Lithium-Ion Battery Charger with Thermal Regulation

Linear Technology 

LTC1733

PI FU CTIO S

CHRG: Open-Drain Charge Status Output. When the

battery is being charged, the CHRG pin is pulled low by an

internal N-channel MOSFET. When the charge current

drops to 10% of the full-scale current, the N-channel

MOSFET latches off and a 25µA current source is con-

nected from the CHRG pin to ground. The C/10 latch can

be cleared by momentarily pulling the PROG pin above the

2.15V shutdown threshold, or by toggling VCC. When the

timer runs out or the input supply is removed, the current

source is disconnected and the CHRG pin is forced to a

high impedance state.

VCC: Positive Input Supply Voltage. When VCC is within

30mV of VBAT or less than the undervoltage lockout

threshold, the LTC1733 enters sleep mode, dropping IBAT

to less than 5µA. VCC can range from 4.5V to 6.5V. Bypass

this pin with at least a 4.7µF ceramic capacitor to ground.

FAULT: Open-Drain Fault Status Output. The FAULT open-

drain logic signal indicates that the charger has timed out

under trickle charge conditions (1/4 of total time period) or

the NTC comparator is indicating an out-of-range battery

temperature condition. When VBAT is less that 2.48V,

trickle charging activates whereby the charge current

drops to one tenth of its programmed value and the timer

period is reduced by a factor of four. When one fourth of

the timing period has elapsed, if VBAT is still less than

2.48V, trickle charging stops and the FAULT pin latches to

ground. The fault can be cleared by toggling VCC, momen-

tarily pulling the PROG pin above the 2.15V shutdown

threshold, or pulling the BAT pin above 2.48V. If the NTC

comparator is indicating an out-of-range battery tempera-

ture condition, then the FAULT pin will pull to ground until

the temperature returns to the acceptable range.

TIMER: Timer Capacitor. The timer period is set by placing

a capacitor, CTIMER, to ground. The timer period is:

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

Short the TIMER pin to ground to disable the internal timer

function.

GND: Ground. Connect exposed back package to ground.

NTC: Input to the NTC (Negative Temperature Coefficient)

Thermistor Temperature Monitoring Circuit. With an ex-

ternal 10kΩ NTC thermistor to ground and a 1% resistor

to VCC, this pin can sense the temperature of the battery

pack and stop charging when it is out of range. When the

voltage at this pin drops below (0.5)•(VCC) at hot tempera-

tures or rises above (0.875)•(VCC) at cold, charging is

suspended and the internal timer is frozen. The CHRG pin

output status is not affected in this hold state. The FAULT

pin is pulled to ground, but not latched. When the tempera-

ture returns to an acceptable range, charging will resume

and the FAULT pin is released. The NTC feature can be

disabled by grounding the NTC pin.

PROG: Charge Current Program, Shutdown Input and

Charge Current Monitor Pin. The charge current is pro-

grammed by connecting a resistor, RPROG to ground.

When in constant-current mode, the LTC1733 servos the

PROG pin voltage to 1.5V. In all modes the voltage on the

PROG pin can be used to measure the charge current as

follows:

ICHG = (VPROG/RPROG) • 1000.

The IC can be forced into shutdown by pulling the PROG

pin above the 2.15V shutdown threshold voltage (note: it

will not be pulled up when allowed to float).

SEL: 4.1V/4.2V Battery Selection Input. Grounding this

pin sets the battery float voltage to 4.1V, while connecting

to VCC sets the voltage to 4.2V.

BAT: Charge Current Output. A bypass capacitor of at least

1µF with a 1Ω series resistor is required to minimize ripple

voltage when the battery is not present. A precision

internal resistor divider sets the final float potential on this

pin. The internal resistor divider is disconnected in sleep

and shutdown modes.

ACPR: Open-Drain Power Supply Status Output. When

VCC is greater than the undervoltage lockout threshold

and at least 30mV above VBAT, the ACPR pin will pull to

ground. Otherwise, the pin is forced to a high impedance

state.

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sn1733 1733fs

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