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LTC1733

Monolithic Linear Lithium-Ion Battery Charger with Thermal Regulation

Linear Technology 

LTC1733

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OPERATIO

The LTC1733 is a linear battery charger designed primarily

for charging single cell lithium-ion batteries. Featuring an

internal P-channel power MOSFET, the charger uses a

constant-current/constant-voltage charge algorithm with

programmable current and a programmable timer for

charge termination. Charge current can be programmed

up to 1.5A with a final float voltage accuracy of ±1%. No

blocking diode or sense resistor is required thus dropping

the external component count to three for the basic

charger circuit. The CHRG, ACPR, and FAULT open-drain

status outputs provide information regarding the status of

the LTC1733 at all times. An NTC thermistor input

provides the option of charge qualification using battery

temperature.

An internal thermal limit reduces the programmed charge

current if the die temperature attempts to rise above a

preset value of approximately 105°C. This feature protects

the LTC1733 from excessive temperature, and allows the

user to push the limits of the power handling capability of

a given circuit board without risk of damaging the LTC1733

or the external components. Another benefit of the LTC1733

thermal limit is that charge current can be set according to

typical, not worst-case, ambient temperatures for a given

application with the assurance that the charger will auto-

matically reduce the current in worst-case conditions.

The charge cycle begins when the voltage at the VCC pin

rises above the UVLO level and a program resistor is

connected from the PROG pin to ground. At the beginning

of the charge cycle, if the battery voltage is below 2.48V,

the charger goes into trickle charge mode to bring the cell

voltage up to a safe level for charging. The charger goes

into the fast charge constant-current mode once the

voltage on the BAT pin rises above 2.48V. In constant-

current mode, the charge current is set by RPROG.

When the battery approaches the final float voltage, the

charge current begins to decrease as the LTC1733 switches

to constant-voltage mode. When the current drops to 10%

of the full-scale charge current, an internal comparator

latches off the MOSFET at the CHRG pin and connects a

weak current source to ground to indicate a near end-of-

charge (C/10) condition. The C/10 latch can be cleared by

momentarily pulling the PROG pin above the 2.15V

shutdown threshold, or momentarily removing and reap-

plying VCC.

An external capacitor on the TIMER pin sets the total

charge time. When this time elapses the charge cycle

terminates and the CHRG pin assumes a high impedance

state. To restart the charge cycle, simply remove the input

voltage and reapply it, or force the PROG pin above the

2.15V shutdown threshold (note: simply floating the PROG

pin will not restart the charging cycle.

For lithium-ion and similar batteries that require accurate

final float potential, the internal reference, voltage ampli-

fier and the resistor divider provide regulation with ±1%

(max) accuracy.

When the input voltage is not present, the charger goes

into a sleep mode, dropping battery drain current, IBAT, to

less than 5µA. This greatly reduces the current drain on the

battery and increases the standby time. The charger can be

shut down (ICC = 0.9mA) by forcing the PROG pin above

2.15V.

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sn1733 1733fs

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