LTC1733

12

sn1733 1733fs

If a thermistor with a tolerance less than

±10%isused, the

trip point errors begin to depend on errors other than

thermistor tolerance including the input offset voltage of

the internal comparators of the LTC1733 and the effects of

internal voltage drops due to high charging currents.

Constant-Current/Constant-Voltage/

Constant-Temperature

The LTC1733 uses a unique architecture to charge a

battery in a constant-current, constant-voltage, constant-

temperature fashion. Figure 1 shows a simplified block

diagram of the LTC1733. Three of the amplifier feedback

loops shown control the constant-current, CA, constant-

voltage, VA, and constant-temperature, TA modes. A

fourth amplifier feedback loop, MA, is used to increase the

output impedance of the current source pair, M1 and M2

(note that M1 is the internal P-channel power MOSFET). It

ensures that the drain current of M1 is exactly 1000 times

greater than the drain current of M2.

Amplifiers CA, TA, and VA are used in three separate

feedback loops to force the charger into constant-current,

temperature, or voltage mode, respectively. Diodes, D1,

D2, and D3 provide priority to whichever loop is trying to

reduce the charging current the most. The outputs of the

other two amplifiers saturate low which effectively re-

moves their loops from the system. When in constant-

current mode, CA servos the voltage at the PROG pin to be

precisely 1.50V (or 0.15V when in trickle-charge mode).

TA limits the die temperature to approximately 105

°C

when in constant-temperature mode and the PROG pin

voltage gives an indication of the charge current as dis-

cussed in “Programming Charge Current” . VA servos its

inverting input to precisely 2.485V when in constant-

voltage mode and the internal resistor divider made up of

R1 and R2 ensures that the battery voltage is maintained

at either 4.1V or 4.2V. Again, the PROG pin voltage gives

an indication of the charge current.

In typical operation, the charge cycle begins in constant-

current mode with the current delivered to the battery

LTC1733 results in the junction temperature approaching

105

°C, the amplifier (TA) will begin decreasing the charge

current to limit the die temperature to approximately

105

°C. As the battery voltage rises, the LTC1733 either

returns to constant-current mode or it enters constant-

voltage mode straight from constant-temperature mode.

Regardless of mode, the voltage at the PROG pin is

proportional to the current being delivered to the battery.

Power Dissipation

The conditions that cause the LTC1733 to reduce charge

current due to the thermal protection feedback can be

approximated by considering the power dissipated in the

IC. For high charge currents, the LTC1733 power dissipa-

tion is approximately:

charge current. It is not necessary to perform any worst-

case power dissipation scenarios because the LTC1733

will automatically reduce the charge current to maintain

the die temperature at approximately 105

°C. However, the

approximate ambient temperature at which the thermal

feedback begins to protect the IC is:

Example: Consider an LTC1733 operating from a 5V wall

adapter providing 1.2A to a 3.75V Li-Ion battery. The

ambient temperature above which the LTC1733 will begin

to reduce the 1.2A charge current is approximately:

The LTC1733 can be used above 45

°C, but the charge

current will be reduced below 1.2A. The approximate

charge current at a given ambient temperature can be

approximated by:

I

CT

VV

BAT

A

CC

BAT

JA

=

°

105

–

(–

)•

θ

Consider the above example with an ambient temperature

of 55

°C. The charge current will be reduced to approxi-

mately:

APPLICATIO S I FOR ATIO