LTC4064 datasheet(12/16 Pages) LINER

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LTC4064

sn4064 4064fs

Constant-Current/Constant-Voltage/

Constant-Temperature

The LTC4064 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 LTC4064. 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 charge 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 4V. 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

equal to 1500V/RPROG. If the power dissipation of the

LTC4064 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 LTC4064 either

returns to constant-current mode or it enters constant-

voltage mode straight from constant-temperature mode.

APPLICATIO S I FOR ATIO

NTC Trip Point Errors

When a 1% resistor is used for RHOT, the major error in

the 50°C trip point is determined by the tolerance of

the NTC thermistor. A typical 10k NTC thermistor has

a ±10% tolerance. By looking up the temperature

coefficient of the thermistor at 50°C, the tolerance error

can be calculated in degrees centigrade. Consider the

Vishay NTHS0603N02N1002J thermistor which has a

temperature coefficient of –3.3%/°C at 50°C. Dividing

the tolerance by the temperature coefficient, ±10%/

(3.3%/°C) = ±3°C, gives the temperature error of the hot

trip point.

The cold trip point is a little more complicated because its

error depends on the tolerance of the NTC thermistor and

the degree to which the ratio of its value at 0°C and its value

at 50°C varies from 7 to 1. Therefore, the cold trip point

error can be calculated using the tolerance, TOL, the

temperature coefficient of the thermistor at 0°C, TC

(in %/°C), the value of the thermistor at 0°C, RCOLD, and

the value of the thermistor at 50°C, RHOT. The formula is:

Temperature Error (°C) =

1 100

⎛

⎝

⎜

⎞

⎠

⎟

TOL R

COLD

HOT

•–

•

For example, the Vishay NTHS0603N02N1002J thermistor

with a tolerance of ±10%, TC of –4.5%/°C, and RCOLD/

RHOT of 6.89, has a cold trip point error of:

Temperature Error (°C) =

10 10

6 89 1 100

⎛

⎝

⎜

⎞

⎠

⎟

•.

–

•

–.

= –1.8°C, +2.5°C

If a thermistor with a tolerance less than ±10% is used, the

trip point errors begin to depend on errors other than

thermistor tolerance including the input offset voltage of

the internal comparators of the LTC4064 and the effects of

internal voltage drops due to high charging currents.

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