1996 Feb 26

5

Philips Semiconductors

Objective specification

Cost effective battery monitor and fast

charge IC for NiCd and NiMH chargers

TEA1104; TEA1104T

Open battery protection

When the rechargeable battery is removed, the output

protection’ block will detect this voltage and the charge

current will be switched off. A digital filter prevents false

open battery protection. The open battery signal

4 clock pulses.

Battery monitor

(battery connection) without an external resistor divider. At

larger cell packs the battery voltage must be scaled down

to a voltage range of 0.81 to 3.6 V. It is also possible to

take a tap on the chain of batteries. Battery full is

decrease of 0.25% (typ.) with respect to the top value.

Keeping in mind a battery voltage range of 0.81 to 3.6 V

detection, means that the internal ADC has to be 13 bits.

Several filters are included to prevent false full detection.

The series resistance of the battery and battery connection

can cause battery voltage fluctuations and therefore it is

necessary to stop the charging before sensing; this is

called the ‘inhibit time’. This will be performed

automatically via the regulation output pin LED. The

charging is stopped for ten oscillator periods at the end of

which sampling is performed. The battery voltage will now

be sensed in a currentless way.

Timer/oscillator

The oscillator has a sawtooth shape.

The oscillator frequency is used in the timer block. In this

block several important signals are created.

• Time-out for protecting the fast charge process in time.

Time-out is normally chosen to be 25% longer than the

associated fast charge time. So for a one hour charge

time, time-out = 1.25 hours. The relationship with the

oscillator period time is:

– Time-out = 2 exp28

• The duty factor in the trickle charge mode: The duty

• The battery voltage is sensed each ‘cycle time’. The

cycle time is defined as:

• The ‘inhibit time’ is the time that the charger current is

disabled, after which the battery voltage is sensed in a

currentless way.

Battery sampling takes one oscillator period for each

cycle interval.

• The ‘disable time’ is present to correct start-up with flat

or polarized batteries. During the disable time, the

battery full detection is not active.

The timer is reset by battery full detection, but is on hold

during the temperature and battery-low protection modes.

Temperature protection block

Temperature sensing is achieved by using a cheap

thermistor. Two temperature windows are built in:

• If the temperature at power-on reset is above the

maximum temperature protection level, the trickle

charge current is active. The same applies for

temperatures below the minimum temperature. Fast

charging starts when the temperature is in between the

minimum and the maximum temperature levels.

• If the temperature is between the maximum and

minimum temperature at power-on reset, the fast charge

current level is active. If the temperature sinks below the

minimum temperature level, again the trickle charge

level is active. At rising temperature, the fast charge

current is latched off at the ‘cut off’ temperature level.

To avoid switching on and off with temperature, a

hysteresis is built in for low temperature level. If the

temperature protection is not necessary, pin ‘Negative

Temperature Coefficient resistor’ (NTC) must be

Battery low protections

When the battery voltage is less than 0.81 V, the circuit

assumes that there are short circuited batteries and the

charge current is reduced to the trickle charge level. If the

batteries are flat, the trickle charge current is able to raise

the battery voltage within an acceptable period of time,

after which fast charging starts.