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
voltage V
bat
will rise to a high level. The ‘open battery
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
(V
bat
> 3.6 V) must be present for a duration of at least
4 clock pulses.
Battery monitor
One or two cell packs can be connected directly to V
bat
(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
recognized by voltage peak detection (V
peak
), meaning a
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
and an accuracy of 10% at V
bat
= 2.4 V for battery full
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 period time is defined by: t
osc
= K
×
R
ref
×
C
osc
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
×
t
osc
•
The duty factor in the trickle charge mode: The duty
factor is fixed to
1
⁄
40
, meaning that the average:
– I
trickle
=
1
⁄
40
×
I
fast
– t
on
=
3
⁄
4
×
2 exp9
×
t
osc
– t
off
= 2 exp14
×
t
osc
.
•
The battery voltage is sensed each ‘cycle time’. The
cycle time is defined as:
– T
cycle
= 2 exp16
×
t
osc
•
The ‘inhibit time’ is the time that the charger current is
disabled, after which the battery voltage is sensed in a
currentless way.
– t
inhibit
= 10
×
t
osc
Battery sampling takes one oscillator period for each
cycle interval.
– t
sample
= t
osc
•
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.
– t
disable
= 2 exp
−
5
×
time-out
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
connected to pin R
ref
.
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.
