SAA1501T Datasheet, PDF (7/20 Pages) NXP Semiconductors – Battery charge level indicator

Philips Semiconductors
Battery charge level indicator
Objective specification
SAA1501T
Mode detector
This block differentiates between the available modes of
operation. The modes are given below:
Charge mode; power charge (POCH).
Discharge mode; battery load (BATLD).
Power load mode (POLD); the batteries are charged
while the load is also active.
Self discharge mode; (STANDBY).
To detect power in a regulated system (see Fig.8) the EN
signal is used for sensing. The POCH mode is recognized
when the converted charge current I_c > I_ref2 (when in the
power mode, change of mode can only be recognised if
EN is HIGH). The BATLD mode is recognized when
I_d > I_ref2; the POLD mode is recognized when I_c > I_ref2 and
I_d > I_ref2; the standby mode is recognized when I_c < I_ref2
and I_d < I_ref2. In the standby mode, if the advised frequency
(4 kHz) is applied, it takes 0.5 s to determine another
mode (in all other modes, a change of mode is sensed
continuously). In all other modes an eventual change of
mode is done continuously. To save supply current during
standby, the V/I converters are switched off. With the
specific fixed intervals, the SAA1501T checks whether
power or load is connected again. This checking is
synchronized by the sensing signal of the V/I converters.
The SAA1501T can handle a DC charge current as well as
a discontinuous charge current (SMSP charger). The load
current can also be DC or interrupted, e.g. produced by a
motor. The digital filtering of both signals, to overcome
faulty mode detections, restricts the conditions in which
power and load are recognized. Because of the very
sensitive input detection level of the mode detector for a
charge current (power) in combination with the high
interference levels of motor driving, the detection level for
power (I_c > I_ref2) is raised by a factor of 25 when the
batteries are loaded.
Prescaler/controller
In the prescaler, a new system clock is created (CLK)
which is used for all timing blocks. Many frequencies are
derived from the basic oscillator at the standard frequency
of 4 kHz (1/T_osc), such as the self-discharge times and the
modulation frequency for the buzzer, the drive voltage
frequency for the LCD screen and the pulse trains for
temperature measurements and power/load sensing
measurements in the mode detector.
Temperature
In the temperature control block two temperature
measurements are performed. In order to switch off fast
charging when the battery temperature exceeds an
adjustable maximum temperature (T_max), a maximum
temperature measurement is performed. A second
temperature measurement is performed in the standby
mode. This temperature measurement is input to the
temperature control block to switch over the self-discharge
rate from a count down of 200 days (based on f_osc = 4 kHz)
if T_battery < T_self, to a count down rate of 100 days (based
on f_osc = 4 kHz) if T_battery > T_self. In all modes the
temperature is measured periodical. The temperature
circuit which controls the above mentioned functions is a
bridge configuration synthesis, as illustrated in Fig.3.
Oscillator
As the oscillator has to operate in all modes, including the
standby mode, the current consumption of the oscillator
must be very low. The same applies for the band gap
generator block, because the band gap delivers accurate
reference voltages and currents to the oscillator block.
Apart from the low current consumption, the accuracy of
the period time is important. The period time of the
oscillator is:
t_osc = 2 × C_osc × -⁽---V----^H-I-_r--_e-^–--_f--₃^V-----L--⁾-- = 5.6 × C_osc × R_ref
Fig.3 Temperature circuit.
December 1994
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