SAA1501T Datasheet, PDF (5/20 Pages) NXP Semiconductors – Battery charge level indicator

Philips Semiconductors
Battery charge level indicator
Objective specification
SAA1501T
FUNCTIONAL DESCRIPTION
The most important function of the SAA1501T is the
charge account in rechargeable battery systems. Both
NiCd and NiMH batteries in all sizes can be used. The
system can operate alone as a charge monitor with a
charge amount display function or, can operate in
conjunction with a charger. If the SAA1501T operates
together with a charger, it delivers a control signal at output
EN, for charge current regulation or for battery voltage
regulation.
Fast charging systems and charge current regulation
The SAA1501T is especially designed to be used in fast
charging systems. In fast charging systems, the charge
time is lowered by raising the charge current. Signal EN
controls the charger current. The counters register the
state of charge of the batteries and at the 80% level the
charge current is reduced via a smaller duty cycle
regulation of signal EN. The second (slow) level fully
charges the batteries which is not possible with the first
(fast) level. After the slow charge mode the counter
switches over to an even smaller duty cycle of EN and thus
enters the third (trickle) charge mode, to overcome the
self-discharge of the batteries.
Current sensing and charge account
The charge current is sensed by means of a very low
resistance (e.g. 70 mΩ) sense resistor R_sc (see Fig.8) to
save power at high charge rates. Via the V/I charge
converter and external resistor R_CCC (see Fig.8), the
sensed voltage is converted into a charge current I_c (the
same is applicable for the discharge current). In the I/F
converter the charge current is converted into a frequency
for up-counting the counter. For the discharge current (I_d)
the converted frequency is used for down-counting. The
up and down counting is registered in counters CNT1 and
CNT2, depending on the actual charge and discharge
current levels of the batteries. This is called dynamic
charge account.
Charge display
The charge amount represented by the Coulomb counter
can be displayed via an LCD screen or via an LED
bargraph. If the charge amount is reduced to 0%, the
battery low indicator (BLI) LED is turned on at the end of a
battery discharge session. A flashing BLI, in combination
with a repeating buzzer alarm, informs the user about the
low charge state. A new charge session should then be
started.
Protections
In the temperature control block, the absolute temperature
is used as a protection to end the fast charge cycle. Fast
charging at high temperature is not permitted because of
degradation of the battery cells. If the batteries are
disconnected, an open-battery condition is recognized and
the SAA1501T enters the standby mode.
Mode detection
The mode detector detects whether there are any charge
or discharge currents, whether the system is powered,
whether loads are connected or whether the system is in
the standby mode. If power is connected, the power-on
LED (POL) is on. In the standby mode, the Coulomb
counter will count down in accordance with the
self-discharge speed of the batteries, which is temperature
controlled. The following subsections describe the various
blocks of the block diagram in more detail.
Supply and reference
During the period when V_CC rises from 0 V to the internal
reset level, all counters are reset. The internal reset is
released before V_CC reaches 1.7 V. The operating supply
voltage ranges from 2 V to the open battery level of
4.3 V (min). The characteristics are guaranteed at
V_CC = 3 V. In order to protect the SAA1501T against high
supply voltages during open battery in a flyback converter,
a voltage clamp circuit is made active at 6.35 V (typ). The
clamping current must not exceed 80 mA. A band gap
reference block is included to generate accurate voltages
i.e. for the oscillator. Moreover, together with R_ref, accurate
currents are generated which are used in the I/F and V/I
converters and the oscillator block. In the standby mode
only the oscillator and the digital parts are active to limit the
discharge current of the batteries to a current level of less
than 100 mA. The circuits that are needed temporarily are
switched on and off during standby (see “Timing
characteristics” t_som).
Voltage-to-current charge and discharge
In the V/I converter, the input charge current is translated
into acceptable levels for the circuit. The conversion
formula is:
I_c = -⁽---I--^c---^h---^a-R-^r--^g-_C--^e--_C--^×-_C---R-----^s--^c---⁾-- ; where R_CCC > R_sc (see Fig.7)
December 1994
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