BQ2003_12 Datasheet, PDF (6/18 Pages) Texas Instruments – Fast-Charge IC

bq2003
taken on the new battery is compared to ones taken be-
fore the original battery was removed and any that may
have been taken while no battery was present. If the IC
is configured for ∆T/∆t termination, this may result in a
premature fast-charge termination on the newly in-
serted battery.
Maximum temperature termination occurs anytime the
voltage on the TS pin falls below the temperature cut-off
threshold VTCO. Charge is also terminated if VTEMP rises
above the minimum temperature fault threshold, VLTF,
after fast charge begins.
Maximum charge time is configured using the TM pin.
Time settings are available for corresponding charge
rates of C/4, C/2, 1C, and 2C. Maximum time-out termi-
nation is enforced on the fast-charge phase, then reset,
and enforced again on the top-off phase, if selected.
There is no time limit on the trickle-charge phase.
Top-off Charge
An optional top-off charge phase may be selected to
follow fast charge termination for the C/2 through 4C
rates. This phase may be necessary on NiMH or other
battery chemistries that have a tendency to terminate
charge prior to reaching full capacity. With top-off en-
abled, charging continues at a reduced rate after
fast-charge termination for a period of time selected
by the TM1 and TM2 input pins. (See Table 2.) During
top-off, the MOD pin is enabled at a duty cycle of 4s
active for every 30s inactive. This modulation results
in an average rate 1/8th that of the fast charge rate.
Maximum voltage, time, and temperature are the only
termination methods enabled during top-off.
External Trickle Resistor
Maintenance charging is provided by the use of an exter-
nal trickle resistor between the high side of the battery
pack and VDC, the input charging supply voltage. (See
Figure 1.) This resistor is sized to meet two criteria.
n With the battery removed, the resistor must pull the
voltage at the BAT input above MCV for battery
insertion and removal detection.
n With the battery at its fully charged voltage, the
trickle current should be approximately equal to the
self-discharge rate of the battery.
Charge Status Indication
Charge status is indicated by the CHG output. The state
of the CHG output in the various charge cycle phases is
shown in Figure 3 and illustrated in Figure 1.
Temperature status is indicated by the TEMP output.
TEMP is in the high state whenever VTEMP is within the
temperature window defined by the VLTF and VHTF tem-
perature limits, and is low when the battery tempera-
ture is outside these limits.
In all cases, if VCELL exceeds the voltage at the MCV
pin, both CHG and TEMP outputs are held high regard-
less of other conditions. CHG and TEMP may both be used
to directly drive an LED.
Charge Current Control
The bq2003 controls charge current through the MOD
output pin. The current control circuitry is designed to
support implementation of a constant-current switching
regulator or to gate an externally regulated current
source.
When used in switch-mode configuration, the nominal
regulated current is:
IREG = 0.235V/RSNS
Charge current is monitored at the SNS input by the
voltage drop across a sense resistor, RSNS, between the
low side of the battery pack and ground. RSNS is sized to
provide the desired fast-charge current.
If the voltage at the SNS pin is less than VSNSLO, the
MOD output is switched high to pass charge current to
the battery.
When the SNS voltage is greater than VSNSHI, the MOD
output is switched low—shutting off charging current to
the battery.
VSNSLO = 0.044 ∗ VCC ± 25mV
VSNSHI = 0.05 ∗ VCC ± 25mV
When used to gate an externally regulated current
source, the SNS pin is connected to VSS, and no sense re-
sisitor is required.
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