BQ2000T_15 Datasheet, PDF (9/29 Pages) Texas Instruments – Programmable Multi-Chemistry Fast-Charge Management IC

bq2000T
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SLUS149D – MAY 1999 – REVISED JANUARY 2010
Maximum Temperature (NiCd, NiMH, Li-Ion)
A negative-coefficient thermistor, referenced to V_SS and placed in thermal contact with the battery, may be used
as a temperature-sensing device. Figure 6 shows a typical temperature-sensing circuit.
V_CC
2 _VSS
V_CC ⁷
bq2000T
5
TS
R_T1
R_T2
N
T
Battery
Pack
C
Figure 6. Temperature Monitoring Configuration
During fast charge, the bq2000T compares the battery temperature to an internal high-temperature cutoff
threshold, V_TCO, and a low-temperature threshold, V_LTF. During fast charge only, the V_HTF fault comparator is
disabled. When the voltage at the TS pin is lower than V_TCO, the bq2000T terminates fast charge, moves to the
charge suspended state, and turns off the LED. When V_TS rises above V_HTF, the bq2000T resumes charging in
the trickle maintenance charge state, per Figure 2. In fast charge (either constant current or constant voltage fast
charge), when the voltage on the TS pin is higher than V_LTF, the charger enters the battery conditioning state, as
described in the previous section. Fast charge is resumed when V_TS is less than V_LTF.
Rate of Temperature Rise (NiCd, NiMH)
The bq2000T uses a rate of temperature rise (ΔT/Δt) scheme to terminate fast charge for NiCd and NiMH
batteries. During fast charge, it samples the TS pin voltage every 8 seconds and compares it to the value
measured 2 samples earlier. This feature terminates fast charge if this voltage declines at a rate of V_CC/161
(V/min). Figure 6 shows a typical connection diagram. In preparation for sampling the TS pin voltage, the
bq2000T briefly turns off most circuits (the MOD and RC pins both go low) in order to get the cleanest possible,
noise-free measurement. While the monitoring of the TS pin voltage is continuous, the sampling of the TS pin
voltage with the internal ADC only occurs during the constant current regulation phase of fast charge. If the cell
voltage reaches V_MCV, the pack is assumed to be Li-Ion and the TS pin voltage sampling is disabled, as ΔT/Δt is
not a termination criterion for Lithium cells.
Minimum Current (Li-Ion Only)
The bq2000T monitors the charging current during the voltage-regulation phase of Li-Ion batteries. Fast charge is
terminated when the current is tapered off to 7% of the maximum charging current. Note that this threshold is
different for the bq2000.
Once constant-current fast charge has ended, the bq2000T either measures the value of the C_MTO capacitor (in
the case of Nickel batteries) and then proceeds to either top-off or trickle maintenance charge or simply
completes the constant-voltage stage of fast charge (in the case of a Li-Ion cell).
Top-Off and Pulse-Trickle Maintenance Charge
An optional top-off charge is available for NiCd or NiMH batteries. Top-off may be desirable on batteries that
have a tendency to terminate charge before reaching full capacity. To enable this option, the capacitance value
of C_MTO connected between the RC pin and V_CC (Figure 5) should be greater than 0.13 µF , and the value of the
resistor connected to this pin should be less than 250 kΩ . To disable top-off, the capacitance value should be
less than 0.07 µF. The tolerance of the capacitor needs to be taken into account in component selection.
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