BQ2000 Datasheet, PDF (5/18 Pages) Texas Instruments – Programmable Multi-Chemistry Fast-Charge Management IC

bq2000
Battery Chemistry
NiCd or NiMH
Li-Ion
Table 1. Charge Algorithm
Charge Algorithm
1. Charge qualification
2. Trickle charge, if required
3. Fast charge (constant current)
4. Charge termination (peak voltage, maximum charge time)
5. Top-off (optional)
6. Trickle charge
1. Charge qualification
2. Trickle charge, if required
3. Two-step fast charge (constant current followed by constant voltage)
4. Charge termination (minimum current, maximum charge time)
Lithium-Ion Batteries
The bq2000 uses a two-phase fast-charge algorithm for
Li-Ion batteries (Figure 3). In phase one, the bq2000
regulates constant current until VBAT rises to VMCV. The
bq2000 then moves to phase two, regulates the battery
with constant voltage of VMCV, and terminates when the
charging current falls below the IMIN threshold. A new
charge cycle is started if the cell voltage falls below the
VRCH threshold.
During the current-regulation phase, the bq2000
monitors charge time, battery temperature, and battery
voltage for adherence to the termination criteria. During
the final constant-voltage stage, in addition to the
charge time and temperature, it monitors the charge
current as a termination criterion. There is no
post-charge maintenance mode for Li-Ion batteries.
Charge Termination
Maximum Charge Time (NiCD, NiMH, and
Li-Ion)
The bq2000 sets the maximum charge-time through pin
RC. With the proper selection of external resistor and
capacitor, various time-out values may be achieved. Fig-
ure 4 shows a typical connection.
The following equation shows the relationship between
the RMTO and CMTO values and the maximum charge
time (MTO) for the bq2000:
MTO = RMTO ∗ CMTO ∗ 35,988
MTO is measured in minutes, RMTO in ohms, and CMTO
in farads. (Note: RMTO and CMTO values also determine
other features of the device. See Tables 2 and 3 for de-
tails.)
For Li-Ion cells, the bq2000 resets the MTO when the
battery reaches the constant-voltage phase of the
charge. This feature provides the additional charge time
required for Li-Ion cells.
Maximum Temperature (NiCd, NiMH, Li-Ion)
A negative-coefficient thermistor, referenced to VSS and
placed in thermal contact with the battery, may be used
as a temperature-sensing device. Figure 5 shows a typi-
cal temperature-sensing circuit.
During fast charge, the bq2000 compares the battery
temperature to an internal high-temperature cutoff
threshold, VTCO. As shown in Table 4, high-temperature
termination occurs when voltage at pin TS is less than
this threshold.
Peak Voltage (NiCd, NiMH)
The bq2000 uses a peak-voltage detection (PVD) scheme
to terminate fast charge for NiCd and NiMH batteries.
The bq2000 continuously samples the voltage on the
BAT pin, representing the battery voltage, and triggers
the peak detection feature if this value falls below the
maximum sampled value by as much as 3.8mV (PVD).
As shown in Figure 6, a resistor voltage-divider between
the battery pack’s positive terminal and VSS scales the
battery voltage measured at pin BAT.
For Li-Ion battery packs, the resistor values RB1 and
RB2 are calculated by the following equation:
RB1
RB2
=


N∗
VCELL
VMCV


−
1
where N is the number of cells in series and VCELL is the
manufacturer-specified charging voltage. The end-to-
end input impedance of this resistive divider network
should be at least 200kΩ and no more than 1MΩ.
A NiCd or NiMH battery pack consisting of N series-
cells may benefit by the selection of the RB1 value to be
N-1 times larger than the RB2 value.
In a mixed-chemistry design, a common voltage-divider
is used as long as the maximum charge voltage of the
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