Features
Safe management of fast charge for NiCd, NiMH, or LiIon battery packs
High-frequency switching controller for efficient and simple charger design
Pre-charge qualification for detecting shorted, damaged, or overheated cells
Fast-charge termination by peak voltage (PVD), minimum current (Li-Ion), maximum temperature, and maximum charge time
Selectable top-off mode for achieving maximum capacity in NiMH batteries
Programmable trickle-charge mode for reviving deeply discharged batteries and for postcharge maintenance
Built-in battery removal and insertion detection
Sleep mode for low power consumption
bq2000
Programmable Multi-Chemistry
Fast-Charge Management IC
General Description
The bq2000 is a programmable, monolithic IC for fast-charge management of nickel cadmium (NiCd), nickel metal-hydride (NiMH), or lith- ium-ion (Li-Ion) batteries in singleor multi-chemistry applications. The bq2000 detects the battery chemistry and proceeds with the optimal charging and termination algorithms. This process eliminates undesirable undercharged or overcharged conditions and allows accurate and safe termination of fast charge.
Depending on the chemistry, the bq2000 provides a number of charge termination criteria:
Peak voltage, PVD (for NiCd and NiMH)
Minimum charging current (f or Li-Ion)
Maximum temperature
Maximum charge time
For safety, the bq2000 inhibits fast charge until the battery voltage and temperature are within user-defined limits. If the battery voltage is below the low-voltage threshold, the bq2000 uses trickle-charge to condition the battery. For NiMH batteries, the bq2000 provides an optional top-off charge to maximize the battery capacity.
The integrated high-frequency comparator allows the bq2000 to be the basis for a complete, high-efficiency power-conversion circuit for both nickel-based and lithium-based chemistries.
Pin Connections
SNS |
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8 |
MOD |
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VSS |
2 |
7 |
VCC |
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3 |
6 |
RC |
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LED |
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BAT |
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5 |
TS |
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8-Pin DIP or Narrow SOIC or TSSOP
PN-2000.eps
SLUS138A–FEBRUARY 2000 E
Pin Names
SNS |
Current-sense input |
TS |
Temperature-sense |
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VSS |
System ground |
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input |
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RC |
Timer-program input |
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Charge-status |
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LED |
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VCC |
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output |
Supply-voltage input |
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BAT |
Battery-voltage |
MOD |
Modulation-control |
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input |
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output |
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1
bq2000
Pin Descriptions
SNS |
Current-sense input |
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Enables the bq2000 to sense the battery cur- |
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rent via the voltage developed on this pin by |
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an external sense-resistor connected in se- |
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ries with the battery pack |
VSS |
System Ground |
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Charge-status output |
LED |
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Open-drain output that indicates the charg- |
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ing status by turning on, turning off, or |
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flashing an external LED |
BAT |
Battery-voltage input |
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Battery-voltage sense input. A simple resistive |
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divider, across the battery terminals, generates |
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this input. |
TS |
Temperature-sense input |
RC |
Timer-program input |
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RC input used to program the maximum |
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charge-time, hold-off period, and trickle |
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rate during the charge cycle, and to disable |
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or enable top-off charge |
VCC |
Supply-voltage input |
MOD |
Modulation-control output |
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Push-pull output that controls the charging |
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current to the battery. MOD switches high |
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to enable charging current to flow and low to |
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inhibit chargingcurrent flow. |
Functional Description
The bq2000 is a versatile, multi-chemistry batterycharge control device. See Figure 1 for a functional block diagram and Figure 2 for a state diagram.
Input for an external battery-temperature monitoring circuit. An external resistive divider network with a negative tempera- ture-coefficient thermistor sets the lower and upper temperature thresholds.
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TS |
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Voltage |
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Voltage |
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Reference |
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Comparator |
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BAT |
ADC |
PVD |
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ALU |
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OSC |
Clock |
Timer |
Charge |
LED |
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Phase |
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Control |
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Generator |
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RC |
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Internal |
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Voltage |
MOD |
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OSC |
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Comparator |
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SNS |
VCC |
VSS |
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bq2000BD.eps |
Figure 1. Functional Block Diagram
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bq2000 |
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4.0V < VCC < 6.0V |
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Charge |
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Initialization |
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VBAT < VSLP |
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Battery Voltage |
VMCV < VBAT < VSLP |
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Sleep |
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(checked at all times) |
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Mode |
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VSLP |
< VBAT |
< VCC |
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VBAT < VMCV |
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VTS > VHTF |
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Charge |
VTS < VHTF |
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Battery Temperature |
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Suspended |
(checked at all times) |
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VBAT |
< VLBAT or |
VLBAT < VBAT < VMCV and |
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Battery |
VTS > VLTF |
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VHTF < VTS < VLTF |
Current |
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Conditioning |
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Regulation |
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VLBAT < VBAT < VMCV and |
PVD (after hold-off period), |
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V |
HTF |
< V |
TS |
< V |
LTF |
or VTS < |
VTCO or |
Time < MTO |
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Time = MTO |
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and |
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VBAT > VMCV |
VCC |
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No |
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Reset |
Maintenance |
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Top-Off |
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Voltage |
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Charge |
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Regulation |
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Selected? |
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Yes |
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Current Taper |
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or |
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Time = MTO |
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Time = MTO or |
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VTS < VTCO |
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Top-Off |
VBAT |
VMCV |
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Done |
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VBAT |
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VMCV |
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VCC Reset or Battery Replacement or Capacity Depletion (Li-Ion) |
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SD2000.eps |
Figure 2. State Diagram
3
bq2000
Initiation and Charge Qualification
The bq2000 initiates a charge cycle when it detects
Application of power to VCC
Battery replacement
Exit from sleep mode
Capacity depletion (Li-Ion only)
Immediately following initiation, the IC enters a charge-qualification mode. The bq2000 charge qualification is based on battery voltage and temperature. If voltage on pin BAT is less than the internal threshold, VLBAT, the bq2000 enters the charge-pending state. This condition indicates the possiblility of a defective or shorted battery pack. In an attempt to revive a fully depleted pack, the bq2000 enables the MOD pin to trickle-charge at a rate of once every 1.0s. As explained in the section “Top-Off and Pulse-Trickle Charge,” the trickle pulse-width is user-selectable and is set by the value of the resistance connected to pin RC.
During this period, the LED pin blinks at a 1Hz rate, indicating the pending status of the charger.
Similarly, the bq2000 suspends fast charge if the battery temperature is outside the VLTF to VHTF range. (See Table 4.) For safety reasons, however, it disables the pulse trickle, in the case of a battery over-temperature condition (i.e., VTS < VHTF). Fast charge begins when the battery temperature and voltage are valid.
Battery Chemistry
The bq2000 detects the battery chemistry by monitoring the battery-voltage profile during the initial stage of the fast charge. If the voltage on BAT input rises to the internal VMCV reference, the IC assumes a Li-Ion battery. Otherwise the bq2000 assumes NiCd/NiMH chemistry.
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. In a mixed-chemistry design, a common voltage-divider is used as long as the maximum charge voltage of the nickel-based pack is below that of the Li-Ion pack. Otherwise, different scaling is required.
Once the chemistry is determined, the bq2000 completes the fast charge with the appropriate charge algorithm (Table 1). The user can customize the algorithm by programming the device using an external resistor and a capacitor connected to the RC pin, as discussed in later sections.
NiCd and NiMH Batteries
Following qualification, the bq2000 fast-charges NiCd or NiMH batteries using a current-limited algorithm. During the fast-charge period, it monitors charge time, temperature, and voltage for adherence to the termination criteria. This monitoring is further explained in later sections. Following fast charge, the battery is topped off, if top-off is selected. The charging cycle ends with a trickle maintenance-charge that continues as long as the voltage on pin BAT remains below VMCV.
IMAX
Current
Trickle
IMIN
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Qualification |
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VMCV |
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Fast |
Charge |
Voltage |
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VLBAT |
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Phase 1 |
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Phase 2 |
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Current |
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Voltage |
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Time |
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GR2000CA.eps |
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Figure 3. Lithium-Ion Charge Algorithm
4
bq2000
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Table 1. Charge Algorithm |
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Battery Chemistry |
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Charge Algorithm |
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1. |
Charge qualification |
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2. |
Trickle charge, if required |
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NiCd or NiMH |
3. |
Fast charge (constant current) |
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4. |
Charge termination (peak voltage, maximum charge time) |
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5. |
Top-off (optional) |
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6. |
Trickle charge |
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1. |
Charge qualification |
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Li-Ion |
2. |
Trickle charge, if required |
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3. |
Two-step fast charge (constant current followed by constant voltage) |
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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. Figure 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 details.)
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 typical 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 |
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VCELL |
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= N |
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− 1 |
RB2 |
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VMCV |
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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 seriescells 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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