"Programmable Multi-Chemistry Fast-Charge Management IC"
1
Features
Safe management of fast
charge for NiCd, NiMH, or
Li-Ion 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
D /
t minimum current
(Li-Ion), maximum tempera-
ture, and maximum charge
time
Selectable top-off mode for
achieving maximum capacity
in NiMH batteries
Programmable trickle-charge
mode for reviving deeply dis-
charged batteries and for
post-charge maintenance
Built-in battery removal and
insertion detection
Sleep mode for low power
consumption
General Description
The bq2000T is a programmable,
monolithic IC for fast-charge man-
agement of nickel cadmium
(NiCd), nickel metal-hydride
(NiMH), or lithium-ion (Li-Ion)
batteries
in
single-
or
multi-chemistry applications. The
bq2000T detects the battery chem-
istry and proceeds with the opti-
mal charging and termination al-
gorithms. This process eliminates
undesirable undercharged or
overcharged conditions and allows
accurate and safe termination of
fast charge.
Depending on the chemistry, the
bq2000T provides a number of
charge termination criteria:
n
Rate of temperature rise,
D /
t
(for NiCd and NiMH)
n
Minimum charging current (for
Li-Ion)
n
Maximum temperature
n
Maximum charge time
For safety, the bq2000T inhibits
fast charge until the battery volt-
age and temperature are within
user-defined limits. If the battery
voltage is below the low-voltage
threshold, the bq2000T uses
trickle-charge to condition the
battery. For NiMH batteries, the
bq2000T provides an optional
top-off charge to maximize the
battery capacity.
The integrated high-frequency
comparator allows the bq2000T to
be the basis for a complete,
high-efficiency power-conversion
circuit for both nickel-based and
lithium-based chemistries.
bq2000T
SNS
Current-sense
input
V
SS
System ground
LED
Charge-status
output
BAT
Battery-voltage
input
1
PN-2000.eps
8-Pin DIP or Narrow SOIC
or TSSOP
2
3
4
8
7
6
5
SNS
LED
BAT
VSS
MOD
VCC
TS
RC
Pin Connections
Pin Names
Programmable Multi-Chemistry
Fast-Charge Management IC
SLUS149BFEBRUARY 2001
TS
Temperature-sense
input
RC
Timer-program
input
V
CC
Supply-voltage
input
MOD
Modulation-control
output
2
Pin Descriptions
SNS
Current-sense input
Enables the bq2000T to sense the battery
current via the voltage developed on this
pin by an external sense-resistor con-
nected in series with the battery pack
V
SS
System Ground
LED
Charge-status output
Open-drain output that indicates the
charging status by turning on, turning off,
or flashing an external LED
BAT
Battery-voltage input
Battery-voltage sense input. A simple resis-
tive divider, across the battery terminals,
generates this input.
TS
Temperature-sense input
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.
RC
Timer-program input
RC input used to program the maximum
charge-time, hold-off period, and trickle
rate during the charge cycle, and to dis-
able or enable top-off charge
V
CC
Supply-voltage input
MOD
Modulation-control output
Push-pull output that controls the charg-
ing current to the battery. MOD switches
high to enable charging current to flow
and low to inhibit charging- current flow.
Functional Description
The bq2000T is a versatile, multi-chemistry battery-
charge control device. See Figure 1 for a functional
block diagram and Figure 2 for the state diagram.
bq2000T
Figure 1. Functional Block Diagram
3
bq2000T
Figure 2. State Diagram
Initiation and Charge Qualification
The bq2000T initiates a charge cycle when it detects
n
Application of power to V
CC
n
Battery replacement
n
Exit from sleep mode
n
Capacity depletion (Li-Ion only)
Immediately following initiation, the IC enters a
charge-qualification mode. The bq2000T charge
qualification is based on battery voltage and
temperature. If voltage on pin BAT is less than the
internal threshold, V
LBAT
, the bq2000T 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
bq2000T 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 bq2000T suspends fast charge if the
battery temperature is outside the V
LTF
to V
HTF
range.
(See Table 4.) For safety reasons, however, it disables
the pulse trickle, in the case of a battery
over-temperature condition (i.e., V
TS
< V
HTF
). Fast
charge begins when the battery temperature and
voltage are valid.
Battery Chemistry
The bq2000T detects the battery chemistry by moni-
toring the battery-voltage profile during fast charge.
If the voltage on BAT input rises to the internal V
MCV
reference, the IC assumes a Li-Ion battery. Otherwise
the bq2000T assumes NiCd/NiMH chemistry.
As shown in Figure 6, a resistor voltage-divider be-
tween the battery pack's positive terminal and V
SS
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 bq2000T
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 bq2000T fast-charges
NiCd or NiMH batteries using a current-limited algo-
rithm. During the fast-charge period, it monitors
charge time, temperature, and voltage for adherence
to the termination criteria. This monitoring is further
4
bq2000T
GR2000CA.eps
I
MAX
I
MIN
Trickle
V
LBAT
V
MCV
Current
Voltage
Qualification
Time
Phase 1
Phase 2
Fast Charge
Voltage
Current
Figure 3. Lithium-Ion Charge Algorithm
explained in later sections. Following fast charge, the
battery is topped off, if top-off is selected. The charg-
ing cycle ends with a trickle maintenance-charge that
continues as long as the voltage on pin BAT remains
below V
MCV
.
Lithium-Ion Batteries
The bq2000T uses a two-phase fast-charge algorithm
for Li-Ion batteries (Figure 3). In phase one, the
bq2000T regulates constant current until V
BAT
rises to
V
MCV
. The bq2000T then moves to phase two, regu-
lates the battery with constant voltage of V
MCV
, and
terminates when the charging current falls below the
I
MIN
threshold. A new charge cycle is started if the
cell voltage falls below the V
RCH
threshold.
During the current-regulation phase, the bq2000T
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 bq2000T 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 be-
tween the R
MTO
and C
MTO
values and the maximum
charge time (MTO) for the bq2000T:
MTO = R
MTO
C
MTO
35,988
MTO is measured in minutes, R
MTO
in ohms, and
C
MTO
in farads. (Note: R
MTO
and C
MTO
values also de-
termine other features of the device. See Tables 2
and 3 for details.)
For Li-Ion cells, the bq2000T 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 V
SS
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 bq2000T compares the bat-
tery temperature to an internal high-temperature
cutoff threshold, V
TCO
. As shown in Table 4, high-tem-
perature termination occurs when voltage at pin TS
is less than this threshold.
T/
t (NiCd, NiMH)
When fast charging, the bq2000T monitors the volt-
age at pin TS for rate of temperature change detec-
tion,
T/
t. The bq2000T samples the voltage at the
TS pin every 16s and compares it to the value mea-
sured 2 samples earlier. This feature terminates fast
charge if this voltage declines at a rate of
V
161
V
Min
CC
Figure 5 shows a typical connection diagram.
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. Please note
that this threshold is different for the bq2000.
Initial Hold-Off Period
5
Battery Chemistry
Charge Algorithm
NiCd or NiMH
1. Charge qualification
2. Trickle charge, if required
3. Fast charge (constant current)
4. Charge termination (
T/
t, time)
5. Top-off (optional)
6. Trickle charge
Li-Ion
1. Charge qualification
2. Trickle charge, if required
3. Two-step fast charge (constant current followed by constant voltage)
4. Charge termination (minimum current, time)
Table 1. Charge Algorithm
bq2000T
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