1
Features
Safe management of fast charge
for NiCd and NiMH
High-frequency switching con-
troller for efficient and simple
charger design
Pre-charge qualification for
detecting shorted, damaged, or
overheated cells
Fast-charge termination by
T/t maximum temperature,
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 in-
sertion detection
Sleep mode for low power con-
sumption
General Description
The bq24401 is a programmable,
monolithic IC for fast-charge manage-
ment of nickel cadmium (NiCd) and
nickel metal-hydride (NiMH).
Depending on the chemistry, the
bq24401 provides a number of
charge termination criteria:
l
Rate of temperature rise, T/t
(for NiCd and NiMH)
l
Maximum temperature
l
Maximum charge time
For safety, the bq24401 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
bq24401 uses trickle-charge to
condition the battery. For NiMH
batteries, the bq24401 provides an
optional top-off charge to maximize
the battery capacity.
The integrated high-frequency com-
parator allows the bq24401 to be the
basis for a complete, high-efficiency
power-conversion circuit.
bq24401
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 NiCd/NiMH
Fast-Charge Management Device
SLUS497SEPTEMBER 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 bq24401 to sense the battery cur-
rent via the voltage developed on this pin by
an external sense-resistor connected in series
with the battery pack
V
SS
System Ground
LED
Charge-status output
Open-drain output that indicates the charg-
ing status by turning on, turning off, or flash-
ing an external LED
BAT
Battery-voltage input
Battery-voltage sense input. A simple resistive
divider, across the battery terminals, generates
this input.
TS
Temperature-sense input
Input for an external battery-temperature
monitoring circuit. An external resistive di-
vider 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 disable or
enable top-off charge
V
CC
Supply-voltage input
MOD
Modulation-control output
Push-pull output that controls the charging
current to the battery. MOD switches high to
enable charging current to flow and low to
inhibit charging- current flow.
Functional Description
The bq24401 is a versatile, NiCd, NiMH battery-charge
control device. See Figure 1 for a functional block dia-
gram and Figure 2 for the state diagram.
bq24401
BD2000T.eps
Voltage
Reference
Internal
OSC
Clock
Phase
Generator
ADC
OSC
SNS
TS
MOD
RC
BAT
LED
V
CC
V
SS
Timer
T/t
ALU
Voltage
Comparator
Voltage
Comparator
Charge
Control
Figure 1. Functional Block Diagram
3
bq24401
4.0 V < V
CC
< 6.0 V
Battery Voltage
(checked at all times)
Sleep
Mode
Charge
Suspended
Charge
Initialization
Battery
Conditioning
Battery Temperature
(sampled every 16
seconds for dT/dt)
V
BAT
< V
SLP
V
SLP
< V
BAT
< V
CC
V
BAT
< V
MCV
V
MCV
< V
BAT
< V
SLP
Current
Regulation
V
BAT
< V
LBAT
or
V
TS
> V
LTF
V
TS
> V
HTF
V
TS
<V
HTF
V
LBAT
< V
BAT
< V
MCV
and
V
HTF
< V
TS
< V
LTF
V
LBAT
< V
BAT
< V
MCV
and
V
HTF
< V
TS
< V
LTF
T/t (after hold-off period),
or V
TS
< V
TCO
or Time = MTO
Top-Off
Selected?
Maintenance
Charge
No
Top-Off
Yes
Done
Time = MTO or
V
TS
< V
TCO
V
BAT
> V
MCV
V
BAT
> V
MCV
V
CC
Reset or Battery Replacement
V
CC
Reset
Figure 2. State Diagram
UDG-01087
Initiation and Charge Qualification
The bq24401 initiates a charge cycle when it detects
l
Application of power to V
CC
l
Battery replacement
l
Exit from sleep mode
Immediately following initiation, the IC enters a
charge-qualification mode. The bq24401 charge qualifica-
tion is based on battery voltage and temperature. If
voltage on pin BAT is less than the internal threshold,
V
LBAT
, the bq24401 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 bq24401 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 bq24401 suspends fast charge if the battery
temperature is outside the V
LTF
to V
HTF
range. (See Ta-
ble 4.) For safety reasons, however, it disables the pulse
trickle, in the case of a battery over-temperature condi-
tion (i.e., V
TS
< V
HTF
). Fast charge begins when the bat-
tery temperature and voltage are valid.
NiCd and NiMH Batteries
Following qualification, the bq24401 fast-charges NiCd or
NiMH batteries using a current-limited algorithm. Dur-
ing the fast-charge period, it monitors charge time, tem-
perature, and voltage for adherence to the termination
criteria. This monitoring is further explained in later sec-
tions. 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 V
MCV
.
Charge Termination
Maximum Charge Time
The bq24401 sets the maximum charge-time through pin
RC. With the proper selection of external resistor and ca-
pacitor, various time-out values may be achieved. Figure
3 shows a typical connection.
The following equation shows the relationship between
the R
MTO
and C
MTO
values and the maximum charge
time (MTO) for the bq24401:
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 determine other fea-
tures of the device. See Tables 2 and 3 for details.)
Maximum Temperature
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 4 shows a typical
temperature-sensing circuit.
During fast charge, the bq24401 compares the battery
temperature to an internal high-temperature cutoff
threshold, V
TCO
. As shown in Table 4, high-temperature
termination occurs when voltage at pin TS is less than
this threshold.
DT/Dt
When fast charging, the bq24401 monitors the voltage at
pin TS for rate of temperature change detection, T/t.
The bq24401 samples the voltage at the TS pin every 16s
and compares it to the value measured 2 samples earlier.
This feature terminates fast charge if this voltage de-
clines at a rate of
V
161
V
Min
CC
Figure 4 shows a typical connection diagram.
4
bq24401
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
Table 1. Charge Algorithm
Initial Hold-Off Period
The values of the external resistor and capacitor con-
nected to pin RC set the initial hold-off period. During
this period, the bq24401 avoids early termination by dis-
abling the T/t feature. This period is fixed at the pro-
grammed value of the maximum charge time divided by
32.
hold-off period =
maximum time - out
32
5
bq24401
Parameter
Value
Maximum cell voltage (V
MCV
)
2V
Minimum pre-charge qualification voltage (V
LBAT
)
950mV
High-temperature cutoff voltage (V
TCO
)
0.225 V
CC
High-temperature fault voltage (V
HTF
)
0.25 V
CC
Low-temperature fault voltage (V
LTF
)
0.5 V
CC
bq24401 fast-charge maximum time out (MTO)
R
MTO
C
MTO
35,988
Fast-charge charging current (I
MAX
)
0.05/R
SNS
Hold-off period
MTO/32
Top-off charging current (optional)
I
MAX
/16
Top-off period (optional)
MTO
Trickle-charge frequency
1Hz
Trickle-charge pulse-width
See Figure 6
Table 2. Summary of NiCd or NiMH Charging Characteristics
Temperature
Condition
Action
V
TS
> V
LTF
Cold battery--checked at all times
Suspends fast charge or top-off and timer
Allows trickle charge--LED flashes at 1Hz rate
during pre-charge qualification and fast charge
V
HTF
< V
TS
< V
LTF
Optimal operating range
Allows charging
V
TS
< V
HTF
Hot battery--checked during charge qualifi-
cation and top-off and trickle-charge
Suspends fast-charge initiation, does not allow
trickle charge--LED flashes at 1Hz rate during
pre-charge qualification
V
TS
< V
TCO
Battery exceeding maximum allowable tem-
perature--checked at all times
Terminates fast charge or top-off
Table 3. Temperature-Monitoring Conditions
Table 4. Charge Status Display
Charge Action State
LED Status
Battery absent
High impedance
Pre-charge qualification
1Hz flash
Trickle charge (before fast charge)
1Hz flash
Fast charging
Low
Top-off or trickle
High impedance
Charge complete
High impedance
Sleep mode
High impedance
Charge suspended (V
TS
> V
LTF
)
1Hz flash
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