1
Features
Ø
Safe management of fast charge
for NiCd and NiMH battery
packs.
Ø
High-frequency switching con-
troller for efficient and simple
charger design
Ø
Pre-charge qualification for
detecting shorted, damaged, or
overheated cells
Ø
Fast-charge termination by
peak voltahge (PVD), maximum
temperature, and maximum
charge time
Ø
Selectable top-off mode for
achieving maximum capacity in
NiMH batteries
Ø
Programmable trickle-charge
mode for reviving deeply dis-
charged batteries and for post-
charge maintenance
Ø
Built-in battery removal and in-
sertion detection
Ø
Sleep mode for low power con-
sumption
General Description
The bq24400 is a programmable,
monolithic IC for fast-charge manage-
ment of nickel cadmium (NiCd) and
nickel metal-hydride (NiMH) batter-
ies.
The bq24400 provides a number of
charge termination criteria:
l
Peak voltage, PVD (for NiCd and
NiMH)
l
Maximum temperature
l
Maximum charge time
For safety, the bq24400 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
bq24400 uses trickle-charge to
condition the battery. For NiMH
batteries, the bq24400 provides an
optional top-off charge to maximize
the battery capacity.
The integrated high-frequency com-
parator allows the bq24400 to be the
basis for a complete, high-efficiency
power-conversion circuit.
bq24400
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
SLUS498 -SEPTEMBER 2001
TS
Temperature-sense
input
RC
Timer-program input
V
CC
Supply-voltage input
MOD
Modulation-control
output
Pin Descriptions
SNS
Current-sense input
Enables the bq24400 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 bq24400 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.
2
bq24400
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
Initiation and Charge Qualification
The bq24400 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 bq24400 charge qualifica-
tion is based on battery voltage and temperature. If
voltage on pin BAT is less than the internal threshold,
V
LBAT
, the bq24400 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 bq24400 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, in-
dicating the pending status of the charger.
Similarly, the bq24400 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.
NiCd and NiMH Batteries
Following qualification, the bq24400 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 bq24400 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 bq24400:
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 features of the device. See Tables 2 and 3 for de-
tails.)
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 bq24400 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.
Peak Voltage
The bq24400 uses a peak-voltage detection (PVD) scheme
to terminate fast charge for NiCd and NiMH batteries.
The bq24400 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 3.8 mV (PVD).
As shown in figure 5, a resistor voltage-divider between
the battery pack's positive terminal and V
SS
scales the
battery voltage measured at pin BAT.
The resistor values R
B1
And R
B2
are calculated by the fol-
lowing equation:
4
bq24400
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
PDF 
ZIP
