Features

Fast charge of nickel cadmium
or nickel-metal hydride batter-
ies

Direct LED output displays
charge status

Fast-charge termination by -

maximum voltage, maximum
temperature, and maximum
time

Internal band-gap voltage ref-
erence

Selectable pulse-trickle charge
rates

Low-power mode

8-pin 300-mil DIP or 150-mil
SOIC

General Description

The bq2002C Fast-Charge IC is a low-
cost CMOS battery-charge controller
providing reliable charge termination
for both NiCd and NiMH battery appli-
cations.

Controlling a current-limited

or constant-current supply allows the
bq2002C to be the basis for a cost-
effective stand-alone or system-inte-
grated charger.

The bq2002C inte-

grates fast charge with pulsed-trickle
control in a single IC for charging one
or more NiCd or NiMH battery cells.

Fast charge is initiated on application
of the charging supply or battery
replacement. For safety, fast charge is
inhibited if the battery temperature
and voltage are outside configured
limits.

Fast charge is terminated by any of
the following:

Peak voltage detection (PVD)

Negative delta voltage (-

Maximum voltage

Maximum temperature

Maximum time

After fast charge, the bq2002C pulse-
trickles the battery per the pre-
configured limits. Fast charge may be
inhibited using the INH pin.

The

bq2002C may also be placed in low-
standby-power mode to reduce
system power consumption.

NiCd/NiMH Fast-Charge Management IC

bq2002C

Timer mode select input

LED

Charging status output

BAT

Battery voltage input

System ground

PN-200201.eps

8-Pin DIP or

Narrow SOIC

LED

BAT

VSS

INH

VCC

Temperature sense input

Supply voltage input

INH

Charge inhibit input

Charge control output

Pin Connections

Pin Names

9/97 B

Pin Descriptions

Timer mode input

A three-level input that controls the settings
for the fast charge safety timer, voltage ter-
mination mode, pulse-trickle, and voltage
hold-off time.

LED

Charging output status

Open-drain output that indicates the charging
status.

BAT

Battery input voltage

The battery voltage sense input. The input to
this pin is created by a high-impedance re-
sistor divider network connected between
the positive and negative terminals of the
battery.

System ground

Temperature sense input

Input for an external battery temperature
monitoring thermistor.

Supply voltage input

5.0V

20% power input.

INH

Charge inhibit input

When high, INH suspends the fast charge in
progress.

When returned low, the IC re-

sumes operation at the point where initially
suspended.

Charge control output

An open-drain output used to control the
charging current to the battery. CC switch-
ing to high impedance (Z) enables charging
current to flow, and low to inhibit charging
current. CC is modulated to provide pulse
trickle.

Functional Description

Figure 2 shows a state diagram and Figure 3 shows a
block diagram of the bq2002C.

Battery Voltage and Temperature
Measurements

Battery voltage and temperature are monitored for
maximum allowable values. The voltage presented on
the battery sense input, BAT, should represent a
single-cell potential for the battery under charge.

resistor-divider ratio of

RB1

RB2

= N - 1

is recommended to maintain the battery voltage within
the valid range, where N is the number of cells, RB1 is
the resistor connected to the positive battery terminal,
and RB2 is the resistor connected to the negative
battery terminal. See Figure 1.

Note: This resistor-divider network input impedance to
end-to-end should be at least 200k

and less than 1 M

A ground-referenced negative temperature coefficient
thermistor placed near the battery may be used as a low-
cost temperature-to-voltage transducer. The temperature
sense voltage input at TS is developed using a resistor-
thermistor network between V

and V

. See Figure 1.

bq2002C

Fg2002/C.eps

bq2002C

BAT

VSS

N
T
C

bq2002C

VCC

PACK +

VSS

BAT pin connection

Thermistor connection

NTC = negative temperature coefficient thermistor.

RB1

RB2

Mid-level

setting for TM

Figure 1. Voltage and Temperature Monitoring and TM Pin Configuration

Starting A Charge Cycle

Either of two events starts a charge cycle (see Figure 4):

1. Application of power to V

2. Voltage at the BAT pin falling through the maximum
cell voltage V

MCV

where

MCV

= 2V

5%.

If the battery is within the configured temperature and
voltage limits, the IC begins fast charge.

The valid

battery voltage range is V

LBAT

< V

BAT

< V

MCV,

where

LBAT

= 0.175

20%

The valid temperature range is V

> V

HTF

where

HTF

= 0.6

5%.

If V

BAT

LBAT

or V

HTF

, the IC enters the charge-

pending state.

In this state pulse trickle charge is

applied to the battery and the LED flashes until the
voltage and temperature come into the allowed fast
charge range or V

BAT

rises above V

MCV

. Anytime V

BAT

MCV

, the IC enters the Charge Complete/Battery

Absent state. In this state the LED is off and trickle
charge is applied to the battery until the next new
charge cycle begins.

Fast charge continues until termination by one or more of
the five possible termination conditions:

Peak voltage detection (PVD)

Negative delta voltage (-

Maximum voltage

Maximum temperature

Maximum time

Corresponding

Fast-Charge

Rate

Termination

Typical

Fast-

Charge

Time Limits

(minutes)

Typical PVD

and -

Hold-Off

Time (seconds)

Pulse-

Trickle

Rate

Pulse-

Trickle

Pulse Width

(ms)

Maximum

Synchronized

Sampling

Period

(seconds)

C/2

Mid

PVD

160

300

C/32

18.7

Low

PVD

150

C/32

18.7

High

C/32

9.4

Notes:

Typical conditions = 25°C, V

= 5.0V

Mid = 0.5 * V

±0.5V

Tolerance on all timing is ±12%.

Table 1. Fast-Charge Safety Time/Hold-Off Table

bq2002C

TD2002C1.eps

Fast Charging

VCC = 0

Fast Charging

CC Output

LED

Charge initiated by application of power

Charge initiated by battery replacement

Pulse-Trickle

See

Table 1

Figure 4. Charge Cycle Phases

PVD and -

V Termination

There are two modes for voltage termination, depend-
ing on the state of TM. For -

V (TM = high), if V

BAT

lower than any previously measured value by 12mV

3mV, fast charge is terminated. For PVD (TM = low or

mid), a decrease of 2.5mV

2.5mV terminates fast

charge. The PVD and -

V tests are valid in the range

1V < V

BAT

< 2V.

Synchronized Voltage Sampling

Voltage sampling at the BAT pin for PVD and -

V termi-

nation may be synchronized to an external stimulus
using the INH input.

Low-high-low input pulses

between 100ns and 3.5ms in width must be applied at
the INH pin with a frequency greater than the "maxi-
mum synchronized sampling period" set by the state of
the TM pin as shown in Table 1. Voltage is sampled on
the falling edge of such pulses.

If the time between

pulses is greater than the synchronizing period, voltage
sampling "free-runs" at once every 17 seconds. A sample
is taken by averaging together voltage measurements
taken 57

s apart. The IC takes 32 measurements in

PVD mode and 16 measurements in -

V mode.

The

resulting sample periods (9.17 and 18.18ms, respec-
tively) filter out harmonics centered around 55 and
109Hz. This technique minimizes the effect of any AC
line ripple that may feed through the power supply from
either 50 or 60Hz AC sources. If the INH input remains
high for more than 12ms, the voltage sample history
kept by the IC and used for PVD and -

V termination

decisions is erased and a new history is started. Such a
reset is required when transitioning from free-running
to synchronized voltage sampling. The response of the
IC to pulses less than 100ns in width or between 3.5ms
and 12ms is indeterminate. The tolerance on all timing
is

12%.

Voltage Termination Hold-off

A hold-off period occurs at the start of fast charging.
During the hold-off time, the PVD and -

V terminations

are disabled. This avoids premature termination on the
voltage spikes sometimes produced by older batteries
when fast-charge current is first applied.

Maximum

voltage and temperature terminations are not affected
by the hold-off period.

Maximum Voltage, Temperature, and Time

Any time the voltage on the BAT pin exceeds the maxi-
mum cell voltage, V

MCV

, fast charge is terminated.

Maximum temperature termination occurs anytime the
voltage on the TS pin falls below the temperature cut-off
threshold V

TCO,

where

TCO

= 0.5

5%.

Maximum charge time is configured using the TM pin.
Time settings are available for corresponding charge
rates of C/2, 1C, and 2C. Maximum time-out termina-
tion is enforced on the fast-charge phase, then reset, and
enforced again on the top-off phase, if selected. There is
no time limit on the trickle-charge phase.

Pulse-Trickle Charge

Pulse-trickle is used to compensate for self-discharge
while the battery is idle in the charger. The battery is
pulse-trickle charged by driving the CC pin active once
per second for the period specified in Table 1.

This

results in a trickle rate of C/32.

TM Pin

The TM pin is a three-level pin used to select the
charge timer, top-off, voltage termination mode, trickle
rate, and voltage hold-off period options.

Table 1

describes the states selected by the TM pin.

The

mid-level selection input is developed by a resistor
divider between V

and ground that fixes the volt-

age on TM at V

0.5V. See Figure 4.

Charge Status Indication

A fast charge in progress is uniquely indicated when the
LED pin goes low. In the charge pending state, the LED
pin is driven low for 500ms, then to high-Z for 500ms.
The LED pin is driven to the high-Z state for all other
conditions. Figure 2 outlines the state of the LED pin
during charge.

Charge Inhibit

Fast charge and top-off may be inhibited by using the
INH pin. When high, INH suspends all fast charge and
top-off activity and the internal charge timer.

INH

freezes the current state of LED until inhibit is
removed. Temperature monitoring is not affected by the
INH pin. During charge inhibit, the bq2002C continues
to pulse-trickle charge the battery per the TM selection.
When INH returns low, charge control and the charge
timer resume from the point where INH became active.

Low-Power Mode

The IC enters a low-power state when V

BAT

is driven

above the power-down threshold (V

) where

= V

- (1V

0.5V)

Both the CC pin and the LED pin are driven to the
high-Z state. The operating current is reduced to less
than 1

A in this mode. When V

BAT

returns to a value

below V

, the IC pulse-trickle charges until the next

new charge cycle begins.

bq2002C

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