Features

Fast charge and conditioning of

nickel cadmium or nickel-metal
hydride batteries

Hysteretic PWM switch-mode

current regulation or gated con-
trol of an external regulator

Easily integrated into systems or

used as a stand-alone charger

Pre-charge qualification of tem-

perature and voltage

Configurable, direct LED outputs

display battery and charge status

Fast-charge termination by tem-

perature/

time, peak volume de-

tection, -

V, maximum voltage,

maximum temperature, and maxi-
mum time

Optional top-off charge and

pu ls ed cu rre nt mai nten anc e
charging

Logic-level controlled low-power

mode (< 5

µA standby current)

General Description

The bq2004 Fast Charge IC pro-
vides comprehensive fast charge con-
trol functions together with high-speed
switching power control circuitry on a
monolithic CMOS device.

Integration of closed-loop current
control circuitry allows the bq2004
to be the basis of a cost-effective so-
lution for stand-alone and system-
integrated chargers for batteries of
one or more cells.

Switch-activated discharge-before-
charge allows bq2004-based chargers
to support battery conditioning and
capacity determination.

High-efficiency power conversion is
accomplished using the bq2004 as a
hysteretic PWM controller for
switch-mode regulation of the charg-
ing current. The bq2004 may alterna-
tively be used to gate an externally
regulated charging current.

Fast charge may begin on applica-
tion of the charging supply, replace-

ment of the battery, or switch de-
pression. For safety, fast charge is
inhibited unless/until the battery
temperature and voltage are within
configured limits.

Temperature, voltage, and time are
monitored throughout fast charge.
Fast charge is terminated by any of
the following:

Rate of temperature time
(

T/ t)

Peak voltage detection (PVD)

Negative delta voltage (-

Maximum voltage

Maximum temperature

Maximum time

After fast charge, optional top-off
and pulsed current maintenance
phases are available.

Fast-Charge IC

bq2004

DCMD

Discharge command

DSEL

Display select

VSEL

Voltage termination
select

Timer mode select 1

Timer mode select 2

TCO

Temperature cutoff

Temperature sense

BAT

Battery voltage

PN2004E01.eps

16-Pin Narrow DIP

or Narrow SOIC

INH

DIS

MOD

VCC

VSS

LED2

LED1

SNS

DCMD

DSEL

VSEL

TM1

TM2

TCO

BAT

SNS

Sense resistor input

LED

Charge status output 1

LED

Charge status output 2

System ground

5.0V

±10% power

MOD

Charge current control

DIS

Discharge control
output

INH

Charge inhibit input

Pin Connections

SLUS063AFEBRUARY 2001 G

Pin Names

Pin Descriptions

DCMD

Discharge-before-charge control input

The DCMD input controls the conditions
that enable discharge-before-charge. DCMD
is pulled up internally.

A negative-going

pulse on DCMD initiates a discharge to end-
of-discharge voltage (EDV) on the BAT pin,
followed by a new charge cycle start. Tying
D C M D t o g r o u n d e n a b l e s a u t o m a t i c
discharge-before-charge on every new charge
cycle start.

DSEL

Display select input

This three-state input configures the charge
status display mode of the LED

and LED

outputs. See Table 2.

VSEL

Voltage termination select input

This three-state input controls the voltage-
termination technique used by the bq2004.
When high, PVD is active.

When floating,

V is used. When pulled low, both PVD and

V are disabled.

Timer mode inputs

and TM

are three-state inputs that

configure the fast charge safety timer, voltage
termination hold-off time, "top-off", and
trickle charge control. See Table 1.

TCO

Temperature cut-off threshold input

Input to set maximum allowable battery
temperature.

If the potential between TS

and SNS is less than the voltage at the TCO
input, then fast charge or top-off charge is ter-
minated.

Temperature sense input

Input, referenced to SNS,

for an external

thermister monitoring battery temperature.

BAT

Battery voltage input

BAT is the battery voltage sense input, refer-
enced to SNS.

This is created by a high-

impedance resistor-divider network con-
nected between the positive and the negative
terminals of the battery.

SNS

Charging current sense input

SNS controls the switching of MOD based on
an external sense resistor in the current
path of the battery. SNS is the reference po-
tential for both the TS and BAT pins.

SNS is connected to V

, then MOD switches

high at the beginning of charge and low at
the end of charge.

LED

Charge status outputs

Push-pull outputs indicating charging
status. See Table 2.

Ground

supply input

5.0V,

±10% power input.

MOD

Charge current control output

MOD is a push-pull output that is used to
control the charging current to the battery.
MOD switches high to enable charging cur-
rent to flow and low to inhibit charging
current flow.

DIS

Discharge control output

Push-pull output used to control an external
transistor to discharge the battery before
charging.

INH

Charge inhibit input

When low, the bq2004 suspends all charge
actions, drives all outputs to high imped-
ance, and assumes a low-power operational
state. When transitioning from low to high,
a new charge cycle is started.

bq2004

Functional Description

Figure 3 shows a block diagram and Figure 4 shows a
state diagram of the bq2004.

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
two-cell potential for the battery under charge.

resistor-divider ratio of:

RB1

RB2

- 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 bat-
tery terminal. See Figure 1.

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

and less than 1M.

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

and V

See

Figure 1. Both the BAT and TS inputs are referenced to
SNS, so the signals used inside the IC are:

BAT

- V

SNS

= V

CELL

and

- V

SNS

= V

TEMP

Discharge-Before-Charge

The DCMD input is used to command discharge-before-
charge via the DIS output.

Once activated, DIS be-

comes active (high) until V

CELL

falls below V

EDV,

which time DIS goes low and a new fast charge cycle be-
gins.

The DCMD input is internally pulled up to V

(its inac-

tive state). Leaving the input unconnected, therefore,
results in disabling discharge-before-charge. A negative
going pulse on DCMD initiates discharge-before-charge
at any time regardless of the current state of the
bq2004.

If DCMD is tied to V

, discharge-before-

charge will be the first step in all newly started charge
cycles.

Starting a Charge Cycle

A new charge cycle (see Figure 2) is started by:

rising above 4.5V

CELL

falling through the maximum cell voltage,

MCV

where:

MCV

= 0.8

± 30mV

A transition on the INH input from low to high.

If DCMD is tied low, a discharge-before-charge is exe-
cuted as the first step of the new charge cycle. Other-
wise, pre-charge qualification testing is the first step.

The battery must be within the configured temperature
and voltage limits before fast charging begins.

The valid battery voltage range is V

EDV

< V

BAT

< V

MCV

where:

EDV

= 0.4

± 30mV

bq2004

Fg2004a.eps

N
T
C

bq2004

VCC

PACK +

PACK -

SNS

RT1

RT2

RB2

RB1

bq2004

Negative Temperature

Coefficient Thermister

PACK+

PACK-

BAT

SNS

Figure 1. Voltage and Temperature Monitoring

The valid temperature range is V

HTF

< V

TEMP

< V

LTF

where:

LTF

= 0.4

± 30mV

HTF

= [(1/4

LTF

) + (3/4

TCO

)]

± 30mV

Note: The low temperature fault (LTF) threshold is not
enforced if

the IC is configured for PVD termination

(VSEL = high).

TCO

is the voltage presented at the TCO input pin, and is

configured by the user with a resistor divider between V

and ground. The allowed range is 0.2 to 0.4

If the temperature of the battery is out of range, or the
voltage is too low, the chip enters the charge pending
state and waits for both conditions to fall within their al-
lowed limits. The MOD output is modulated to provide
the configured trickle charge rate in the charge pending
state.

There is no time limit on the charge pending

state; the charger remains in this state as long as the
voltage or temperature conditons are outside of the al-
lowed limits. If the voltage is too high, the chip goes to
the battery absent state and waits until a new charge
cycle is started.

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

Delta temperature/delta time (

T/t)

Peak voltage detection (PVD)

Negative delta voltage (-

Maximum voltage

Maximum temperature

Maximum time

PVD and - V Termination

The bq2004 samples the voltage at the BAT pin once
every 34s. When -

V termination is selected, if V

CELL

lower than any previously measured value by 12mV
±4mV (6mV/cell), fast charge is terminated. When PVD
termination is selected, if V

CELL

is lower than any previ-

ously measured value by 6mV

±2mV (3mV/cell), fast

charge is terminated. The PVD and -

V tests are valid

in the range 0.4

< V

CELL

< 0.8

Voltage Sampling

Each sample is an average of voltage measurements
taken 57

µs apart. The IC takes 32 measurements in

PVD mode and 16 measurements in -

V mode. The re-

sulting sample periods (9.17ms and 18.18ms, respec-
tively) filter out harmonics centered around 55Hz and
109Hz. This technique minimizes the effect of any AC
line ripple that may feed through the power supply from
either 50Hz or 60Hz AC sources. Tolerance on all tim-
ing is

±16%.

Voltage Termination Hold-off

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

V termination is disabled.

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

T/t, maximum

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

T/ t Termination

The bq2004 samples at the voltage at the TS pin every
34s, and compares it to the value measured two samples
earlier. If V

TEMP

has fallen 16mV

±4mV or more, fast

charge is terminated. If VSEL = high, the

T/t termi-

nation test is valid only when V

TCO

< V

TEMP

< V

TCO

0.2

. Otherwise the

T/t termination test is valid

only when V

TCO

< V

TEMP

< V

LTF

Temperature Sampling

Each sample is an average of 16 voltage measurements
taken 57

µs apart. The resulting sample period

(18.18ms) filters out harmonics around 55Hz. This tech-
nique minimizes the effect of any AC line ripple that
may feed through the power supply from either 50Hz or
60Hz AC sources. Tolerance on all timing is

±16%.

Maximum Voltage, Temperature, and Time

Anytime V

CELL

rises above V

MCV,

the LEDs go off and

charging ceases immediately. If V

CELL

then falls back be-

low V

MCV

before t

MCV

= 1.5s

±0.5s, the chip transitions to

the Charge Complete state (maximum voltage termina-
tion). If V

CELL

remains above V

MCV

at the expiration of

MCV,

the bq2004 transitions to the Battery Absent state

(battery removal). See Figure 4.

Maximum temperature termination occurs anytime
V

TEMP

falls below the temperature cutoff threshold

TCO

Unless PVD termination is enabled (VSEL =

high), charge will also be terminated if V

TEMP

rises

above the low temperature fault threshold, V

LTF

, after

fast charge begins. The V

LTF

threshold is not enforced

when the IC is configured for PVD termination.

Maximum charge time is configured using the TM pin.
Time settings are available for corresponding charge
rates of C/4, C/2, 1C, and 2C. Maximum time-out termi-
nation is enforced on the fast-charge phase, then reset,

bq2004

VSEL Input

Voltage Termination

Low

Disabled

Float

High

PVD

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