Texas Instruments BQ2040SN-C408TR, BQ2040SN-C408 Datasheet

bq2040

Gas Gauge IC With SMBus Interface

Features

Provides accurate measurement

➤

of available charge in NiCd,
NiMH, and Li-Ion batteries

Supports SBS v1.0 data set and

➤

two-wire interface

Monitors charge FET in Li-Ion

➤

pack protection circuit

Designed for battery pack inte

➤

gration

Low operating current

-

Complete circuit can fit on less

-

than ¾ square inch of PCB
space

➤ Supports SBS charge control

commands for NiCd, NiMH, and
Li-Ion

➤ Drives a four-segment LED dis-

play for remaining capacity

General Description

The bq2040 Gas Gauge IC With
SMBus Interface is intended for
battery-pack or in-system installa
tion to maintain an accurate record
of available battery charge. The
bq2040 directly supports capacity
monitoring for NiCd, NiMH, and Li­Ion battery chemistries.

The bq2040 uses the System Man

­agement Bus v1.0 (SMBus) protocol

and supports the Smart Battery
Data (SBData) commands. The
bq2040 also supports the SBData
charge control functions. Battery
state-of-charge, remaining capacity,
remaining time, and chemistry are
available over the serial link.
Battery-charge state can be directly
indicated using a four-segment LED
display to graphically depict battery
full-to-empty in 25% increments.

indication

➤ 16-pin narrow SOIC

Pin Connections Pin Names

The bq2040 estimates battery self­discharge based on an internal
timer and temperature sensor and
user-programmable rate informa
tion stored in external EEPROM.

­The bq2040 also automatically re

calibrates or “learns” battery capac
ity in the full course of a discharge
cycle from full to empty.

The bq2040 may operate directly
from three nickel chemistry cells.

­With the REF output and an exter

nal transistor, a simple, inexpensive
regulator can be built to provide
V

for other battery cell configu

CC

rations.

An external EEPROM is used to
program initial values into the
bq2040 and is necessary for proper
operation.

-

-

-

-

-

V

ESCL

ESDA

LED

LED

LED

LED

V

SLUS005–JUNE 1999 E

1

CC

2

3

4

1

5

2

6

3

7

4

8

SS

16-Pin Narrow SOIC

V

16

V

OUT

15

REF

14

SMBC

13

SMBD

12

PSTAT

11

SB

10

DISP

9

SR

PN204001.eps

ESCL EEPROM clock
ESDA EEPROM data
LED
V
SR Sense resistor input
DISP Display control input

CC

1-4

SS

3.0–6.5V

LED segment 1-4
System ground

SB Battery sense input
PSTAT Protector status input
SMBD SMBus data input/output
SMBC SMBus clock
REF Voltage reference output
V

EEPROM supply output

OUT

1

bq2040

Pin Descriptions

V

CC

ESCL

ESDA Serial memory data and address

LED
LED

V

SS

SR

Supply voltage input

Serial memory clock

Output used to clock the data transfer be
tween the bq2040 and the external non
volatile configuration memory.

Bidirectional pin used to transfer address
and data to and from the bq2040 and the ex
ternal nonvolitile configuration memory.

LED display segment outputs

–

1

4

Each output may drive an external LED.

Ground

Sense resistor input

The voltage drop (V

) across pins SR and

SR

VSSis monitored and integrated over time
to interpret charge and discharge activity.
The SR input is connected to the sense re­sistor and the negative terminal of the
battery. VSR<VSSindicates discharge, and
VSR>VSSindicates charge. The effective
voltage drop, V

, as seen by the bq2040

SRO

is VSR+VOS. (See Table 3.)

DISP

Display control input

DISP high disables the LED display. DISP
floating allows the LED display to be active
during charge if the rate is greater than
100mA. DISP low activates the display for

-

-

SB

4 seconds.

Secondary battery input

Monitors the pack voltage through a high­impedance resistor divider network. The
pack voltage is reported in the SBD register

-

function Voltage (0x09) and is monitored for
end-of-discharge voltage and charging volt
age parameters.

PSTAT

Protector status input

Provides overvoltage status from the Li-Ion
protector circuit and can initiate a charge sus
pend request.

SMBD

SMBus data

Open-drain bidirectional pin used to transfer
address and data to and from the bq2040.

SMBC

SMBus clock

Open-drain bidirectional pin used to clock
the data transfer to and from the bq2040.

REF Reference output for regulator

REF provides a reference output for an op­tional FET-based micro-regulator.

V

OUT

Supply output

Supplies power to the external EEPROM con
figuration memory.

-

-

-

2

bq2040

Functional Description

General Operation

The bq2040 determines battery capacity by monitoring
the amount of charge put into or removed from a re
chargeable battery. The bq2040 measures discharge
and charge currents, estimates self-discharge, and
monitors the battery for low-battery voltage thresholds.
The charge is measured by monitoring the voltage
across a small-value series sense resistor between the
battery's negative terminal and ground. The available
battery charge is determined by monitoring this voltage
over time and correcting the measurement for the envi
ronmental and operating conditions.

V

V

CC

ESCL

SMBC

ESDA

SMBD

LED1
LED2

PS TAT
LED3
LED4
V

SS

bq2040

Figure 1 shows a typical battery pack application of the
bq2040 using the LED capacity display, the serial port,
and an external EEPROM for battery pack program
ming information. The bq2040 must be configured and
calibrated for the battery-specific information to ensure
proper operation. Table 1 outlines the configuration in

­formation that must be programmed in the EEPROM.

An internal temperature sensor eliminates the need
for an external thermistor—reducing cost and compo
nents. An internal, temperature-compensated time­base eliminates the need for an external resonator,
further reducing cost and components. The entire cir
cuit in Figure 1 can occupy less than

­board space.

(Optional)

OUT
REF

SB

DISP

SR

3

square inch of

4

-

-

-

-

No. of Cells

Li-IonNiMH

Chart 1

For bq2040 With No D8

R5 R4 Q1R11

301K BSS138

2

499K

3
4

698K

6

499K

8

698K

9

806K

10

909K

12

909K

Figure 1. Battery Pack Application Diagram—LED Display

604K
806K
604K

499K
806K
499K
604K
909K

100K
100K
100K

100K
100K
100K
100K

86.5K

BSS138
2N7002

BSS138
BSS138
2N7002
2N7002
2N7002

(Optional)

2040LED.eps

3

bq2040

Table 1. Configuration Memory Map

Parameter Name Address Description Length Units

EEPROM length 0x00

EEPROM check1 0x01 EEPROM data integrity check byte, must = 0x5b 8 bits NA

Remaining time alarm 0x02/0x03 Sets RemainingTimeAlarm (0x02) 16 bits minutes

Remaining capacity alarm 0x04/0x05 Sets RemainingCapacityAlarm (0x01) 16 bits mAh

Reserved 0x06/0x07 Reserved for future use 16 bits NA

Initial charging current 0x08/0x09 Sets the initial charging current 16 bits mA

Charging voltage 0x0a/0x0b Sets ChargingVoltage (0x15) 16 bits mV

Battery status 0x0c/0x0d Initializes BatteryStatus (0x16) 16 bits NA

Cycle count 0x0e/0x0f Initializes and stores CycleCount (0x17) 16 bits cycles

Design capacity 0x10/0x11 Sets DesignCapacity (0x18) 16 bits mAh

Design voltage 0x12/0x13 Sets DesignVoltage (0x19) 16 bits mV

Specification information 0x14/0x15 Programs SpecificationInfo (0x1a) 16 bits NA

Manufacture date 0x16/0x17 Programs ManufactureDate (0x1b) 16 bits NA

Serial number 0x18/0x19 Programs SerialNumber (0x1c) 16 bits NA

Fast-charging current 0x1a/0x1b Sets ChargingCurrent (0x14) 16 bits mA

Maintenance-charge current 0x1c/0x1d Sets the trickle current request 16 bits mA

Reserved 0x1e/0x1f Reserved must = 0x0000 16 bits mAh

Manufacturer name 0x20-0x2b Programs ManufacturerName (0x20) 96 bits NA

Current overload 0x2c/0x2d Sets the overload current threshold 16 bits mA

Battery low % 0x2e Sets the battery low amount 8 bits %

Reserved 0x2f Reserved for future use 8 bits NA

Device name 0x30-0x37 Programs DeviceName (0x21) 64 bits NA

Li-Ion taper current 0x38/0x39

Maximum overcharge limit 0x3a/0x3b Sets the maximum amount of overcharge 16 bits NA

Reserved 0x3c Reserved must = 0x00 8 bits NA

Access protect 0x3d Locks commands outside of the SBS data set 8 bits NA

FLAGS1 0x3e Initializes FLAGS1 8 bits NA

FLAGS2 0x3f Initializes FLAGS2 8 bits NA

Device chemistry 0x40-0x45 Programs DeviceChemistry (0x22) 48 bits NA

Current measurement gain 0x46/0x47 Sense resistor calibration value 16 bits NA

Battery voltage offset 0x48 Voltage calibration value 8 bits NA

Temperature offset 0x49 Temperature calibration value 8 bits NA

Maximum temperature and
∆T step

0x4a

Number of EEPROM data locations
must = 0x64

Sets the upper limit of the taper current for charge
termination

Sets the maximum charge temperature and the ∆T
step for ∆T/∆t termination

8 bits NA

16 bits mA

8 bits NA

4

bq2040

Table 1. Configuration Memory Map (Continued)

Parameter Name Address Description Length Units

Charge efficiency 0x4b Sets the high/low charge rate efficiencies 8 bits NA

Full charge percentage 0x4c

Digitial filter 0x4d Sets the minimum charge/discharge threshold 8 bits NA

Current integration gain 0x4e

Self-discharge rate 0x4f Sets the battery’s self-discharge rate 8 bits NA

Manufacturer data 0x50-0x55 Programs ManufacturerData (0x23) 48 bits NA

Voltage gain1 0x56/0x57 Battery divider calibration value 16 bits NA

Reserved 0x58-0x59 Reserved 16 bits NA

EDVF charging current 0x5a/0x5b

End of discharge voltage1 0x5c/0x5d Sets EDV1 16 bits NA

End of discharge voltage final 0x5e/0x5f Sets EDVF 16 bits NA

Full-charge capacity 0x60/0x61 Initializes and stores FullChargeCapacity (0x10) 16 bits mAh
∆t step

Hold-off time 0x63

EEPROM check 2 0x64

Reserved 0x65-0x7f Reserved for future use NA

0x62

Sets the percent at which the battery is consid
ered fully charged

Programs the current integration gain to the
sense resistor value

Sets the charge current request when the battery
voltage is less than EDVF

Sets the ∆t step for ∆T/∆t termination
Sets ∆T/∆t hold-off timer

EEPROM data integrity check byte
must = 0xb5

­8 bits NA

8 bits NA

16 bits NA

8 bits NA

8 bits NA

8 bits NA

5

bq2040

Voltage Thresholds

In conjunction with monitoring VSRfor charge/discharge
currents, the bq2040 monitors the battery potential
through the SB pin. The voltage potential is deter
mined through a resistor-divider network per the fol
lowing equation:

R

MBV

5

R

2.25

4

where MBV is the maximum battery voltage, R

1=−

is con

5

nected to the positive battery terminal, and R4is con
nected to the negative battery terminal. R5/R4should be
rounded to the next higher integer. The voltage at the
SB pin (VSB) should never exceed 2.4V.

The battery voltage is monitored for the end-of­discharge voltages (EDV1 and EDVF) and for alarm
warning conditions. EDV threshold levels are used to de
termine when the battery has reached a programmable
“empty” state. The bq2040 generates an alarm warning
when the battery voltage exceeds the maximum charg­ing voltage by 5% or if the voltage is below EDVF. The
battery voltage gain, the two EDV thresholds, and the
charging voltage are programmable in the EEPROM.

is below either of the two EDV thresholds, the associ-

If V

SB

ated flag is latched and remains latched, independent of
VSB, until the next valid charge.

EDV monitoring may be disabled under certain condi­tions. If the discharge current is greater than the value
stored in location 0x2c and 0x2d in the EEPROM (EE
0x2c/0x2d), EDV monitoring is disabled and resumes af­ter the current falls below the programmed value.

Reset

The bq2040 is reset when first connected to the battery
pack. On power-up, the bq2040 initializes and reads the
EEPROM configuration memory. The bq2040 can also
be reset with a command over the SMBus. The software
reset sequence is the following: (1) write MaxError
(0x0c) to 0x0000; (2) write the reset register (0x64) to
0x8009. A software reset can only be performed if the
bq2040 is in an unlocked state as defined by the value in
location 0x3d of the EEPROM (EE 0x3d) on power-up.

Temperature

The bq2040 monitors temperature sensing using an in
ternal sensor. The temperature is used to adapt charge
and self-discharge compensations as well as to monitor
for maximum temperature and∆T/∆t during a bq2040
controlled charge. Temperature may also be accessed
over the SMBus with command 0x08.

Layout Considerations

The bq2040 measures the voltage differential between
the SR and VSSpins. VOS(the offset voltage at the SR

­pin) is greatly affected by PC board layout. For optimal

­results, the PC board layout should follow the strict rule

of a single-point ground return. Sharing high-current
ground with small signal ground causes undesirable
noise on the small signal nodes. Additionally, in refer
ence to Figure 1:

-

The capacitors (C1 and C2) should be placed as close as

n

-

possible to the SB and VCCpins, and their paths to V
should be as short as possible. A high-quality ceramic
capacitor of 0.1µf is recommended for VCC.

The sense resistor capacitor (C3) should be placed as

n

close as possible to the SR pin.

-

The bq2040 should be in thermal contact with the

n

cells for optimum temperature measurement.

Gas Gauge Operation

The operational overview diagram in Figure 2 illus­trates the operation of the bq2040. The bq2040 accumu­lates a measure of charge and discharge currents, as
well as an estimation of self-discharge. Charge currents
are compensated for temperature and state-of-charge of
the battery. Self-discharge is temperature-compensated.

The main counter, RemainingCapacity (RM), represents
the available battery capacity at any given time. Battery
charging increments the RM register, whereas battery dis­charging and self-discharge decrement the RM register
and increment the internal Discharge Count Register
(DCR).

The Discharge Count Register is used to update the
FullChargeCapacity (FCC) register only if a complete
battery discharge from full to empty occurs without any
partial battery charges. Therefore, the bq2040 adapts
its capacity determination based on the actual condi
tions of discharge.

The battery's initial full capacity is set to the value stored
in EE 0x60-0x61. Until FCC is updated, RM counts up to,
but not beyond, this threshold during subsequent charges.

The battery’s empty state is also programmed in the
EEPROM. The battery low percentage (EE 0x2e) stores

­the percentage of FCC that will be written to RM when
the battery voltage drops below the EDV1 threshold.

1. FullChargeCapacity or learned-battery

capacity:

FCC is the last measured discharge capacity of the
battery. On initialization (application of V
FCC is set to the value stored in the EEPROM. Dur

CC

-

SS

-

or reset),

-

6

bq2040

Inputs

Main Counters

and Capacity

Reference (FCC)

Outputs

Charge
Current

State-of-charge

Temperature

Compensation

Figure 2. Operational Overview

ing subsequent discharges, FCC is updated with the
latest measured capacity in the Discharge Count Reg­ister plus the battery low amount, representing a dis­charge from full to below EDV1. A qualified dis­charge is necessary for a capacity transfer from the
DCR to the FCC register. Once updated, the bq2040
writes the new FCC to the EEPROM. The FCC also
serves as the 100% reference threshold used by the
relative state-of-charge calculation and display.

2. DesignCapacity (DC):

The DC is the user-specified battery capacity and is
programmed from external EEPROM. The DC also
provides the 100% reference for the absolute dis
play mode.

3. RemainingCapacity (RM):

RM counts up during charge to a maximum value of
FCC and down during discharge and self-discharge to

0. RM is set to the battery low amount after the
EDV1 threshold has been reached. If RM is already
equal to or less than the battery low amount, RM is
not modified. If RM reaches the battery low amount
before the battery voltage falls below EDV1 on dis
charge, RM stops counting down until the EDV1
threshold is reached. RM is set to 0 when the battery
voltage reaches EDVF. To prevent overstatement of
charge during periods of overcharge, RM stops in
crementing when RM = FCC. RM may optionally
be written to a user-defined value when fully
charged if the battery pack is under bq2040 charge
control. On initialization,RM is set to 0.

and

Remaining

+

Chip-Controlled

Available Charge

LED Display

Capacity

(RM)

Discharge

Current

--

<

Serial Interface

Full

Charge

Capacity

(FCC)

Two-Wire

Self-Discharge

Timer

Temperature

Compensation

+

Discharge

Count

Qualified

Register

Transfer

(DCR)

Temperature, Other Data

FG294501.eps

4. Discharge Count Register (DCR):

The DCR counts up during discharge independent
of RM and can continue increasing after RM has
decremented to 0. Prior to RM = 0, both discharge
and self-discharge increment the DCR. After RM
= 0, only discharge increments the DCR. The DCR
resets to 0 when RM = FCC and stops counting at
EDV1 on discharge. The DCR does not roll over but
stops counting when it reaches FFFFh.

FCC is updated on the first charge after a qualified
discharge to EDV1. The updated FCC equals the
battery low percentage times the current FCC plus

-

the DCR value. A qualified discharge to EDV1 oc
curs if all of the following conditions exist:

n

No valid charge initiations (charges greater than
10mAh, where V
the period between RM = FCC and EDV1 de
tected.

n

The self-discharge count is not more than
256mAh.

n

-

The low temperature fault bit in FLAGS2 is not
set when the EDV1 level is reached during dis
charge.

n

-

Battery voltage is not more than 256mV below
the EDV1 threshold when EDV1 is set.

The valid discharge flag (VDQ) in FLAGS1 indi
cates whether the present discharge is valid for an
FCC update. FCC cannot be reduced by more than
256mAh during any single cycle.

+

SRO

>+V

occurred during

SRD

-

-

-

-

7

bq2040

Charge Counting

Charge activity is detected based on a positive voltage
on the SR input. If charge activity is detected, the
bq2040 increments RM at a rate proportional to V
and, if enabled, activates an LED display. Charge ac
tions increment the RM after compensation for charge
state and temperature.

The bq2040 determines charge activity sustained at a
continuous rate equivalent to V

charge equates to sustained charge activity
greater than 10 mAh. Once a valid charge is detected,

charge threshold counting continues until V
low V
scribed in the Digital Magnitude Filter section.

SRD

.V

SRD

is a programmable threshold as de

SRO

>+V

SRD

SRO

SRO

. A valid

falls be

Discharge Counting

All discharge counts where V
register to decrement and the DCR to increment. V
is a programmable threshold as described in the Digital
Magnitude Filter section.

SRO

<-V

cause the RM

SRD

SRD

Self-Discharge Estimation

The bq2040 continuously decrements RM and incre­ments DCR for self-discharge based on time and temper­ature provided that the discharge flag in BatteryStatus
is set (charge not detected). The bq2040 self-discharge
estimation rate is programmed in EE 0x4f and can be
set from 0 to 25% per day for 20–30°C. This rate approx­imately doubles for every 10°C increase until the tem­perature is ≥ 70°C or halves every 10°C decrease until
the temperature is < 10°C.

Charge Control

The bq2040 supports SBS charge control by broadcast
ing the ChargingCurrent and the ChargingVoltage to
the Smart Charger address. The bq2040 broadcasts
charging commands every 10 seconds; the broadcasts
can be disabled by writing bit 14 of BatteryMode to 1.
On reset, the initial charging current broadcast to the
charger is set to the value programmed in EE 0x08­0x09. The bq2040 updates the value used in the charg
ing current broadcasts based on the battery’s state of
charge, voltage, and temperature.

The bq2040 internal charge control is compatible with
nickel-based and Li-Ion chemistries. The bq2040 uses
current taper detection for Li-Ion primary charge termi
nation and ∆T/∆t for nickel based primary charge termi
nation. The bq2040 also provides a number of safety
terminations based on battery capacity, voltage, and
temperature.

Current Taper

For Li-Ion charge control, the ChargingVoltage must be
set to the desired pack voltage during the constant volt
age charge phase. The bq2040 detects a current taper
termination when it measures the pack voltage to be

­within 128mV of the requested charging voltage and
when the AverageCurrent is less than the programmed
threshold in EE 0x38—0x39 and non-zero for at least
100s.

∆T/∆t

-

The ∆T/∆t used by the bq2040 is programmable in both

-

the temperature step (1.6°C–4.6°C) and time step (20
seconds–320seconds). Typical settings for 1°C/min in
clude 2°C over 120 seconds and 3°C over 180 seconds.
Longer times are required for increased slope resolution.

∆

T

is set by the formula:

∆

t

[]

(lower nibble of EE 0x4a) 2 + 16 / 10

[(

In addition to the ∆T/∆t timer, there is a hold-off timer,
which starts when the battery is being charged at more
than 255mA and the temperature is above 25°C. Until
this timer expires, ∆T/∆t is suspended. If the tempera­ture falls below 25°C, or if charging current falls below
255mA, the timer is reset and restarts only if these con­ditions are once again within range. The hold-off time is
programmed in EE 0x63.

EE 0x62

−320

∆

T

∆

=

t

) 20)]

∗

Charge Termination

Once the bq2040 detects a valid charge termination, the
Fully_Charged, Terminate_Charge_Alarm, and the

­Over_Charged_Alarm bits are set in BatteryStatus, and

the requested charge current is set to zero. Once the
terminating conditions cease, the Termi
nate_Charge_Alarm and the Over_Charged_Alarm are
cleared, and the requested charging current is set to the
maintenance rate. The bq2040 requests the mainte
nance rate until RM falls below the amount determined

­by the programmable full- charge percentage. Once this

occurs, the Fully_Charged bit is cleared, and the re
quested charge current and voltage are set to the
fast-charge rate.

Bit 4 (CC) in FLAGS2 determines whether RM is modi

-

fied after a ∆T/∆t or current taper termination occurs. If

­CC = 1, RM may be set from 0 to 100% of the FullChar

geCapacity as defined in EE 0x4c. If RM is below the
full-charge percentage, RM is set to the full-charge per
centage of FCC. If RM is above the full-charge percent
age, RM is not modified.

-

-

o

C









s∗





-

-

-

-

-

-

-

8

bq2040

Charge Suspension

The bq2040 may temporarily suspend charge if it detects
a charging fault. The charging faults include the follow
ing conditions:

Maximum Overcharge: If charging continues for

n

more than the programmed maximum overcharge
limit as defined in EE 0x3a—0x3b beyond RM=FCC,
the Fully_Charged bit is set, and the requested
charging current is set to the maintenance rate.

Overvoltage: An over-voltage fault exists when the

n

bq2040 measures a voltage more than 5% above the
ChargingVoltage. When the bq2040 detects an
overvoltage condition, the requested charge current is
set to 0 and the Terminate_Charge_Alarm bit is set
in BatteryStatus. The alarm bit is cleared when the
current drops below 256mA and the voltage is less
than 105% of ChargingVoltage.

Overcurrent: An overcurrent fault exists when the

n

bq2040 measures a charge current more than 25%
above the ChargingCurrent. If the ChargingCurrent
is less than 1024mA, an overcurrent fault exists if the
charge current is more than 1mA above the lowest
multiple of 256mA that exceeds the ChargingCurrent.
When the bq2040 detects an overcurrent condition, the
requested charge current is set to 0 and the
Terminate_Charge_Alarm bit is set in Battery Status.
The alarm bit is cleared when the current drops below
256mA.

n

Maximum Temperature: When the battery
temperature equals the programmed maximum
temperature, the requested charge current is set to
zero and the Over_Temp_Alarm and the
Terminate_Charge_Alarm bits are set in Battery
Status. The Over_Temp_Alarm bit is cleared when
the temperature drops to 43°C below the maximum
temperature threshold minus 5°C.

n

PSTAT: When the PSTAT input is ≥1.5V, the
requested charge current is set to 0 and the
Terminate_Charge_Alarm bit is set in BatteryStatus
if the Discharging flag is not set. The alarm bit is
cleared when the PSTAT input is <1.0V or the
Discharging flag is set.

n

Low Temperature: When the battery temperature
is less than 12°C (LTF bit in FLAGS2 set), the
requested charge current is set to the maintenance
rate. Once the temperature is above 15°C, the
requested charge current is set to the fast rate.

n

Undervoltage: When the battery voltage is below
the EDVF threshold, the requested charge current is
set to the EDVF rate stored in EE0x5a/0x5b. Once
the voltage is above EDVF, the requested charge
current is set to the fast or maintenance rate
depending on the state of the LTF bit.

Count Compensations

Charge activity is compensated for temperature and
state-of-charge before updating the RM and/or DCR.

­Self-discharge estimation is compensated for tempera
ture before updating RM or DCR.

Charge Compensation

Charge efficiency is compensated for state-of-charge,
temperature, and battery chemistry. The charge effi
ciency is adjusted using the following equations:

1.)

RM RM * Q Q

=−()

EFC ET

where RelativeStateOfCharge < FullChargePercentage,
and
ing from 0.75 to 1.0.

is the programmed fast-charge efficiency vary

Q

EFC

2.)

RM RM Q Q

=−*( )

ETC ET

where RelativeStateOfCharge ≥ FullChargePercentage
and
charge efficiency varying from 0.75 to 1.0.

Q

ET

temperature increases according to the following:

QifT

QCTC

QTC

Q

ET

is the programmed maintenance (trickle)

Q

ETC

is used to adjust the charge efficiency as the battery

=<030

ET

=°≤<°002 30 40.if

ET

=≥°005 40.if

ET

°C

is 0 over the entire temperature range for Li-Ion.

Digital Magnitude Filter

The bq2040 has a programmable digital filter to elimi
nate charge and discharge counting below a set
threshold, V

. Table 2 shows typical digital filter

SRD

settings. The proper digital filter setting can be calcu
lated using the following equation.

DMF =

45

SRD

V

Table 2. Typical Digital Filter Settings

DMF DMF Hex. V

75 4B

100 64 0.45

150 96 0.30

175 AF 0.26

200 C8 0.23

SRD

(mV)

0.60

-

-

-

-

-

9

bq2040

Table 3. bq2040 Current-Sensing Errors

Symbol Parameter Typical Maximum Units Notes

V

OS

INL

INR

Offset referred to V

Integrated non-linearity
error

Integrated non­repeatability error

SR

75

±

1

±

0.5

±

150

±

4

±

1

±

V DISP

µ

%

%

= VCC.

Add 0.1% per °C above or below 25°C
and 1% per volt above or below 4.25V.

Measurement repeatability given
similar operating conditions.

Error Summary

Capacity Inaccurate

The FCC is susceptible to error on initialization or if no
updates occur. On initialization, the FCC value includes
the error between the design capacity and the actual ca
pacity. This error is present until a qualified discharge
occurs and FCC is updated (see the DCR description).
The other cause of FCC error is battery wear-out. As the
battery ages, the measured capacity must be adjusted to
account for changes in actual battery capacity. Periodic
qualified discharges from full to empty will minimize er­rors in FCC.

Current-Sensing Error

Table 3 illustrates the current-sensing error as a func­tion of VSR. A digital filter eliminates charge and dis­charge counts to the RM register when -V
+V

.

SRD

Display

The bq2040 can directly display capacity information us­ing low-power LEDs. The bq2040 displays the battery
charge state in either absolute or relative mode. In rela
tive mode, the battery charge is represented as a per
centage of the FCC. Each LED segment represents 25%
of the FCC.

In absolute mode, each segment represents a fixed
amount of charge, 25% of the DesignCapacity. As the
battery wears out over time, it is possible for the FCC to
be below the design capacity. In this case, all of the
LEDs may not turn on in absolute mode, representing
the reduction in the actual battery capacity.

When DISP is tied to VCC, the LED
tive. When DISP is left floating, the display becomes ac
tive whenever the bq2040 detects a charge rate of
100mA or more. When pulled low, the segment outputs
become active immediately for a period of approximately
4 seconds. The DISP pin must be returned to float or
VCCto reactivate the display.

LED1blinks at a 4Hz rate indicating a low battery con
dition whenever the display is active, EDVF is not set,

SRD<VSRO

outputs are inac

1-4

and Remaining_Capacity_Alarm is set. V
(EDVF = 1) disables the display output.

below EDVF

SB

Microregulator

The bq2040 can operate directly from three nickel chem

-

istry cells. To facilitate the power supply requirements
of the bq2040, an REF output is provided to regulate an
external low-threshold n-FET. A micropower source for
the bq2040 can be built inexpensively using a 2N7002 or
BSS138 FET and an external resistor. (See Figure 1.)
The value of R11 depends on the battery pack’s nominal
voltage.

Communicating With the bq2040

The bq2040 includes a simple two-pin (SMBC and
SMBD) bi-directional serial data interface. A host proc-

<

essor uses the interface to access various bq2040 regis­ters; see Table 4. This method allows battery character­istics to be monitored easily. The open-drain SMBD and
SMBC pins on the bq2040 are pulled up by the host sys­tem, or may be connected to VSS, if the serial interface is
not used.

-

The interface uses a command-based protocol, where the

-

host processor sends the battery address and an eight­bit command byte to the bq2040. The command directs
the bq2040 to either store the next data received to a
register specified by the command byte or output the
data specified by the command byte.

bq2040 Data Protocols

The host system, acting in the role of a Bus master, uses
the read word and write word protocols to communicate

­integer data with the bq2040. (See Figure 3).

-

Host-to-bq2040 Message Protocol

The Bus Host communicates with the bq2040 using one
of three protocols:

n

Read word

-

n

Write word

-

10