TEXAS INSTRUMENTS bq2012 Technical data

bq2012

Gas Gauge IC With

Slow-Charge Control

Features

Conservative and repeatable

➤

measurement of available charge
in rechargeable batteries

Charge control output

➤

Designed for battery pack inte

➤

gration

120µA typical standby current

-

(self-discharge estimation mode)

Small size enables imple-

-

mentations in as little as
square inch of PCB

➤ Integrate within a system or as a

stand-alone device

Display capacity via single-

-

wire serial communication
port or direct drive of LEDs

➤ Measurements compensated for

current and temperature

➤ Self-discharge compensation us-

ing internal temperature sensor

➤

16-pin narrow SOIC

1

2

General Description

The bq2012 Gas Gauge IC is in
tended for battery-pack or in-system
installation to maintain an accurate
record of available battery charge.
The IC monitors a voltage drop
across a sense resistor connected in

­series between the negative battery

terminal and ground to determine
charge and discharge activity of the
battery.

Self-discharge of NiMH and NiCd
batteries is estimated based on an
internal timer and temperature sen
sor. Compensations for battery tem
perature and rate of charge or dis­charge are applied to the charge,
discharge, and self-discharge calcu­lations to provide available charge
information across a wide range of
operating conditions. Battery capac­ity is automatically recalibrated, or
“learned,” in the course of a dis­charge cycle from full to empty.

The bq2012 includes a charge con­trol output that, when used with
other full-charge safety termination
methods, can provide a cost-effective

means of controlling charge based
on the battery's charge state.

-

Nominal available charge may be di
rectly indicated using a five- or six­segment LED display. These seg
ments are used to graphically indi
cate nominal available charge.

The bq2012 supports a simple
single-line bidirectional serial link to
an external processor (common
ground). The bq2012 outputs battery
information in response to external
commands over the serial link.

Internal registers include available

­charge, temperature, capacity, battery

­ID, battery status, and programming

pin settings. To support subassembly
testing, the outputs may also be con­trolled. The external processor may
also overwrite some of the bq2012 gas
gauge data registers.

The bq2012 may operate directly
from three or four cells. With the
REF output and an external transis­tor, a simple, inexpensive regulator
can be built to provide V

CC

greater number of cells.

-

-

-

across a

Pin Connections Pin Names

LCOM LED common output

SEG

/PROG1LED segment 1/

SEG

SEG

SEG

SEG

SEG

1

2

3

4

5

6

program 1 input

/PROG2LED segment 2/

program 2 input

/PROG3LED segment 3/

program 3 input

/PROG4LED segment 4/

program 4 input

/PROG5LED segment 5/

program 5 input

/PROG6LED segment 6/

program 6 input

9/96 B

LCOM

SEG1/PROG

SEG2/PROG

SEG3/PROG

SEG4/PROG

SEG5/PROG

SEG6/PROG

1

2

3

4

5

6

V

SS

16-Pin Narrow SOIC

16

15

14

13

12

11

10

9

PN201201.eps

V

CC

REF

CHG

DQ

EMPTY

SB

DISP

SR

1

2

3

4

5

6

7

8

REF Voltage reference output

CHG

Charge control output

DQ Serial communications

input/output

EMPTY Empty battery indicator

output

SB Battery sense input

DISP

Display control input

SR Sense resistor input

V

CC

V

SS

3.0–6.5V

System ground

1

bq2012

Pin Descriptions

LCOM

SEG
SEG

PROG
PROG

PROG
PROG

PROG

PROG

CHG

LED common output

Open-drain output switches V
current for the LEDs. The switch is off dur
ing initialization to allow reading of the soft
pull-up or pull-down program resistors.
LCOM is also high impedance when the dis
play is off.

LED display segment outputs (dual func

–

1

tion with PROG

6

Each output may activate an LED to sink
the current sourced from LCOM.

Programmed full count selection inputs

–

1

(dual function with SEG

2

These three-level input pins define the pro
grammed full count (PFC) thresholds de
scribed in Table 2.

Gas gauge rate selection inputs (dual

–

3

function with SEG

4

These three-level input pins define the scale
factor described in Table 2.

Self-discharge rate selection (dual func-

5

tion with SEG

This three-level input pin defines the selfdis­charge compensation rate shown in Table 1.

Display mode selection (dual function

6

with SEG

)

6

This three-level pin defines the display op
eration shown in Table 1.

Charge control output

This open-drain output becomes active low
when charging is allowed. Valid charging
conditions are described in the Charge Con
trol section.

–PROG6)

1

)

5

–SEG4)

3

–SEG2)

1

to source

CC

-

-

-

-

-

-

-

SR

DISP

SB

EMPTY

DQ

REF

V

CC

V

SS

Sense resistor input

The voltage drop (V
sistor R

is monitored and integrated over

S

) across the sense re

SR

time to interpret charge and discharge activ
ity. The SR input is tied to the high side of
the sense resistor. V
charge, and V

SR>VSS

SR<VSS

indicates charge. The
effective voltage drop (V
bq2012 is V

SR+VOS

(see Table 5).

indicates dis

) as seen by the

SRO

Display control input

high disables the LED display. DISP

DISP
tied to VCCallows PROGXto connect directly
to V

or VSSinstead of through a pull-up or

CC

pull-down resistor. DISP

floating allows the
LED display to be active during a valid
charge or during discharge if the NAC regis
ter is updated at a rate equivalent to V

-4mV. DISP

low activates the display. See

SRO

Table 1.

Secondary battery input

This input monitors the single-cell voltage
potential through a high-impedance resis­tive divider network for end-of-discharge
voltage (EDV) thresholds, maximum charge
voltage (MCV), and battery removed.

Battery empty output

This open-drain output becomes high­impedance on detection of a valid end-of­discharge voltage (V

) and is low following

EDVF

the next application of a valid charge.

Serial I/O pin

This is an open-drain bidirectional pin.

Voltage reference output for regulator

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

Supply voltage input

Ground

-

-

-

-

≤

2

bq2012

Functional Description

General Operation

The bq2012 determines battery capacity by monitoring
the amount of charge input to or removed from a re
chargeable battery. The bq2012 measures discharge and
charge currents, estimates self-discharge, monitors the
battery for low-battery voltage thresholds, and compen
sates for temperature and charge/discharge rates. The
charge measurement is made by monitoring the voltage
across a small-value series sense resistor between the
negative battery 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.

bq2012

Gas Gauge IC

LCOM

SEG1/PROG

SEG2/PROG

SEG3/PROG

SEG4/PROG

SEG5/PROG

SEG6/PROG

CHG

REF

V

CC

SB

1

2

DISP

3

4

SR

5

V

SS

6

EMPTY

DQ

Figure 1 shows a typical battery pack application of the
bq2012 using the LED display capability as a charge­state indicator. The bq2012 can be configured to display
capacity in either a relative or an absolute display mode.
The relative display mode uses the last measured dis
charge capacity of the battery as the battery “full” refer
ence. The absolute display mode uses the programmed

­full count (PFC) as the full reference, forcing each seg

ment of the display to represent a fixed amount of
charge. A push-button display feature is available for

­momentarily enabling the LED display.

The bq2012 monitors the charge and discharge currents
as a voltage across a sense resistor (see R

in Figure 1).

S

A filter between the negative battery terminal and the
SR pin may be required if the rate of change of the bat

-

tery current is too great.

R

1

1M

Q1
ZVNL110A

C1

0.1 F
V

V

CC

CC

RB

RB

1

2

R

S

-

-

-

-

Indicates optional.

Directly connect to VCC across 3 or 4 cells (3 to 4.8V nominal,should not
exceed 6.5V) with a resistor and a Zener diode to limit voltage during charge.
Otherwise, R1, C1, and Q1 are needed for regulation of >4 cells.
The value of R1 depends on the number of cells.

Programming resistors (6 max.) and ESD-protection diodes are not shown.

R-C on SR may be required (application-specific), where the R
should not exceed 100k.

Figure 1. Battery Pack Application Diagram—LED Display

3

Charger

Load

FG201201.eps

bq2012

Voltage Thresholds

In conjunction with monitoring VSRfor charge/discharge
currents, the bq2012 monitors the single-cell battery po
tential through the SB pin. The single-cell voltage po
tential is determined through a resistor/divider network
per the following equation:

RB

1

N

RB

2

where N is the number of cells, RB
positive battery terminal, and RB

1=−

is connected to the

1

is connected to the

2

negative battery terminal. The single-cell battery volt
age is monitored for the end-of-discharge voltage (EDV)
and for maximum cell voltage (MCV). EDV threshold
levels are used to determine when the battery has
reached an “empty” state, and the MCV threshold is used
for fault detection during charging.

Two EDV thresholds for the bq2012 are fixed at:

EDV1 (early warning) = 1.05V

EDVF (empty) = 0.95V

If V

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

SB

sociated flag is latched and remains latched, independ­ent of V

During discharge and charge, the bq2012 monitors V

, until the next valid charge.

SB

SR

for various thresholds. These thresholds are used to
compensate the charge and discharge rates. Refer to the
count compensation section for details. EDV monitoring
is disabled if V
ond after V

SR

-250mV typical and resumes

≤

SR

> -250mV.

1

sec-

2

EMPTY Output

The EMPTY output switches to high impedance when
V

SB<VEDF

curs. The bq2012 also monitors V

2.25V. V

and remains latched until a valid charge oc

relative to V

falling from above V

SB

SB

resets the device.

MCV

MCV

Reset

The bq2012 recognizes a valid battery whenever VSBis
greater than 0.1V typical. V

rising from below 0.25V

SB

or falling from above 2.25V resets the device. Reset can
also be accomplished with a command over the serial
port as described in the Register Reset section.

Temperature

The bq2012 internally determines the temperature in
10°C steps centered from -35°C to +85°C. The tempera
ture steps are used to adapt charge and discharge rate
compensations, self-discharge counting, and available
charge display translation. The temperature range is

available over the serial port in 10°C increments as
shown below:

-

TMPGG (hex) Temperature Range

-

0x < -30°C

1x -30°C to -20°C

2x -20°C to -10°C

-

3x -10°C to 0°C

4x 0°C to 10°C

5x 10°C to 20°C

6x 20°C to 30°C

7x 30°C to 40°C

8x 40°C to 50°C

9x 50°C to 60°C

Ax 60°C to 70°C

Bx 70°C to 80°C

Cx > 80°C

Layout Considerations

The bq2012 measures the voltage differential between
the SR and V

­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:

■

The capacitors (SB and VCC) should be placed as
close as possible to the SB and V
and their paths to V
A high-quality ceramic capacitor of 0.1µf is
recommended for V

■

The sense resistor (RS) should be as close as possible
to the bq2012.

-

■

The R-C on the SR pin should be located as close as
possible to the SR pin. The maximum R should not
exceed 100K.

pins. VOS(the offset voltage at the SR

SS

pins, respectively,

should be as short as possible.

SS

.

CC

CC

4

bq2012

Gas Gauge Operation

The operational overview diagram in Figure 2 illustrates
the operation of the bq2012. The bq2012 accumulates a
measure of charge and discharge currents, as well as an
estimation of self-discharge. Charge and discharge cur
rents are temperature and rate compensated, whereas
self-discharge is only temperature compensated.

The main counter, Nominal Available Charge (NAC),
represents the available battery capacity at any given
time. Battery charging increments the NAC register,
while battery discharging and self-discharge decrement
the NAC register and increment the DCR (Discharge
Count Register).

The Discharge Count Register (DCR) is used to update
the Last Measured Discharge (LMD) register only if a
complete battery discharge from full to empty occurs
without any partial battery charges. Therefore, the
bq2012 adapts its capacity determination based on the
actual conditions of discharge.

The battery’s initial capacity is equal to the programmed
full count (PFC) shown in Table 2. Until LMD is up­dated, NAC counts up to but not beyond this threshold
during subsequent charges. This approach allows the
gas gauge to be charger-independent and compatible
with any type of charge regime.

1. Last Measured Discharge (LMD) or learned

battery capacity:

LMD is the last measured discharge capacity of the
battery. On initialization (application of V

-

tery replacement), LMD = PFC. During subsequent
discharges, the LMD is updated with the latest
measured capacity in the Discharge Count Register
(DCR) representing a discharge from full to below
EDV1. A qualified discharge is necessary for a ca
pacity transfer from the DCR to the LMD register.
The LMD also serves as the 100% reference thresh
old used by the relative display mode.

2. Programmed Full Count (PFC) or initial bat

tery capacity:

The initial LMD and gas gauge rate values are pro
grammed by using PROG
provides the 100% reference for the absolute dis

–PROG4. The PFC also

1

play mode. The bq2012 is configured for a given ap
plication by selecting a PFC value from Table 2.
The correct PFC may be determined by multiplying
the rated battery capacity in mAh by the sense re
sistor value:

Battery capacity (mAh)*sense resistor (Ω) =

PFC (mVh)

Selecting a PFC slightly less than the rated capac­ity for absolute mode provides capacity above the
full reference for much of the battery’s life.

CC

or bat

-

-

-

-

-

-

-

-

Inputs

Main Counters

and Capacity

Reference (LMD)

Outputs

Charge
Current

Rate and

Rate and

Temperature

Temperature

Compensation

Compensation

+

Available Charge

Nominal

Available

Charge

(NAC)

Temperature

Translation

Chip-Controlled

LED Display

Discharge

Current

Rate and

Temperature

Compensation

--

<

Discharged

Last

Measured

(LMD)

Serial

Port

Qualified
Transfer

Temperature Step,
Other Data

Figure 2. Operational Overview

5

Self-Discharge

Timer

Temperature

Compensation

+

+

Discharge

Count

Register

(DCR)

FG201002.eps

bq2012

Example: Selecting a PFC Value

Given:

Sense resistor = 0.1

Ω

Number of cells = 6
Capacity = 2200mAh, NiCd battery
Current range = 50mA to 2A
Absolute display mode
Serial port only
Self-discharge =
Voltage drop over sense resistor = 5mV to 200mV

C

64

Therefore:

Select:

PFC = 33792 counts or 211mVh
PROG

= float

1

PROG

= float

2

PROG

= float

3

PROG

= low

4

PROG

= float

5

PROG

= float

6

The initial full battery capacity is 211mVh
(2110mAh) until the bq2012 “learns” a new capac
ity with a qualified discharge from full to EDV1.

2200mAh*0.1Ω= 220mVh

Table 1. bq2012 Programming

Pin

Connection

H Self-discharge disabled NAC = PFC on reset LED disabled

Z

L

Note: PROG5and PROG6states are independent.

PROG

5

Self-Discharge Rate

NAC

64

NAC

47

PROG

6

Display Mode

Absolute

LED enabled on discharge when

V

< -4mV or during a valid charge

SRO

Relative LED on

Table 2. bq2012 Programmed Full Count mVh Selections

-

DISP

Display State

Programmed

PROG

1 2 PROG

-- -

Full

x

Count

(PFC)

Scale =

1/80

PROG

= L PROG4= Z

4

= H PROG3= Z PROG3= L PROG3= H PROG3= Z PROG3= L

3

Scale =

1/160

Scale =

1/320

Scale =

1/640

Scale =

1/1280

Scale =

1/2560

Units

mVh/
count

H H 49152 614 307 154 76.8 38.4 19.2 mVh

H Z 45056 563 282 141 70.4 35.2 17.6 mVh

H L 40960 512 256 128 64.0 32.0 16.0 mVh

Z H 36864 461 230 115 57.6 28.8 14.4 mVh

Z Z 33792 422 211 106 53.0 26.4 13.2 mVh

Z L 30720 384 192 96.0 48.0 24.0 12.0 mVh

L H 27648 346 173 86.4 43.2 21.6 10.8 mVh

L Z 25600 320 160 80.0 40.0 20.0 10.0 mVh

L L 22528 282 141 70.4 35.2 17.6 8.8 mVh

V

is equivalent to 2

SR

counts/sec. (nom.)

90 45 22.5 11.25 5.56 2.8 mV

6

bq2012

3. Nominal Available Charge (NAC):

NAC counts up during charge to a maximum
value of LMD and down during discharge and
self-discharge to 0. NAC is reset to 0 on initializa
tion (PROG
is set to PFC on initialization if PROG
prevent overstatement of charge during periods of

= Z or low) and on reaching EDV1. NAC

6

= high. To

6

overcharge, NAC stops incrementing when NAC =
LMD.

4. Discharge Count Register (DCR):

The DCR counts up during discharge independent
of NAC and could continue increasing after NAC
has decremented to 0. DCR stops counting when
EDV1 is reached. Prior to NAC = 0 (empty battery),
both discharge and self-discharge increment the
DCR. After NAC = 0, only discharge increments the
DCR. The DCR resets to 0 when NAC = LMD. The
DCR does not roll over but stops counting when it
reaches FFFFh.

The DCR value becomes the new LMD value on the
first charge after a valid discharge to V

EDV1

if:

No valid charge initiations (charges greater than
256 NAC counts; where V

SRO>VSRQ

) occurred dur­ing the period between NAC = LMD and EDV1 de­tected.

The self-discharge count is not more than 4096
counts (8% to 18% of PFC, specific percentage
threshold determined by PFC).

The temperature is ≥ 0°C when the EDV1 level is
reached during discharge.

The valid discharge flag (VDQ) indicates whether
the present discharge is valid for LMD update.

Charge Counting

Charge activity is detected based on a positive voltage
on the V
bq2012 increments NAC at a rate proportional to V
(VSR+VOS) and, if enabled, activates the LED display if
the rate is equivalent to V
crement the NAC after compensation for charge rate
and temperature.

The bq2012 determines charge activity sustained at a
continuous rate equivalent to V
charge equates to sustained charge activity greater than
256 NAC counts. Once a valid charge is detected, charge
counting continues until V
a programmable threshold as described in the Digital
Magnitude Filter section. The default value for V
375µV.

input. If charge activity is detected, the

SR

> 4mV. Charge actions in

SRO

. A valid

SRQ.VSRQ

falls below V

SRO

SRO>VSRQ

SRO

SRQ

Charge Control

Charge control is provided by the CHG output. This
output is asserted continuously when:

-

NAC < 0.94*LMD and

0.95V < V
0°C < Temp < 50°C and
BRM=0

This output is asserted at a
and high for 7.5 sec) when the above conditions are not
met and:

NAC < LMD and

0.95V < V
Temp < 50°C and
BRM=0

This output is also asserted at a
and high for 7.5 sec) for a 2-hour top-off period after:

NAC = LMD and
Temp < 50°C and

0.95V < V
BRM=0

This output is inactive when:

NAC = LMD (after a 2-hour top-off period) or
Temp > 50°C or
V

SB

V

SB

BRM=1

The top-off timer (2 hours) is reset to allow another top­off after the battery is discharged to 0.8*LMD (PROG
=L)or0.8*PFC (PROG6=ZorH).

Caution: The charge control output (CHG
be used with other forms of charge termination
such as∆T/∆t and -∆V.

If charge terminates due to maximum temperature, the
battery temperature must fall typically 10°C below 50°C
before the charge output becomes active again.

-

Discharge Counting

All discharge counts where V
register to decrement and the DCR to increment. Ex
ceeding the fast discharge threshold (FDQ) if the rate is
equivalent to V
abled. The display becomes inactive after V
above -4mV. V

is

described in the Digital Magnitude Filter section. The
default value for V

is

SB

SB

SB

< 0.95V or
> 2.25V or

SRO

SRD

< 2.25V and

1

duty cycle (low for 0.5 sec

16

< 2.25V and

1

duty cycle (low for 0.5 sec

16

< 2.25V and

SRO<VSRD

cause the NAC

< -4mV activates the display, if en

is a programmable threshold as

is -300µV.

SRD

) should

rises

SRO

6

-

-

7