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Features

➤

Conservative and repeatable

measurement of available charge

in rechargeable batteries

➤

Designed for portable equipment

such as power tools with high dis

charge rates

➤

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

➤ Direct drive of LEDs for capacity

display

➤ Self-discharge compensation us-

ing internal temperature sensor

➤ Simple single-wire serial commu-

nications port for subassembly

testing

➤

16-pin narrow SOIC

General Description

The bq2011 Gas Gauge IC is intended

for battery-pack installation to main

tain an accurate record of available

battery charge. The IC monitors a

voltage drop across a sense resistor

connected in series between the nega

tive battery terminal and ground to

determine charge and discharge ac

tivity of the battery. The bq2011 is

designed for systems such as power

tools with very high discharge rates.

Battery self-discharge is estimated

based on an internal timer and tem

perature sensor. Compensations for

battery temperature and rate of

charge or discharge are applied to

the charge, discharge, and

selfdischarge calculations to provide

available charge information across

a wide range of operating conditions.

Initial battery capacity is set using

the PFC and MODE pins. Actual

battery capacity is automatically

“learned” in the course of a dis-

charge cycle from full to empty and

may be displayed depending on the

display mode.

Nominal available charge may be di

rectly indicated using a five-seg

ment LED display. These segments

are used to indicate graphically the

nominal available charge.

The bq2011 supports a simple single-

line bidirectional serial link to an exter

nal processor (common ground). The

bq2011 outputs battery information in

response to external commands over the

serial link. To support subassembly

testing, the outputs may also be con

trolled by command. The external proc

essor may also overwrite some of the

bq2011 gas gauge data registers.

The bq2011 may operate directly

from four cells. With the REF output

and an external transistor, a simple,

inexpensive regulator can be built to

provide V

from a greater number

of cells.

Internal registers include available

charge, temperature, capacity, battery

ID, and battery status.

Gas Gauge IC for

High Discharge Rates

2/96 E

Pin Connections Pin Names

PN201101.eps

16-Pin Narrow SOIC

REF

RBI

DISP

MODE

SEG

PFC

MODE Display mode output

SEG

LED segment 1

SEG

LED segment 2

SEG

LED segment 3

SEG

LED segment 4

SEG

LED segment 5

PFC Programmed full count

selection input

REF Voltage reference output

NC No connect

DQ Serial communications

input/output

RBI Register backup input

SB Battery sense input

DISP

Display control input

SR Sense resistor input

3.0–6.5V

Negative battery terminal

bq2011

Functional Description

General Operation

The bq2011 determines battery capacity by monitoring

the amount of charge input to or removed from a re

chargeable battery. The bq2011 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.

Figure 1 shows a typical battery pack application of the

bq2011 using the LED display with absolute mode as a

charge-state indicator. The bq2011 can be configured to

display capacity in either a relative or an absolute dis

play mode. The relative display mode uses the last

measured discharge capacity of the battery as the bat

tery “full” reference. The absolute display mode uses the

programmed full count (PFC) as the full reference, forc

ing each segment of the display to represent a fixed

amount of charge. A push-button display feature is

available for momentarily enabling the LED display.

The bq2011 monitors the charge and discharge currents

as a voltage across a sense resistor (see R

in Figure 1).

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.

bq2011

FG201101.eps

PFC

SEG

DISP

REF

bq2011

Gas Gauge IC

MODE

RBI

0.1 F

ZVNL110A

Load

Charger

Indicates optional.

Directly connect to V

across 4 cells (4.8V nominal and 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.

Programming resistors and ESD-protection diodes are not shown.

R-C on SR may be required, (application-specific), where the maximum

R should not exceed 20K.

Figure 1. Battery Pack Application Diagram—LED Display,

Absolute Mode

The bq2011 RBI input pin is intended to be used with a

storage capacitor to provide backup potential to the inter

nal bq2011 registers when V

momentarily drops below

3.0V. V

is output on RBI when V

is above 3.0V.

After V

rises above 3.0V, the bq2011 checks the inter

nal registers for data loss or corruption. If data has

changed, then the NAC and FULCNT registers are

cleared, and the LMD register is loaded with the initial

PFC.

Voltage Thresholds

In conjunction with monitoring V

for charge/discharge

currents, the bq2011 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:

1=−

where N is the number of cells, RB

is connected to the

positive battery terminal, and RB

is connected to the

negative battery terminal. The single-cell battery volt-

age is monitored for the end-of-discharge voltage (EDV)

and for maximum cell voltage (MCV). The EDV thresh-

old level is used to determine when the battery has

reached an “empty” state, and the MCV threshold is used

for fault detection during charging. The EDV and MCV

thresholds for the bq2011 are fixed at:

EDV

= 0.90V

MCV

= 2.00V

During discharge and charge, the bq2011 monitors V

for various thresholds, V

SR1

–V

SR4

. These thresholds are

used to compensate the charge and discharge rates. Ref

er to the discharge compensation section for details.

EDV monitoring is disabled if V

> V

SR1

(50mV typical)

and resumes 1 second after V

drops back below V

SR1

Reset

The bq2011 recognizes a valid battery whenever V

greater than 0.1V typical. V

rising from below 0.25V

resets the device. Reset can also be accomplished with a

command over the serial port as described in the Reset

Temperature

The bq2011 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:

Layout Considerations

The bq2011 measures the voltage differential between

the SR and V

pins. V

(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:

The capacitors (SB and V

) should be placed as close

as possible to the SB and V

pins, respectively, and

their paths to V

should be as short as possible. A

high-quality ceramic capacitor of 0.1

f is recommended

for V

The sense resistor (R

) should be as close as possible

to the bq2011.

The R-C on the SR pin should be located as close as

possible to the SR pin. The maximum R should not

exceed 20K.

bq2011

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

Gas Gauge Operation

The operational overview diagram in Figure 2 illustrates

the operation of the bq2011. The bq2011 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

bq2011 adapts its capacity determination based on the

actual conditions of discharge.

The battery's initial capacity is equal to the Pro

grammed Full Count (PFC) shown in Table 1. Until

LMD is updated, NAC counts up to but not beyond this

threshold during subsequent charges. This approach al-

lows the gas gauge to be charger-independent and com-

patible 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

or bat

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

EDV. 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

battery capacity:

The initial LMD and gas gauge rate values are pro

grammed by using PFC. The PFC also provides the

100% reference for the absolute display mode. The

bq2011 is configured for a given application by se

lecting a PFC value from Table 1. The correct PFC

may be determined by multiplying the rated bat

tery capacity in mAh by the sense resistor value:

Battery capacity (mAh)

sense resistor (

Ω

) =

PFC (mVh)

Selecting a PFC slightly less than the rated capac-

ity for absolute mode provides capacity above the

full reference for much of the battery's life.

bq2011

FG201104.eps

Temperature

Compensation

Charge

Current

Discharge

Current

Self-Discharge

Timer

Temperature

Translation

Nominal

Available

Charge

(NAC)

Last

Measured

Discharged

(LMD)

Discharge

Count

(DCR)

Qualified

Transfer

Rate and

Temperature

Compensation

Rate and

Temperature

Compensation

Temperature Step,

Other Data

Inputs

Main Counters

and Capacity

Reference (LMD)

Outputs

Serial

Port

Chip-Controlled

Available Charge

LED Display

Rate and

Temperature

Compensation

Figure 2. Operational Overview

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