Texas Instruments EV2011, BQ2011SN-D118 Datasheet

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 implementations 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 discharge cycle from full to empty and may be displayed depending on the display mode.

Nominal available charge may be d i

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 singleline 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

Pin Descriptions

MODE

Display mode output

When left floating, this output selects rela

tive mode for capacity display. If connected to the anode of the LEDs to source current, absolute mode is selected for capacity dis

play. See Table 1.

SEG

–

SEG

LED display segment outputs

Each output may activate an LED to sink the current sourced from MODE, the bat

tery, or V

PFC

Programmed full count selection input

This three-level input pin defines the pro

grammed full count (PFC) thresholds and scale selections described in Table 1. The state of the PFC pin is only read immediate

ly after a reset condition.

Sense resistor input

The voltage drop (V

) across the sense re-

sistor R

is monitored and integrated over time to interpret charge and discharge activity. The SR input is tied to the low side of the sense resistor. V

SR>VSS

indicates dis-

charge, and V

SR<VSS

indicates charge. The

effective voltage drop, V

SRO

, as seen by the

bq2011 is V

SR+VOS

(see Table 3).

No connect

DISP

Display control input

DISP

floating allows the LED display to be active during charge and discharge if V

SRO

< -1mV (charge) or V

SRO

> 2mV (dis

charge). Transitioning DISP

low activates

the display for 4±0.5 seconds.

Secondary battery input

This input monitors the single-cell voltage potential through a high-impedance resis

tive divider network for the end-of-discharge voltage (EDV) threshold and maximum cell voltage (MCV).

RBI

This input is used to provide backup potential to the bq2011 registers during periods when V

≤

3V. A storage capacitor should be con

nected to RBI.

Serial I/O pin

This is an open-drain bidirectional pin.

REF

Voltage reference output for regulator

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

Supply voltage input

Ground

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

Q1 ZVNL110A

Load

Charger

Indicates optional.

Directly connect to VCC 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 VCCis 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 VSRfor 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:

RB RB

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 voltage is monitored for the end-of-discharge voltage (EDV) and for maximum cell voltage (MCV). The EDV threshold 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–VSR4

. 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 VSBis 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 Register section.

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. 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:

The capacitors (SB and VCC) 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 0.1µf is recommended for V

The sense resistor (RS) 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 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

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 capacity 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

Example: Selecting a PFC Value

Given:

Sense resistor = 0.005

Ω

Number of cells = 6 Capacity = 1300mAh, NiCd cells Current range = 1A to 80A Relative display mode Self-discharge =

Voltage drop over sense resistor = 5mV to 400mV

Therefore:

1300mAh*0.005Ω= 6.5mVh

Select:

PFC = 34304 counts or 6.5mVh PFC = Z (float) MODE = not connected

The initial full battery capacity is 6.5mVh (1300mAh) until the bq2011 “learns” a new capac

ity with a qualified discharge from full to EDV.

bq2011

PFC

Programmed

Full Count (PFC) mVh Scale MODE Pin Display Mode

H 27648 10.5

2640

Floating RelativeZ 34304 6.5

5280

L 44800 8.5

5280

H 42240 8.0

5280

Connected to LEDs AbsoluteZ 31744 6.0

5280

L 23808 4.5

5280

Table 1. bq2011 Programmed Full Count mVh Selections

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