TEXAS INSTRUMENTS bq2040 Technical data

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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

rations.

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

ESCL

ESDA

LED

SLUS005–JUNE 1999 E

16-Pin Narrow SOIC

OUT

REF

SMBC

SMBD

PSTAT

DISP

PN204001.eps

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

1-4

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

bq2040

Pin Descriptions

ESCL

ESDA Serial memory data and address

LED LED

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

–

Each output may drive an external LED.

Ground

Sense resistor input

The voltage drop (V

) across pins SR and

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

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.

OUT

Supply output

Supplies power to the external EEPROM con figuration memory.

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.

ESCL

SMBC

ESDA

SMBD

LED1 LED2

PS TAT LED3 LED4 V

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

DISP

square inch of

No. of Cells

Li-IonNiMH

Chart 1

For bq2040 With No D8

R5 R4 Q1R11

301K BSS138

499K

3 4

698K

499K

698K

806K

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

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

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

16 bits NA

8 bits NA

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:

MBV

2.25

where MBV is the maximum battery voltage, R

1=−

is con

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

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

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

close as possible to the SR pin.

The bq2040 should be in thermal contact with the

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

or reset),

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

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:

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

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

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

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

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

is a programmable threshold as de

SRO

>+V

SRD

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

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.

∆

is set by the formula:

∆

[]

(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

∆

) 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.









s∗





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

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

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

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.

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.

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.

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.

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

EFC

2.)

RM RM Q Q

=−*( )

ETC ET

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

temperature increases according to the following:

QifT

QCTC

QTC

is the programmed maintenance (trickle)

ETC

is used to adjust the charge efficiency as the battery

=<030

=°≤<°002 30 40.if

=≥°005 40.if

°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 =

SRD

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

bq2040

Table 3. bq2040 Current-Sensing Errors

Symbol Parameter Typical Maximum Units Notes

INL

INR

Offset referred to V

Integrated non-linearity error

Integrated nonrepeatability error

0.5

150

V DISP

= 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

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:

Read word

Write word

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