Texas Instruments BQ2945SN-C409TR, BQ2945SN-C409, BQ2945SN-C307TR, BQ2945SN-C307, BQ2945SN Datasheet

Features

➤

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

➤

Supports SBS v1.0 data set and two-wire interface

➤

Two programmable general purpose output ports for added flexibility

➤

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 five-segment LED dis-

play for remaining capacity indication

➤ 16-pin narrow SOIC

General Description

The bq2945 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 bq2945 directly supports capacity monitoring for NiCd, NiMH, and Li-Ion battery chemistries.

The bq2945 uses the System Man

agement Bus v1.0 (SMBus) protocol and supports the Smart Battery Data (SBData) commands. The bq2945 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 five-segment LED display to graphically depict battery full-to-empty in 20% increments.

The bq2945 estimates battery selfdischarge based on an internal timer and temperature sensor and user-programmable rate informa

tion stored in external EEPROM. The bq2945 also automatically re

calibrates or “learns” battery capac

ity in the full course of a discharge cycle from full to empty.

The bq2945 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 configurations.

An external EEPROM programs initial values into the bq2945 and is necessary for proper operation.

bq2945

Gas Gauge IC with SMBus Interface

3.0–6.5V

LED

LED segment 1/ EEPROM clock

LED

LED segment 2/ EEPROM data

LED

LED segment 3

LED

LED segment 4

LED

LED segment 5

Control pin 1

System ground

PN294501.eps

16-Pin Narrow SOIC

OUT

REF

SMBC

SMBD

DISP

LED1/ESCL

LED2/ESDA

LED

SR Sense resistor input

DISP

Display control input

SB Battery sense input

Control pin 2

SMBD SMBus data input/output

SMBC SMBus clock

REF Voltage reference output

OUT

EEPROM supply output

6/99 C

Pin Connections Pin Names

Pin Descriptions

Supply voltage input

LED

–

LED

LED display segment outputs

Each output may drive an external LED.

ESCL

Serial memory clock

Output used to clock the data transfer be

tween the bq2945 and the external non

volatile configuration memory.

ESDA Serial memory data and address

Bidirectional pin used to transfer ad

dress and data to and from the bq2945 and the external nonvolitile configura

tion memory.

–

Control pins 1 and 2

These open-drain outputs can be controlled by an SMBus command from the host. CP

can also act as a digital input.

Ground

Sense resistor input

The voltage drop (V

) across pins SR and

is 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. V

SR<VSS

indicates discharge, and

SR>VSS

indicates charge. The effective

voltage drop, V

SRO

, as seen by the bq2945

is V

SR+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.

SMBD

SMBus data

Open-drain bidirectional pin used to trans

fer address and data to and from the bq2945.

SMBC

SMBus clock

Open-drain bidirectional pin used to clock the data transfer to and from the bq2945.

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 configuration memory.

bq2945

Functional Description

General Operation

The bq2945 determines battery capacity by monitoring the amount of charge put into or removed from a re

chargeable battery. The bq2945 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.

Figure 1 shows a typical battery pack application of the bq2945 using the LED capacity display, the serial port, and an external EEPROM for battery pack program

ming information. The bq2945 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 EEROM.

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

square inch of

board space.

bq2945

100K

86.5K

BSS138

2N7002

BSS138

2N7002

R5R4 R11Q1

No. of Cells

Chart 1

Li-IonNiMH

499K

301K

499K

698K

806K

604K

909K

See Chart 1 for resistor values and Q1 FET selection

Figure 1. Battery Pack Application Diagram—LED Display

Notes: R4, R5, and R11 values depend on the battery voltage.

R12 and R13 nominal values must be 10k

bq2945

Parameter Name Address Description Length Units

EEPROM length 0x00

Number of EEPROM data locations must = 0x64

8 bits NA

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

Manufacturer 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 integration gain 0x2c/0x2d Programs the sense resistor scale 16 bits NA

Reserved 0x2e/0x2f Reserved for future use 16 bits NA

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

Li-Ion taper current 0x38/0x39

Sets the upper limit of the taper current for charge termination

16 bits mA

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 0x40-0x47 Programs DeviceChemistry (0x22) 64 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

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

8 bits NA

Table 1. Configuration Memory Map

bq2945

Parameter Name Address Description Length Units

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

Full-charge percentage 0x4c

Sets the percent at which the battery is consid

ered fully charged

8 bits NA

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

Reserved 0x4e Reserved for future use 8 bits NA

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

Current measurement gain 0x5a/0x5b Sense resistor calibration value 16 bits NA

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

0x62

Sets the ∆t step for ∆T/∆t termination

8 bits NA

Hold-off time 0x63

Sets ∆T/∆t hold-off timer

8 bits NA

EEPROM check 2 0x64

EEPROM data integrity check byte must = 0xb5

8 bits NA

Reserved 0x65-0x7f Reserved for future use NA

Table 1. Configuration Memory Map (Continued)

Voltage Thresholds

In conjunction with monitoring VSRfor charge/discharge currents, the bq2945 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 R

MBV

2.25

1=−

where MBV is the maximum battery voltage, R

is con

nected to the positive battery terminal, and R

is con

nected to the negative battery terminal. R

5/R4

should be

rounded to the next higher integer. R

and R4should be

sized so that the voltage at the SB pin (V

) 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 an “empty” state. The bq2945 generates an alarm warning when the bat

tery 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.

If V

is below either of the two EDV thresholds, the associated flag is latched and remains latched, independent of V

, until the next valid charge.

EDV monitoring may be disabled under certain conditions. If the discharge current is greater than approximately 6A, EDV monitoring is disabled and resumes after the current falls below 6A.

Reset

The bq2945 is reset when first connected to the battery pack. On power-up, the bq2945 initializes and reads the EEPROM configuration memory. The bq2945 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 bq2945 is in an unlocked state as defined by the value in location 0x3d of the EEPROM (EE 0x3d) on power-up.

Temperature

The bq2945 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 bq2945 controlled charge. Temperature may also be accessed over the SMBus with command 0x08.

Layout Considerations

The bq2945 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, in refer

ence to Figure 1:

The capacitors (C1 and C2) should be placed as close as possible to the SB and V

pins, 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 capacitor (C3) should be placed as close as possible to the SR pin.

The bq2945 should be in thermal contact with the cells for optimum temperature measurement.

An optional zener (D9) may be necessary to ensure that V

is not above the maximum rating during

operation.

Gas Gauge Operation

The operational overview diagram in Figure 2 illustrates the operation of the bq2945. The bq2945 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 dis

charging 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 bq2945 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.

1. FullChargeCapacity or learned-battery

capacity:

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

or reset),

FCC is set to the value stored in the EEPROM. Dur

ing subsequent discharges, FCC is updated with the

bq2945

latest measured capacity in the Discharge Count Register, 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 bq2945 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 000Ah after the EDV1 threshold has been reached and a valid charge has been detected. To prevent overstatement of charge during periods of overcharge, RM stops incrementing when RM = FCC. RM may optionally be written to a userdefined value when fully charged if the battery pack is under bq2945 charge control. On initializa

tion, RM is set to the value stored in EE 0x1e—0x1f.

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 (empty battery),

both discharge and self-discharge increment the DCR. After RM = 0, only discharge increments the DCR. The DCR resets to 0 when RM = FCC. The DCR does not roll over but stops counting when it reaches FFFFh.

The DCR value becomes the new FCC value on the first charge after a qualified discharge to EDV1. A qualified discharge to EDV1 occurs if all of the following conditions exist:

No valid charge initiations (charges greater than 10mAh), where V

SRO

>+V

SRD

occurred during

the period between RM = FCC and EDV1 de

tected.

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

The temperature is≥273°K (0°C) when the EDV1 level is reached during discharge.

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.

Charge Counting

Charge activity is detected based on a positive voltage on the SR input. If charge activity is detected, the bq2945 increments RM at a rate proportional to V

SRO

and, if enabled, activates an LED display. Charge ac

tions increment the RM after compensation for charge state and temperature.

bq2945

FG294501.eps

Temperature

Compensation

Charge Current

Discharge

Current

Self-Discharge

Timer

Remaining

Capacity

(RM)

Full

Charge

Capacity

(FCC)

Discharge

Count

(DCR)

Qualified

Transfer

Temperature, Other Data

Inputs

Main Counters

and Capacity

Reference (FCC)

Outputs

Two-Wire

Serial Interface

Chip-Controlled

Available Charge

LED Display

State-of-charge

and

Temperature

Compensation

Figure 2. Operational Overview

The bq2945 determines charge activity sustained at a continuous rate equivalent to V

SRO

>+V

SRD

. A valid

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

charge threshold counting continues until V

SRO

falls be

low V

SRD

is a programmable threshold as de

scribed in the Digital Magnitude Filter section.

Discharge Counting

All discharge counts where V

SRO

<-V

SRD

cause the RM

SRD

is a programmable threshold as described in the Digital Magnitude Filter section.

Self-Discharge Estimation

The bq2945 continuously decrements RM and incre

ments DCR for self-discharge based on time and tem

perature. The bq2945 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 doubles every 10°C from 30°C to 70°C and halves every 10°C from 20°Cto 0°C.

Charge Control

The bq2945 supports SBS charge control by broadcasting the ChargingCurrent and the ChargingVoltage to the Smart Charger address. The bq2945 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 bq2945 updates the value used in the charging current broadcasts based on the battery’s state of charge, voltage, and temperature.

The bq2945 internal charge control is compatible with nickel-based and Li-Ion chemistries. The bq2945 uses current taper detection for Li-Ion primary charge termi

nation and ∆T/∆t for nickel based primary charge termi

nation. The bq2945 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 bq2945 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 between the programmed threshold in EE 0x38—0x39 and 100 mA for at least 40s.

∆T/∆t

The ∆T/∆t used by the bq2945 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

EE 0x62)

∗

2 s

20]

s∗

 



 



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 tempera

ture falls below 25°C, or if charging current falls below 255mA, the timer is reset and restarts only if these con

ditions are once again within range. The hold-off time is programmed in EE 0x63.

Charge Termination

Once the bq2945 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 Terminate_Charge_Alarm and the Over_Charged_Alarm are cleared, and the requested charging current is set to the maintenance rate. The bq2945 requests the maintenance rate until RM falls below 95% of full-charge percentage. Once this occurs, the Fully_Charged bit is cleared, and the requested charge current and voltage are set to the fast-charge rate.

Bit 4 (CC) in FLAGS2 determines whether RM is modified 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.

Charge Suspension

The bq2945 may temporarily suspend charge if it de

tects a charging fault. The charging faults include the following conditions:

Maximum Overcharge: If charging continues for more than the programmed maximum overcharge limit as defined in EE 0x3a—0x36 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 bq2945 measures a voltage more than 5% above the ChargingVoltage. When the bq2945 detects an overvoltage 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

bq2945

the current drops below 256mA and the voltage is less than 105% of ChargingVoltage.

Overcurrent: An overcurrent fault exists when the bq2945 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 256mA above the ChargingCurrent. When the bq2945 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 exceeds 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 alarm bits are cleared when the temperature drops below 50°C.

Low Temperature: When the battery temperature is less than 0°C, the requested charge current is set to the maintenance rate. Once the temperature is above 5°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 maintenance rate. Once the voltage is above EDVF, the requested charge current is set to the fast rate.

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

EFC

is the programmed fast-charge efficiency vary

ing from 0.75 to 1.0.

2.)

RM RM Q Q

ETC ET

=−*( )

where RelativeStateOfCharge ≥ FullChargePercentage and

ETC

is the programmed maintenance (trickle)

charge efficiency varying from 0.75 to 1.0.

is used to adjust the charge efficiency as the battery

temperature increases according to the following:

QifT

=<030

°C

QCTC

=°≤<°002 30 40.if

QTC

=≥°005 40.if

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

Digital Magnitude Filter

The bq2945 has a programmable digital filter to elimi

nate charge and discharge counting below a set threshold, V

SRD

. Table 2 shows typical digital filter

settings. The proper digital filter setting can be calcu

lated using the following equation.

DMF =

SRD

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 ad

justed 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 V

. A digital filter eliminates charge and discharge

counts to the RM register when -V

SRD<VSRO

<+V

SRD

Display

The bq2945 can directly display capacity information using low-power LEDs. The bq2945 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 20% of the FCC.

bq2945

DMF DMF Hex. V

SRD

(mV)

75 4B

0.60

100 64 0.45

150 96 0.30

175 AF 0.26

200 C8 0.23

Table 2. Typical Digital Filter Settings

In absolute mode, each segment represents a fixed amount of charge, 20% 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

1-5

outputs are inac

tive. When DISP

is left floating, the display becomes ac

tive whenever the bq2945 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

to reactivate the display.

LED

blinks at a 4Hz rate whenever VSBhas been detected to be below EDV1 (EDV1 = 1), indicating a lowbattery condition. V

below EDVF (EDVF = 1) disables

the display output.

Microregulator

The bq2945 can operate directly from three nickel chem

istry cells. To facilitate the power supply requirements of the bq2945, an REF output is provided to regulate an external low-threshold n-FET. A micropower source for the bq2945 can be inexpensively built 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 bq2945

The bq2945 includes a simple two-pin (SMBC and SMBD) bi-directional serial data interface. A host proc

essor uses the interface to access various bq2945 regis

ters; see Table 4. This method allows battery character

istics to be monitored easily. The open-drain SMBD and SMBC pins on the bq2945 are pulled up by the host sys

tem, or may be connected to V

, 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 bq2945. The command directs the bq2945 to either store the next data received to a register specified by the command byte or output the data specified by the command byte.

bq2945 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 bq2945. (See Figure 3.)

Host-to-bq2945 Message Protocol

The Bus Host communicates with the bq2945 using one of three protocols:

Read word

Write word

Read block

The particular protocol used is a function of the com

mand. The protocols used are shown in Figure 3.

Host-to-bq2945 Messages

(see Table 4)

ManufacturerAccess() (0x00)

This function is used to control CP1and CP2. (See Table

7.)

bq2945

Symbol Parameter Typical Maximum Units Notes

Offset referred to V

150

V DISP

= VCC.

INL

Integrated non-linearity error

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

INR

Integrated nonrepeatability error

0.5

Measurement repeatability given similar operating conditions.

Table 3. bq2945 Current-Sensing Errors

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