Rainbow Electronics DS2740 User Manual

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875

DS2740

High-Precision Coulomb Counter

FEATURES

§ 15-Bit Bidirectional Current Measurement

(DS2740)

§ 1.56mV LSB and ±51.2mV Dynamic

Range

§ 78mA LSB and ±2.56A Dynamic

Range with External 20mW Sense Resistor (R

§ 156mA LSB and ±5.12A Dynamic

Range with External 10mW Sense Resistor (R

§ 13-Bit Bidirectional Current Measurement

(DS2740B)

§ 6.25mV LSB and ±51.2mV Dynamic

Range

§ 312mA LSB and ±2.56A Dynamic

Range with External 20mW Sense Resistor (R

§ 625mA LSB and ±5.12A Dynamic

Range with External 10mW Sense Resistor (R

§ Analog Input Filter (IS1, IS2) Extends

Dynamic Range for Pulse-Load Applications

§ Current Accumulation Register

Resolution

§ 6.25mVhr (Both DS2740 and

DS2740B)

§ 0.3125mAhr with External 20mW

SNS

§ 0.6250mAhr with External 10mW

SNS

§ Dallas 1-Wire® Interface

§ Unique 64-Bit Device Address

§ Standard and Overdrive Timings

(OVD)

§ Low Power Consumption:

§ Active Current: 65mA max

§ Sleep Current: 1mA max

SNS

)

PIN CONFIGURATION

PIO

SNS

MAX

DS2740U, DS2740BU

See Table 1 for Ordering Information. See Table 2 for Detailed Pin Descriptions.

IS1IS2

PIN DESCRIPTION

OVD- 1-Wire Bus Speed Select PIO - Programmable I/O Pin SNS - Sense Resistor Input IS2 - Current-Sense Input IS1 - Current-Sense Input VSS- Device Ground, Current-Sense Resistor Return DQ - Data Input/Output VDD- Power-Supply Input (2.7V to 5.5V)

1-Wire is a registered trademark of Dallas Semiconductor.

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DS2740

Table 1. ORDERING INFORMATION

PART MARKING PIN-PACKAGE

DS2740U DS2740U/T&R DS2740BU DS2740BU/T&R

DS2740

DS2740 DS2740B DS2740B

15-Bit Current Resolution, mMAX 15-Bit Current Resolution, mMAX, Tape-and-Reel 13-Bit Current Resolution, mMAX 13-Bit Current Resolution, mMAX, Tape-and-Reel

DESCRIPTION

The DS2740 provides high-precision current-flow measurement data to support battery-capacity monitoring in cost-sensitive applications. Current is measured bidirectionally over a dynamic range of 15 bits (DS2740U) or 13 bits (DS2740UB), with the net flow accumulated in a separate 16-bit register. Through its 1-Wire interface, the DS2740 allows the host system read/write access to status and current measurement registers. Each device has a unique factory-programmed 64-bit net address that allows it to be individually addressed by the host system, supporting multibattery slot operation. The interface can be operated with standard or overdrive timing.

Although the DS2740 is primarily intended for location on the host system, it is also suited for mounting in the battery pack. The DS2740 and FuelPack™ algorithms, along with host measurements of temperature and voltage, form a complete and accurate solution for estimating remaining capacity.

Figure 1. BLOCK DIAGRAM

1-WIRE

INTERFACE

AND

ADDRESS

10 k

STATUS/CONTROL

ACCUMULATED

CURRENT

15-Bit + Sign

TIMEBASE

10 k

PIO

CHIP

GROUND

SNS IS2

FuelPack is a trademark of Dallas Semiconductor.

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IS1

Table 2. DETAILED PIN DESCRIPTION

PIN SYMBOL DESCRIPTION

1-Wire Bus Speed Control. Input logic level selects the speed of the 1-

OVD 1

Wire bus. Logic 1 selects overdrive (OVD) and Logic 0 selects standard timing (STD). On a multidrop bus, all devices must operate at same speed.

DS2740

PIO 2

SNS 3

IS2

IS1 5

Programmable I/O Pin. Programmed as input or output through internal registers. Open-drain output sufficient for LED or vibrator activation.

Current-Sense Resistor Input Current-Sense Input. Connected to SNS through a 10kΩ resistor to

allow filtering of the current waveform by an external capacitor. Current-Sense Input. Connected to VSS through a 10kΩ resistor to allow

filtering of the current waveform through an external capacitor.

Device Ground, Current-Sense Resistor Return. Connect directly to the negative terminal of the battery cell.

Data I/O Pin. Operates bidirectionally with open-drain output driver.

DQ 7

Internal 1µA pulldown aids in sensing pack removal and sleep-mode activation.

Power-Supply Input. Connects to system voltage supply or positive terminal of battery cell.

Figure 2. APPLICATION EXAMPLE

2.7V to 5.5V System Supply

Battery Pack

150

DATA

PIO

System

150

PIO

SNS

* *

VDD

DS2740

OVD

VSS

IS1IS2

104

GND

SNS

* 5.6V zener recommended for ESD protection when DATA or PIO

contacts exposed, such as a removable battery pack application

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104

Negative Battery Pack Return

DS2740

POWER MODES

The DS2740 has two power modes: active and sleep. While in active mode, the DS2740 operates as a high-precision coulomb counter with current and accumulated current measurement blocks operating continuously and the resulting values updated in the measurement registers. Read and write access is allowed to all registers. PIO pin is active. In sleep mode, the DS2740 operates in a low-power mode with no current measurement activity. Serial access to current, accumulated current, and status/control registers is allowed if VDD > 2V.

The DS2740 operating mode transitions from SLEEP to ACTIVE when:

1) DQ > VIH, and VDD > UV threshold, or

2) VDD rises from below UV threshold to above UV threshold.

The DS2740 operating mode transitions from ACTIVE to SLEEP when:

1) VDD falls to UV threshold, or

2) SMOD = 1 and DQ < VIL for 2s.

CURRENT MEASUREMENT

In the active mode of operation, the DS2740 continually measures the current flow into and out of the battery by measuring the voltage drop across a low-value current-sense resistor, R range for pulse-type load currents, the voltage signal can be filtered by adding a capacitor between the IS1 and IS2 pins. The external capacitor and two internal resistors form a lowpass filter at the input of the ADC. The voltage-sense range at IS1 and IS2 is ±51.2mV. The input converts peak signal amplitudes up to 75mV as long as the continuous or average signal level (post filter) does not exceed ±51.2mV over the conversion cycle period. The ADC samples the input differentially at IS1 and IS2 with an 18.6kHz sample clock and updates the current register at the completion of each conversion cycle. Conversion times for each resolution option are listed in the tables below. Two resolution options are available. Figure 3 describes the current measurement register format and resolution for each option. “S” indicates the sign bit(s).

. To extend the input

SNS

Figure 3. CURRENT REGISTER FORMATS

DS2740: 15-bit + sign resolution, 3.5s conversion period.

MSB—Address 0E LSB—Address 0F

S2142132122112

1029

MSb LSb MSb LSb

DS2740B: 13-bit + sign resolution, 0.875s conversion period.

MSB—Address 0E LSB—Address 0F

SSS2122112

1029

MSb LSb MSb LSb

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272625242322212

Units: 1.5625mV/Rsns

272625242322212

Units: 6.250mV/Rsns

DS2740

CURRENT RESOLUTION (1 LSB)

PART

DS2740 DS2740B

CONVERSION

TIME

3.515s

0.878s

- V

IS1

IS2

20mW 15mW 10mW 5mW

1.5625mV 78.13mA 104.2mA 156.3mA 312.5mA

6.250mV 312.5mA 416.7mA 625mA

SNS

1.250mA

Every 1024th conversion, the ADC measures its input offset to facilitate offset correction. Offset correction occurs approximately once per hour in the DS2740 and four times per hour in the DS2740B. The resulting correction factor is applied to the subsequent 1023 measurements. During the offset correction conversion, the ADC does not measure the IS1 to IS2 signal. A maximum error of 1/1024 in the accumulated current register (ACR) is possible, however, to reduce the error, the current measurement just prior to the offset conversion is displayed in the current register and is substituted for the dropped current measurement in the current accumulation process. The typical error due to offset correction is much less than 1/1024.

CURRENT ACCUMULATOR

Current measurements are internally summed, or accumulated, at the completion of each conversion period with the results displayed in the ACR. The accuracy of the ACR is dependent on both the current measurement and the conversion timebase. The ACR has a range of ±204.8mVh with an LSb of

6.25µVh. Additional registers hold fractional results of each accumulation, however, these bits are not user accessible.

Read and write access is allowed to the ACR. Whenever the ACR is written, fractional accumulation results are cleared. Also, a write forces the ADC to measure its offset and update the offset correction factor. The current measurement and accumulation begin with the second conversion following a write to the ACR. Figure 4 describes the ACR address, format, and resolution.

Figure 4. CURRENT ACCUMULATOR FORMAT

MSB—Address 10 LSB—Address 11

1421321221121029

MSb LSb MSb LSb

PART

DS2740 DS2740B

UPDATE

INTERVAL

3.515s

0.878s

- V

IS1

IS2

6.25mVh 312.5mAh 416.7mAh 625mAh

272625242322212

Units: 6.25mVh/Rsns

ACR LSB

SNS

20mW 15mW 10mW 5mW

1.250mAh

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