PS700 datasheet(6/38 Pages) MICROCHIP

6 / 38 page

PS700

DS21760F-page 6

 2004 Microchip Technology Inc.

3.1.2

CURRENT MEASUREMENT

Charge and discharge currents are measured using a

5 to 600 m

Ω sense resistor that is connected between

the SR and GND pins. The maximum input voltage at

SR is ±150 mV. The sense resistor should be properly

sized to accommodate the systems lowest and highest

expected charge and discharge currents including

suspend and/or standby currents.

In order to perform charge and discharge current

measurements, the Ictrl register must be programmed

with the SR pin as the analog input source. If charge

and discharge accumulation is desired, the Ictrl and

corresponding Ires registers should be used to select

current measurement since the DCA, DTC, CCA and

DCA registers are updated by the measurement results

from the Ires register.

Ictrl programming in a typical application is as follows.

TABLE 3-2:

Ictrl PROGRAMMING

Using the maximum resolution of 16 bits, the voltage

value of the LSB is:

A/D LSB = 340 mV/216 = 5.19

µV

Using a sense resistor value of 20 m

Ω, the value of the

LSB in units of current is:

5.19

µV/20 mΩ = 259 µA

3.1.3

VOLTAGE MEASUREMENTS

Analog mux inputs are provided to support measure-

ment of individual cell and battery pack voltages. A/D

control registers VPctrl, VC1ctrl and VC2ctrl are used

to specify the measurement to be made. In typical

applications, voltage measurement at the cell or pack

level is done using the +340 mV reference and a

resolution of 10 bits plus sign bit.

The value of the LSB in a pack voltage measurement

using a 340 mV reference voltage is given by the formula:

VPACK LSB = 10.2V/2n

Where “n” is the resolution selected. For typical

applications where n = 10:

VPACK LSB = 10.2V/210 = 10.2V/1024 = 9.96 mV

The value of the LSB in a cell voltage measurement

using a 340 mV reference voltage is given by the

formula:

VCELL LSB = 6.23V/2n

Where “n” is the resolution selected. For typical

applications where n = 10:

VCELL LSB = 6.23V/210 = 6.23V/1024 = 6.08 mV

The following table shows LSB values for 9-bit plus

sign resolution, so n = 9.

TABLE 3-3:

LSB VALUES FOR 10-BIT

RESOLUTION

VPctrl programming in a typical application is as

follows.

TABLE 3-4:

VPctrl PROGRAMMING

The input source fields for the VPctrl, VC1ctrl and

VC2ctrl registers must be programmed to select pack

voltage (on VC1), VC1 cell voltage and VC2 cell

voltage in order for these registers to control their

intended measurements.

3.1.4

TEMPERATURE MEASUREMENTS

A/D input channels are provided for temperature mea-

surement using either the internal temperature sensor

or an external thermistor.

3.1.4.1

Internal Temperature

The output of the internal temperature sensor is a volt-

age range with limits that correspond to operating

temperature limits as follows:

-20°C

→ 239 mV

+70°C

→ 312 mV

The output voltage of the internal sensor as a function

of temperature can be given as:

VIT (mV) = 239 + 0.82 * (T + 20)

Defined within the ITctrl registers are the settings for

the reference utilized and the resolution desired for

measurement of temperature using the internal tem-

perature sensor. Because of input voltage range

described above, the 340 mV reference should be

selected. Typically, 10-bit plus sign of resolution are

selected which results in the following temperature

resolution:

LSB (Voltage) = Full Scale Range/# of steps

= 340 mV/210 = 332

µV/LSB

LSB (°C)

= 332

µV/LSB * (1 / 820) °C/µV

= 0.404°C/LSB

Bit(s)

Name

Value

Function

7En

Enables A/D conversion

6-4

Res

111

Selects 16-bit resolution

3Ref

Selects ±170 mV reference

2-0

Sel

000

Selects VSR as converter input

Measurement

Divider

Bits

LSB

VPACK

340

1/30

9 + sign

19.92

VCELL

340

1/18.33

9 + sign

12.17

Bit(s)

Name

Value

Function

7En

Enables A/D conversion

6-4

Res

001

Selects 10-bit resolution

3Ref

Selects ±340 mV reference

2-0

Sel

011

Selects VSR as converter input

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