PS501 datasheet(5/42 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21818C-page 5

PS501

2.0

A/D OPERATION

The PS501 A/D converter measures current, voltage

and temperature and integrates the current over time to

predict State-Of-Charge. Pack voltage and individual

cell voltages are monitored and can be individually cal-

ibrated for the best accuracy. Using an external sense

resistor, current is monitored during both charge and

discharge and is integrated over time using the on-chip

oscillator as the time base. Temperature is measured

from the on-chip temperature sensor or an optional

external thermistor. Current and temperature are also

calibrated for accuracy.

2.1

A/D Converter List

The A/D converter alternately measures pack voltage,

cell voltages, current, temperature and auto-offset as

explained below. The schedule for the sequence and

frequency of these measurements is programmable, as

is the number of bits used. The default scheduling uses

three lists. At near full (above the voltage point

ADLNearFull) and near empty (below the voltage point

ADLNearEmpty), voltage intensive lists are used to

accurately end charge or discharge. In between

ADLNearFull and ADLNearEmpty, a current intensive

schedule is used to more accurately calculate capacity.

2.2

Current Measurement

The A/D input channels for current measurement are

the RSHP and RSHN pins. The current is measured

using an integrating method, which averages over time

to get the current measurement and integrates over

time to get a precise measurement value.

A 5 to 600 milli-Ohm sense resistor is connected to

RSHP and RSHN as shown in the example schematic.

The maximum input voltage at either RSHP or RSHN is

+/-150 mV. The sense resistor should be properly sized

to accommodate the lowest and highest expected

charge and discharge currents, including suspend

and/or standby currents.

Circuit traces from the sense resistor should be as

short as practical without significant crossovers or

feedthroughs. Failure to use a single ground reference

point at the negative side of the sense resistor can

significantly degrade current measurement accuracy.

The EEPROM value, NullCurr, represents the zero

zone current of the battery. This is provided as a

calibration guardband for reading zero current. Cur-

rents below the +/- NullCurr (in mA) limit are read as

zero and are not included in the capacity algorithm

calculations. A typical value for NullCurr is 3 mA, so

currents between -3 mA and +3 mA will be reported as

zero and not included in the capacity calculations.

The equation for current measurement resolution and

sense resistor selection is shown in the following

equation.

EQUATION 2-1:

In-circuit calibration of the current is done using the

SMBus interface at time of manufacture to obtain

absolute accuracy. The current measurement equation

is:

EQUATION 2-2:

COCurr is the “Correction Offset for Current” which

compensates

for

any

offset

error

current

measurement stored in EEPROM.

CFCurr is the “Correction Factor for Current” which

compensates for any variances in the actual sense

resistance over varying currents stored in EEPROM.

Figure 2-1 shows the relationship of the COCurr and

CFCurr values.

FIGURE 2-1:

COCurr AND CFCurr

VALUE RELATIONSHIP

2.3

Auto-Offset Compensation

Accuracy drift is prevented using an automatic auto-

zero self-calibration method, which ‘re-zeroes’ the

current measurement circuit every AOMInt * 0.5 sec-

onds when enabled. This feature can correct for drift in

temperature during operation. The auto-offset compen-

sation circuit works internally by disconnecting the

RSHP and RSHN inputs and internally shorting these

inputs to measure the zero input offset. The EEPROM

and calibration value COD is the true zero offset value

of the particular module.

9.15 mV/RSENSE (milli-Ohms) = Current LSB

(Minimum current measurement if > NullCurr)

Current LSB x 16384 = Maximum current

measurement possible

I(ma) = (I_A/D – COCurr) * CFCurr/16384

where:

I_A/D is the internal measurement

Ideal A/D Response

Actual A/D

Response

CFCurr

COCurr

Actual Current

Raw Measurement

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