PS810 datasheet(7/46 Pages) MICROCHIP

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DS21904C-page 7

PS810

3.0

OPERATIONAL DESCRIPTION

3.1

A/D Operation

The PS810 A/D converter measures current, voltage

and temperature and integrates the current over time to

calculate State-Of-Charge. Cell voltage is measured

with a direct connection to the battery cell without

requiring an external voltage divider. 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. Voltage, current and

temperature can be calibrated for accuracy over the

operational

range.

The

A/D

converter

performs

sampling using a 32 kHz clock.

3.1.1

CURRENT MEASUREMENT

The A/D input channels for current measurement are the

SR and GND pins. The voltage drop across the sense

resistor is measured and converted mathematically into

a current measurement. The current is also integrated

over time to get the amount of charge entering or leaving

the battery.

A sense resistor is connected to SR and GND. The

maximum input voltage that can be measured at SR 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.

The parameter NullCurr represents the zero-zone

current of the battery. This is provided as a calibration

guardband for reading zero current. Currents below

+/- NullCurr (in mA) limit are read as zero and not

included in the capacity algorithm calculations. A

typical value for NullCurr is 3 mA, therefore, currents

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

not included in the capacity calculations. This feature is

provided so that electrical noise on the battery module

current path is not interpreted as actual charge entering

or leaving the battery.

3.1.1.1

Sense Resistor Selection and

Current Measurement Range

The current resolution is based on the smallest amount

of voltage the A/D converter can measure across the

sense resistor. Since this measurement uses 13 bits

(plus sign) and the reference used is the internal

150 mV reference, the smallest voltage across the

sense resistor that can be measured is:

EQUATION 3-1:

3.1.1.2

Current Calibration

In-circuit calibration of the current is done using the

communication interface (SMBus/I2C or SPS) at time

of manufacture to obtain optimal accuracy. A correction

is calculated and stored for both offset and slope.

COD is a constant that is measured at calibration time

and

contains

the

offset

due

external

PCB

components.

CFCurr is the “Correction Factor for Current” which

compensates the A/D gain and any variances in the

actual sense resistance over varying currents. It is

multiplied against the raw A/D measurement.

COCurr is the “Correction Offset for Current” which is

updated in real time by the A/D shorting the inputs and

comparing any result to zero. This is added to COD and

compensates for any offset that varies over time, such

as temperature dependent offsets.

Figure 3-1 shows the relationship of the COCurr and

CFCurr values.

150 mV/(2 ^ 13 – 1) = 150 mV/32767 = 18.3

μV

Thus, the smallest current that can be measured is:

18.3

μV/RSENSE (mΩ)

The largest current that can be measured is:

150 mV/RSENSE (m

Ω)

Example: a 20 milliohm sense resistor will measure

from:

18.3

μV/20 mΩ = 0.915 mA

(though will be recorded as zero if < NullCurr)

up to:

150 mV/20 m

Ω = 7.5 Amps

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