PS402-01XX datasheet(6/46 Pages) MICROCHIP

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PS402-01XX

DS21766A-page 6

 2003 Microchip Technology Inc.

2.2

Auto-Offset Compensation

Accuracy drift is prevented using an automatic auto-

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

current measurement circuit every 30 seconds, when

enabled. This feature can correct for drift in tempera-

ture during operation. The Auto-Offset Compensation

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 IC. When an Auto-Offset Compensation

measurement occurs (once per 30 seconds), the actual

current measurement is skipped and the previous

measurement for current is used for the next capacity

calculation.

2.3

Voltage Measurements

The A/D input channel for pack voltage measurements

is the VC(1) pin. Measurements are taken each

measurement period when the A/D is active. The

maximum voltage at the VC(1) input pin is 18.5V

absolute, but voltages above 18V are not suggested.

The pack voltage is measured with an integration

method to reduce any sudden spikes or fluctuations.

The A/D uses a 11-bit Resolution mode for these

measurements.

The pack voltage input is read once per measurement

period in Run Mode. Voltage readings occur less

frequently in Sample Mode, where A/D measurements

are not activated every measurement period, depend-

ing on the configuration of SampleLimit and NSample

values. (See Section 3.0 Operational Modes for

additional information.)

2.3.1

IMPEDANCE COMPENSATION

Since accurate measurement of pack voltage is critical

to performance, the voltage measurements can be

compensated for any impedance in the power path that

might affect the voltage measurements.

The first compensation point is the current sense resis-

tor. This sense resistor affects the measured voltage

since the ground reference point for the measurement

is on the side of the current sense resistor.

The OTP EPROM value PackResistance is used to

compensate for additional resistance that should be

removed.

The equation for the compensation value (in ohms) is:

PackResistance = Trace resistance * 65535

(This is a 2-byte value so the largest value is 1

ohm.)

This requires modification of overall voltage SBData

function to compensate for pack resistance and shunt

resistance of current sense resistor. Thus, the previous

voltage equation is modified to:

SBData Voltage value = VC(1) + Measured

Current (mA) * PackResistance / 65535)

Figure 2-2 illustrates the compensations provided by

the PackResistance value. The heavy traces are the

portions of the circuit represented by the resistance.

The voltage measurement equation is:

V (mV) = (V_A/D - COVPack) x CFVPack / 2048

where:

V_A/D is the internal measurement output.

COVPack is the "Correction Offset for Pack Voltage"

which compensates for any offset error in voltage

measurement (since the offset of the A/D is less than

the voltage measurement resolution of +/- 16.5 mV, the

COVPack value is typically zero).

FIGURE 2-2:

PACK RESISTANCE

VALUE COMPENSATIONS

Pack

Positive

Pack

Negative

Current

Sense

Resistor

VSSA

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