PS810 查看數據表（PDF） - Microchip Technology

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PS810

Li Ion Single Cell Fuel Gauge

Microchip Technology 

PS810

FIGURE 3-1:

COCurr AND CFCurr

VALUE RELATIONSHIP

Raw Measurement

Ideal A/D Response

COCurr

CFCurr

Actual A/D

Response

Actual Current

3.1.2 AUTO-OFFSET COMPENSATION

Accuracy drift is prevented using an automatic auto-

zero self-calibration method which zeros the current

measurement circuit periodically at a programmable

rate. This feature can correct for drift in temperature

during operation. The Auto-Offset Compensation

circuit works internally by disconnecting the RS input

and internally shorting it to GND to measure the zero

input offset. Furthermore, the calibration factor, COD,

contains the offset factor external to the IC, offset due

to the circuit board, system, etc. COD is added to the

internal offset calculated by the auto-offset cycle to

determine the full offset, COCurr.

3.1.3 VOLTAGE MEASUREMENTS

The A/D input channel for cell voltage measurement is

the VC1 pin. Measurements are taken each measure-

ment period when the A/D is active. The maximum

voltage at the VC1 pin is 5.5V, but voltages above 4.5V

are not suggested since this will saturate the A/D. The

cell voltage is measured with an integration method to

reduce any sudden spikes or fluctuations. The A/D

uses a default of 11-bit plus sign resolution for these

measurements.

The VC1 input circuit contains an internal resistive

divider to reduce the external voltage input to a range

that the internal A/D circuit can accommodate (300 mV

maximum). The divider is 15 to 1 based on a maximum

cell voltage of 4.5 volts. The voltage divider is only

connected to ground when the actual voltage

measurement is occurring.

CFVoltage is the “Correction Factor for Pack Voltage”

which compensates for any variance in the actual A/D

response versus an ideal A/D response over varying

voltage inputs. In-circuit calibration of the voltage is

done at the time of manufacture to obtain accuracy in

addition to high resolution. Cell voltage measurements

can be accurate to within ±20 mV.

3.1.4 TEMPERATURE MEASUREMENTS

The A/D can measure temperature from the internal

temperature sensor or an external thermistor

connected to the NTC pin. The A/D uses a default of

11-bit plus sign resolution for the temperature

measurements.

A standard 10 kOhms at 25°C Negative-Temperature-

Coefficient (NTC) device of the 103ETB type is

suggested for the optional external thermistor. One leg

of the NTC should be connected to the NTC pin and the

other to ground.

A linearization algorithm is used to convert the voltage

measurement seen at the NTC pin to a temperature

value. The external thermistor should be placed as

close as possible to the battery cells and should be

isolated from any other sources of heat that may affect

its operation.

Calibration of the temperature measurements involves

a correction factor and an offset exactly like the current

measurement. The internal temperature measurement

makes use of correction factor, CFTempI and offset,

COTempI, while the NTC pin for the optional external

thermistor makes use of correction factor, CFTempE.

DS21904C-page 8

© 2006 Microchip Technology Inc.

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