PS501 datasheet(7/42 Pages) MICROCHIP

7 / 42 page

 2004 Microchip Technology Inc.

DS21818C-page 7

PS501

Figure 2-2 shows the relationship of the COVPack and

CFVPack values.

FIGURE 2-2:

COVPack AND CFVPack

VALUE RELATIONSHIP

In-circuit calibration of the voltage is done at the time of

manufacture to obtain absolute accuracy in addition to

high resolution. Individual cell voltage measurements

can be accurate to within ±20 mV.

The individual cell voltage inputs are also calibrated the

same way the pack voltage is. There is one offset value,

COVCell, for all individual cells and up to four different

correction factors, CFVCell1 through CFVCell4, one for

each cell input.

2.5

Temperature Measurements

The A/D receives input from the internal temperature

sensor to measure the temperature. Optionally, an

external thermistor can be connected to the VNTC pin

which is also monitored by the A/D converter. An output

reference voltage for use with an external thermistor is

provided on the VREFT pin. The A/D uses an 11-bit

resolution mode 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 VREFT pin and

the other to both the VNTC pin and a 3.65 kOhms resis-

tor to analog ground (VSSA). The resistor forms the

lower leg of a voltage divider circuit. To maintain high

accuracy in temperature measurements, a 1% resistor

should be used.

A look-up table is used to convert the voltage measure-

ment seen at the VNTC pin to a temperature value. The

external thermistor should be placed as close as possi-

ble 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

and voltage measurements. The internal temperature

measurement makes use of correction factor CFTempI

and offset COTempI, while the VNTC and VREFT pins for

the optional external thermistor make use of correction

factor CFTempE and offset COTempE.

Ideal A/D Response

Actual A/D

Response

CFVPack

COVPack

Raw Measurement

Actual Voltage

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