PS501 datasheet(6/42 Pages) MICROCHIP

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PS501

DS21818C-page 6

 2004 Microchip Technology Inc.

2.4

Voltage Measurements

The A/D input channels for cell and pack voltage mea-

surements are the VC(1) to VC(4) pins. Measurements

are taken each measurement period when the A/D is

active. The maximum voltage at any VCELLx input pin is

19V absolute, but voltages above 18V are not sug-

gested. The individual cell voltages are measured with

an integration method to reduce any sudden spikes or

fluctuations. The A/D uses an 11-bit resolution mode

for these measurements.

Cell voltage inputs are read every measurement

period, which is approximately every 500 milliseconds.

This could be further extended by the use of Sample

mode, where A/D measurements are not activated

every measurement period, depending on the configu-

ration of SampleLimit and NSample values. (See

Section 3.0 “Operational Modes” for additional

information.) For Li Ion, Li-based, or even Lead-Acid

applications, up to four (4) series cell voltages may be

monitored individually. The highest voltage cell of the

stack must be connected to VC(1).

For some applications, the actual cell stack arrange-

ment can be altered accordingly. The PS501 voltage

input pins (VCELLx pins) are capable of measuring up to

18V each. Therefore, cell arrangements can be com-

bined and the corresponding cell voltage thresholds

can be adjusted. For example, a 2-cell Li Ion pack could

actually be connected as a single 7.2V cell instead of

two 3.6V cells. The values for the cell voltages would all

be doubled, for example VCELL1 would equal the sum

of two cells and only the VC(1) input pin would be used.

Each VCELLx input circuit contains an internal resistive

divider to reduce the external voltage input to a range

that the internal A/D circuit can accommodate (150 mV

maximum). These dividers are set based on a

maximum cell voltage of 4.5 volts. A range of 340 mV

with dividers, based on a maximum voltage of 20 volts,

is used for pack voltage.

The impedance at each VCELLx input is roughly

100 kOhms, but is only connected to ground (via the

VSSA pins) when the actual voltage measurement is

occurring. This corresponds to an insignificant amount

of capacity drained through this circuit during the brief

voltage measurement period, typically 45 ms every

500 ms.

2.4.1

IMPEDANCE COMPENSATION

Since accurate measurement of pack voltage and cell

voltages are critical to performance, the voltage

measurements can be compensated for any imped-

ance in the power path that might affect the voltage

measurements.

The EEPROM value PackResistance is used to

compensate for additional resistance that should be

removed.

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

EQUATION 2-3:

This requires modification of overall voltage SBData

function to compensate for pack resistance and shunt

resistance of the current sense resistor. Thus, the

previous voltage equation is modified to:

EQUATION 2-4:

The voltage measurement equation is:

EQUATION 2-5:

COVPack is the “Correction Offset for Pack Voltage”

which compensates for any offset error in voltage mea-

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

voltage measurement resolution of +/- 16.5 mV, the

COVPack value is typically zero).

CFVPack 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.

The COVPack and CFVPack are calibration constants

that are stored in EEPROM.

VCELL1 and VCELL4 can also be compensated for

impedance in the lines connecting to the PS501.

VC1Res is the resistance between the PS501 and the

highest series cell. VC4Res is the resistance between

the PS501 and the lowest cell. This allows the PS501

to subtract any voltage drop due to current between the

cell and the PS501.

PackResistance = Trace Resistance * 65535

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

1ohm.)

SBData Voltage Value = VC(1) + Measured

Current (mA) * PackResistance/65535

V (mV) = (V_A/D – COVPack) x CFVPack/16384

where:

V_A/D is the internal measurement output

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