PS700 datasheet(7/38 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21760F-page 7

PS700

3.1.4.2

External Temperature

For temperature measurement using an external sensor,

the NTC pin supplies a constant current source of

12.5

µA. For proper operation, an industry standard

10 kOhm at 25°C negative temperature coefficient

(NTC) device of the 103ETBtype, should be connected

between NTC and GND. The NTC reference output is

only

enabled

during

external

temperature

measurement in order to minimize power consumption.

The output of the current source, connected to the

external thermistor, produces a voltage range with

limits that correspond to operating temperature limits,

as follows:

-20°C

→ 263 mV

+70°C

→ 317 mV

The output voltage of the external sensor as a function

of temperature can be given as:

VEX (mV) = 263 + 0.6 * (T + 20)

Defined within the ETctrl registers are the settings for the

reference utilized and the resolution desired for mea-

surement of temperature using the external temperature

sensor. Again, the 340 mV reference should be

selected. The following temperature results in a 10-bit

conversion:

LSB (Voltage)

= Full Scale Range/# of steps

= 340 mV/210 = 332

µV/LSB

LSB (°C)

= 332

µV/LSB * (1/600)°C/µV

= 0.553°C/LSB

3.1.5

OFFSET COMPENSATION

The host software can perform offset compensation by

using an offset measurement value read from the

PS700. When the offset calibration is enabled within

the OFFSctrl register, the converter inputs are

internally shorted together and an A/D conversion is

performed at the specified resolution. The offset value

is stored in the OFFSres register.

3.1.6

ACCUMULATION/TIMING

The PS700 incorporates four 32-bit accumulators and

four 32-bit elapsed time counters. The Discharge Current

Accumulator (DCA) and the Charge Current Accumula-

tor (CCA) are intended to record discharge and charge

capacity values. The Discharge Time Counter (DTC) and

the Charge Time Counter (CTC) are intended to maintain

the total discharge time and charge time. Accumulated

charge and discharge values can be used to determine

state of charge of the battery as well as cycle count infor-

mation. With information provided by the elapsed time

counters, average charge and discharge currents over

an extended period of time can be calculated.

Each of the four 32-bit accumulator registers is assigned

a fixed “source” A/D Result register. When the accumu-

lator is enabled, it is updated every 500 ms by adding the

contents of the assigned result register value to the

previous accumulator value. The accumulators are

listed in Table 3-5 with their assigned source registers.

TABLE 3-5:

ACCUMULATOR REGISTERS

The resolution of the accumulated value is equal to the

resolution selected for the associated conversion, up to

a converter resolution of 15-bit plus sign. If a 15-bit plus

sign A/D value is being accumulated, then the

accumulator resolution in microvolt seconds is:

Accumulator LSB (

µVs) = (Full Scale Range/# of steps)

* 0.5s = (340 mV/215) * 0.5s = 5.19

µVs

3.1.7

CHARGE/DISCHARGE

ACCUMULATORS

The DCA register is intended to accumulate discharge

current and the CCA register is intended to accumulate

charge current. Both use the Ires register as its source.

For this reason, in most applications, current measure-

ment defined in the A/D Control registers should be

programmed to measure current by reading the voltage

across the Sense Resistor pin (SR).

During charging, a negative voltage will exist across

the SR pin to ground. Following a conversion, a

positive voltage measurement results in the sign bit = 0

in the Ires register. When the sign bit = 0, the measured

result will be added to the CCA register contents and

the sum is returned to CCA.

In this way, the total charge current will be accumulated

in CCA.

Similarly, during discharge, a positive voltage will exist

between the SR pin and ground. In this case, the

conversion will result with sign bit = 1 in the Ires regis-

ter, indicating a negative value or discharge current

condition. Under this condition, the DCA register will be

updated with the discharge current measured during

that conversion.

The value stored in the DCA or CCA register can be

interpreted as illustrated in the following example.

Using a 16-bit signed conversion for current measure-

ment and a 20 m

Ω sense resistor, the LSB can be

expressed in units of capacity in micro amp seconds as

follows:

Accumulator LSB (

µAs) = Voltage LSB/RSENSE =

(2.59

µVs)/20 mΩ = 130 µAs

The “Accum” bit in the Accumctrl register must be

enabled for accumulation to occur in both the CCA and

DCA registers.

Abbr.

Accumulator Name

Source

DCA

Discharge Current

Accumulator

Ires (Sign bit = 1)

CCA

Charge Current

Accumulator

Ires (Sign bit = 0)

Temperature Accumulator

ITres or ETres

VC2A

VC2 Accumulator

VC2res

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