PS401-I/SS Microchip Technology, PS401-I/SS Datasheet - Page 5

• Manufacturer Part Number: PS401-I/SS
• Description: IC FUEL GAUGE BATTERY MGR 28SSOP
• Manufacturer: Microchip Technology

Specifications of PS401-I/SS

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2.0
The PS401 A/D converter measures voltage, current
and temperature and integrates the current over time to
measure state-of-charge. The voltage of all battery
cells and the entire pack is monitored, and the pack and
each cell input are individually calibrated for accuracy.
Using an external sense resistor, current is monitored
during both charge and discharge and is integrated
over time using the on-chip oscillator as the time base.
Temperature is measured from the on-chip tempera-
ture sensor or an optional external thermistor. Current
and temperature are also calibrated for accuracy.
2.1
The A/D input channels for current measurement are
the RSHP and RSHN pins. The current is measured
using an integrating method, which averages over time
to get the current measurement and integrates over
time to get a precise measurement value.
A 5 to 600 milli-Ohm sense resistor is connected to
RSHP and RSHN as shown in the example schematic.
The maximum input voltage at either RSHP or RSHN is
+/-150 mV. The sense resistor should be properly sized
to accommodate the lowest and highest expected
charge and discharge currents, including suspend and/
or standby currents.
Circuit traces from the sense resistor should be as
short as practical without significant crossovers or
feedthroughs. Failure to use a single ground reference
point at the negative side of the sense resistor can
significantly degrade current measurement accuracy.
The OTP EPROM value NullCurr represents the zero-
zone current of the battery. This is provided as a
calibration guard band for reading zero current.
Currents below +/- NullCurr (in mA) limit are read as
zero and not included in the capacity algorithm
calculations. A typical value for NullCurr is 3 mA, so
currents between -3 mA and +3 mA will be reported as
zero and not included in the capacity calculations.
The equation for current measurement resolution and
sense resistor selection is:
 2003 Microchip Technology Inc.
9.15 mV / RSENSE (milli-Ohms) = Current LSB
(Minimum current measurement if > NullCurr)
Current LSB x 16384 = Maximum current
measurement possible
A/D OPERATION
Current Measurement
In-circuit calibration of the current is done using the
SMBus interface at time of manufacture to obtain
absolute accuracy in addition to high resolution. The
current measurement equation is:
where:
COCurr is the "Correction Offset for Current" which
compensates
measurement, stored in OTP EPROM.
CFCurr is the "Correction Factor for Current" which
compensates for any variances in the actual sense
resistance over varying currents, stored in OTP
EPROM
Figure 2-1 shows the relationship of the COCurr and
CFCurr values.
FIGURE 2-1:
2.2
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.
A/D Output
COCurr
I(ma) = (I_A/D – COCurr – COD) * CFCurr /16384
I_A/D is the internal measurement.
Auto-Offset Compensation
for
any
COCurr AND CFCurr
VALUE RELATIONSHIP
offset
Ideal A/D
Response
CFCurr
error
PS401
DS40238B-page 5
Actual A/D
Response
Current Input
in
current