MAX17047 Datasheet, PDF(11/47 Page) Maxim Integrated Products

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MAX17047/MAX17050

ModelGauge m3 Fuel Gauge

â¢ Cell Capacity (FullCapNom). This is the total cell

capacity at full, according to the VFG. This includes

some capacity that is not available to the application

at high loads and/or low temperature. The device

periodically compares percent change based on OCV

measurement vs. coulomb-count change as the cell

charges and discharges. This information allows the

device to maintain an accurate estimation of the cellâs

capacity in mAh as the cell ages.

â¢ Voltage Fuel-Gauge Adaptation. The device

observes the batteryâs relaxation response and

adjusts the dynamics of the VFG. This adaptation

adjusts the RCOMP0 register during qualified cell

relaxation events.

â¢ Empty Compensation. The device updates inter-

nal data whenever cell empty is detected (VCELL <

V_empty) to account for cell age or other cell devia-

tions from the characterization information.

Determining Fuel-Gauge Accuracy

To determine the true accuracy of a fuel gauge, as expe-

rienced by end users, the battery should be exercised

in a dynamic manner. The end-user accuracy cannot be

understood with only simple cycles.

To challenge a correction-based fuel gauge, such as

a coulomb counter, test the battery with partial loading

sessions. For example, a typical user may operate the

device for 10min and then stop use for an hour or more.

A robust test method includes these kinds of sessions

many times at various loads, temperatures, and duration.

Refer to Application Note 4799: Cell Characterization

Procedure for a ModelGauge m3 Fuel Gauge.

Initial Accuracy

The device uses the first voltage reading after power-up

or after cell insertion to determine the starting output of

the fuel gauge. It is assumed that the cell is fully relaxed

prior to this reading; however, this is not always the

case. If the cell was recently charged or discharged, the

voltage measured by the device may not represent the

true state of charge of the cell, resulting in initial error in

the fuel gauge outputs. In most cases, this error is minor

and is quickly removed by the fuel gauge algorithm dur-

ing normal operation.

Typical Operating Circuit

The device is designed to mount outside the cell pack

that it monitors. Voltage of the battery pack is measured

directly at the pack terminals by the VBATT and CSP

connections. Current is measured by an external sense

resistor placed between the CSP and CSN pins. An

external resistor-divider network allows the device to

measure temperature of the cell pack by monitoring the

AIN pin. The THRM pin provides a strong pullup for the

resistor-divider that is internally disabled when tempera-

ture is not being measured.

Communication to the host occurs over a standard I2C

interface. SCL is an input from the host, and SDA is an

open-drain I/O pin that requires an external pullup. The

ALRT pin is an output that can be used as an external

interrupt to the host processor if certain application con-

ditions are detected. ALRT can also function as an input,

allowing the host to shut down the device. This pin is

also open drain and requires an external pullup resistor.

Figure 5 is the typical operating circuit.

Multicell Circuit

The MAX17047 can be used in multicell pack applica-

tions. A resistor-divider network divides the pack voltage

down so that the IC monitors the equivalent voltage of a

single cell. The MAX9910 buffers the divider output so

that loading by the MAX17047 does not affect accuracy.

VTT must be connected to a regulated supply in the sys-

tem to prevent overloading the MAX9910. Contact the

factory for a MAX17050 multicell application circuit. See

Figure 6.

Thermistor Sharing Circuit

The MAX17047 can share the cell thermistor circuit

with the system charger. In this circuit, there is a single

thermistor inside the cell pack and a single bias resistor

external to the cell pack. The device shares the same

external bias as the charger circuit and measurement

point on the thermistor. In this configuration, each device

can measure temperature individually or simultaneously

without interference. Alternatively, if the bias voltage in

the charger circuit is not available to the device, a sepa-

rate bias voltage on the VTT pin can be used. For proper

operation, the separate bias voltage must be larger than

the minimum operating voltage of the device, but no

larger than one diode drop above the charger circuit

bias voltage. The MAX17050 cannot be operated in his

configuration. See Figure 7.

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