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2770EAR

Battery Monitor and Charge Controller

Dallas Semiconductor -> Maxim Integrated 

DS2770

SECONDARY CHARGE TERMINATION

Two secondary charge terminations are included. Independent of battery technology selection, charge

terminates if the maximum charge temperature is exceeded or the charge timer expires. The CSTAT1 and

CSTAT0 bits in the Status Register will both be set to 1 to indicate a “charge sequence completed” status.

The maximum temperature threshold, TMCT, is fixed at +50°C. If the temperature measurement exceeds

TMCT during charge, the charge is terminated.

The maximum charge time is set in the Charge Time Register, CTR. CTR is initialized from lockable

EEPROM location address 34h at the start of fast charge ( CC driven low after initiation). CTR

decrements by one count every 56 seconds (tCTR) during fast charge. If CTR reaches zero, charging is

terminated. Since CTR is writable, the value in CTR may be overwritten at any time during a charge to

modify the maximum charge time. Figure 4 shows the format of CTR.

CHARGE TIME REGISTER Figure 4

Address 06

27 26 25 24 23 22 21 20

MSb

LSb

Units: 0.015625h

CURRENT MEASUREMENT

In the Active Mode of operation, the DS2770 continually measures the current flow into and out of the

battery by measuring the voltage drop across a current-sense resistor. The DS2770 is available in two

configurations: 1) internal 25mW current-sense resistor; 2) external user-selectable sense resistor. In

either configuration, the DS2770 considers the voltage difference between pins IS1 and IS2 (VIS = VIS1 -

VIS2) to be the filtered voltage drop across the sense resistor. A positive VIS value indicates current is

flowing into the battery (charging), while a negative VIS value indicates current is flowing out of the

battery (discharging). Note that when an external sense resistor is used, one end of the resistor must be

wired directly to VSS (the negative terminal of the cell) for proper operation of the current measurement

circuitry.

VIS is measured with a signed resolution of 15 bits. Measurements are updated in the Current Register in

two’s complement format every 3.52 seconds. The Current Register value represents the average current

over 3.52 seconds. Currents outside the register’s range are reported at the limit of the range. Figure 5

shows the format of the Current Register.

For the internal sense resistor configuration, the DS2770 maintains the Current Register in units of amps,

with a resolution of 62.5mA and a full-scale range of ±2.048A. The DS2770 automatically compensates

for internal sense resistor process variations and temperature effects when reporting current.

For the external sense resistor configuration, the DS2770 writes the measured VIS voltage to the Current

Register, with a resolution of 1.56mV and a full-scale range of ±51.2mV.

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