DS2711/DS2712: Loose-Cell NiMH Chargers

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Cell Voltage Monitoring

In the 2-cell series mode, the voltage difference between VP2 and VP1 is used to determine the Vcell2 voltage in

the two-cell series stack. The voltage difference between VP1 and VN1 is used to determine the Vcell1 voltage. In

the 1-cell series mode, the difference between VP1 and VN1 is used as the cell voltage. VP2 can be left

disconnected in the 1-cell series mode. In parallel mode, the difference between VP2 and VN1 is used for the

Vcell2 voltage, and the difference between VP1 and VN1 is used for Vcell1 voltage.

fast charge cycle, and when to transition from precharge to fast charge. Once fast charging commences, cell

control pin (CC1 or CC2) controlling current to the cell is active (low). When the charge control pin is active so

and a fault condition is displayed. While fast charge is in progress, cell voltage measurements are stored and

compared to future measurements for charge termination and cell test purposes.

Two types of tests are performed to detect primary alkaline and lithium cells or defective NiMH or NiCd secondary

cells. Cells are tested individually in the series and parallel configurations, so that a single improper or defective

cell can be detected quickly. In the series configuration, a single defective cell will terminate charge for both cells,

whereas the parallel mode continues charging the good cell and stops charging the defective cell.

an 80k

Ω resistor between CTST and V

formula approximates the setting for the detection threshold.

-

ΔV and Flat Voltage Termination

During fast charge, -

∆V detection is performed by comparing successive voltage measurements for a drop of 2mV

in the cell voltage. A hold-off period for -

∆V detection begins at the start of fast charging and prevents false

termination in the first few minutes of the charge cycle. Once the hold-off period expires, cell voltage

measurements are acquired every 32 clock cycles (during the CCx off time). When a newly acquired voltage

measurement is greater than any previous one, the new value is retained as the maximum value. When the cell

voltage no longer increases, the maximum value is retained and compared against subsequent values. If the cell

voltage drops by the -

∆V threshold, V

-

begins.

Top-Off and Maintenance

In top-off mode, the charger scales the cell current to 25% of the fast charge current. The charge timer is reset and

restarted with a timeout period of one-half the fast-charge duration. When the charge timer expires in top-off, the

charger enters maintenance and delivers 1/64 of the charge source current to the cells. Maintenance charge

continuous until power is removed, the cell(s) are removed or the DS2711/DS2712 is cycled into and out of

suspend mode by disconnecting the TMR pin.

Selecting the Charge Mode

The charge mode configuration is selected by testing the LED2 pin during startup. An internal current source tests

the state of the LED2 pin by pulling up and pulling down on the pin to determine if it is high, low, or open. The

recommended pullup or pulldown resistor value (if used) is 100k

Ω. In the parallel charging circuit diagrams on page

7, no resistor is shown. The current path through the LED and 270

Ω resistor is sufficient to pull the LED2 pin high

Characteristics table to select other pullup values.