DS2711/DS2712: Loose-Cell NiMH Chargers

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DETAILED DESCRIPTION

Charge Algorithm Overview

A charge cycle begins in one of three ways: with the application of power to the DS2711 with cell(s) already

inserted, with the detection of cell insertion after power-up, or when exiting suspend mode with cell(s) inserted. The

charge cycle begins with precharge qualification to prevent fast charging of deeply depleted cells or charging under

extreme temperature conditions. Precharging is performed at a reduced rate until each cell reaches 1V. The

algorithm proceeds to a fast-charge phase, which includes cell tests to avoid accidental charging of alkaline cells or

NiMH cells that are worn-out or damaged. Fast charging continues as long as the cell temperature(s) are less than

50

°C (based on THM1, THM2 voltages) and the open-circuit cell voltage(s) are between 1.0V and 1.75V. Fast

charging terminates by the -

∆V (negative delta voltage) method. The top-off charge phase follows to completely

charge the cells. After the top-off charge timer expires, the maintenance charge phase continues indefinitely to

keep the cells at a full state of charge. Maximum voltage, temperature, and charge-time monitoring during all

charge phases act as secondary or safety termination methods to provide additional protection from overcharge.

Each cell is monitored independently, and in parallel mode the charge phase of each cell is independently

controlled.

Series Charge Configuration

The DS2711/DS2712 series configuration supports one or two-slot stand-alone and one or two cell in-system

chargers. The single-cell-series mode charges one cell while the two-cell-series mode charges two series cells.

Since the cells are charged in series, cell sizes should not be mixed in the series configuration. In the application

example in Figure 3, charge current is gated to the battery cells by a PNP transistor under the control of the CC1

pin of the DS2711. Current regulation is performed outside of this example schematic using the current-sense

feedback provided by the DS2711 CSOUT pin. The DS2712 can also be used in this circuit to provide switch-mode

control on the CSOUT pin. RSNS = 0.125

Ω sets the charge source current, ICHG, to 1A. In series mode, the

effective charge current is 0.969 x ICHG = 969mA.

Figure 3. Series Configuration with External Current Regulation

VDD

LED1

LED2

DMSEL

CTST

TMR

CSOUT

VP1

VP2

THM1

THM2

VN1

VSS

VN0

CC1

CC2

75K

10K

x2

103AT-2

x2

DS2711/12

270

100

0.125

10K

FCX718

+5V

GND

IFB

ICHG

100K

RSNS

0.1