Datasheet

DS2711/DS2712: Loose-Cell NiMH Chargers
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Table 2. Charge Mode Selection
LED2 PIN STRAPPING MODE
Low 1-Cell Series
Open 2-Cell Series
High Parallel
CC1 and CC2 Outputs
The CC1 and CC2 operate as open-drain outputs that drive active low to connect the charge source to the battery
cell. During charge, the behavior of the CC1 and CC2 outputs depends on the charge-mode configuration. In
parallel mode, CC1 and CC2 are driven low in alternating time slots. The charge source is loaded by just one cell
during any time slot. In the 1-cell and 2-cell series mode, only CC1 is driven. Except for the periodic performance of
impedance and -∆V tests, series mode charging is continuous during the fast charge phase rather than pulsed in
parallel mode.
Parallel Mode Fast Charge
Referring to Figure 4, CC1 controls the PNP switch that gates current to the cell in slot 1. CC2 controls the PNP
switch that gates current to the cell in slot 2. During fast charge, current is gated to each slot sequentially, with
charge pulses occurring in alternating time frames. The cell in one slot charges while the other relaxes and the
effective fast-charge current is 48.4% of the magnitude set by the charge-source current limit. The parallel
configuration skips a charge pulse every 32 clock cycles to facilitate independent testing of the open- and closed-
circuit cell voltages (V
OFF
and V
ON
, respectively). Since the charge regime of each cell is independent, one cell may
complete a charge phase before the other. The more fully charged cell of a pair inserted at the same time could
terminate fast charge by -∆V, then charge in top-off while the less charged cell continues in fast charge. In the case
of an improper or faulty cell (e.g., alkaline) being inserted along with a proper cell (NiMH or NiCd), charging of the
faulty cell would be stopped, while the proper cell is charged to full.
Series Mode Fast Charge
Referring to Figure 3, CC1 controls the PNP switch that gates current to the cell(s). In series mode, 1 or 2 cells can
be charged, depending on whether the 1-cell or 2-cell series mode has been selected. During fast charge, current
is gated to the cell(s) almost continuously, with the effective fast-charge current approximately equal to current limit
of the charge source. The series configuration deactivates CC1 briefly every 32 clock cycles to facilitate
independent testing of V
OFF
and V
ON
of each cell. The one second deactivation makes the duty factor 0.969 and
therefore the effective current equals approximately 97% of the charge-source current limit. In the 2-cell series
mode, the characteristics of each cell are evaluated individually; however charging stops if either cell is determined
to be improper or faulty.
In the 1-cell charge series mode, CC1 gates the charge current as in the 2-cell series mode. The cell voltage is
monitored between VP1 and VN1, and temperature is monitored with THM1. The VP2 and THM2 pins can be left
disconnected in the 1-cell series mode.
EXAMPLE CAPACITIES AND CHARGE RATES
Parallel Charging Example
A 1700mAH cell is charged using a 1A regulated charge source. During fast charge, the cell is charged at a duty
factor of 0.484 and receives an effective charge current of 0.484A. In terms of C-rate, this is 484mA/1700mAh =
0.285°C (or C/3.5). During precharge and top-off, the duty factor is 0.125 (i.e., 1/8), for an effective average current
of 125mA, corresponding to a C-rate of 125/1700 = 0.073C (or C/13.6). Similarly, in maintenance mode, the duty
factor is 0.0156 (i.e., 1/64) and the C-rate is 15.6/1700 = 0.0092 ( or C/109). The C-rates for charging 3 different
cell capacities using a 500mA and a 1000mA current source are shown in Table 3.