DS2711/DS2712: Loose Cell NiMH Charger

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Table 3. CHARGE MODE SELECTION

LED2 PIN STRAPPING

MODE

Low

1-Cell Series

Floating

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 -

ΔV 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. PARALLEL CONFIGURATION WITH EXTERNAL CURRENT REGULATION, 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

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 -

ΔV, 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. SERIES CONFIGURATION WITH EXTERNAL CURRENT REGULATION, 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

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

floating 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 4.