DS2714: Quad Loose Cell NiMH Charger

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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 DS2714 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 Pre-charge qualification to prevent Fast charging of deeply depleted cells or charging

under extreme temperature conditions. Pre-charging is performed at a reduced rate until the cell being charged

charging of alkaline cells or NiMH cells which are worn-out or damaged. Fast charging continues as long as the cell

temperature is less than 50°C (based on Thermistor sensors THM 1, 2), the open circuit cell voltage(s) are

Fast charging terminates by the -

DV (negative delta voltage) or flat voltage method. The Top-Off charge phase

follows to completely charge the cell. After the Top-off charge timer expires, the Maintenance charge phase

continues indefinitely to keep the cells fully charged. Maximum voltage, temperature and charge time monitoring

during all charge phases act as secondary or safety termination methods to provide additional protection from

than 50°C (see Table 1) will result in either Fault or Maintenance depending on which charge state the device was

last in. Each cell is monitored independently, and the charge phase of each cell is independently controlled. If a cell

is removed while being charged, the algorithm for that cell slot is completely reset to its Presence Test state without

affecting the charge control states of the other cells.

CHARGE CONFIGURATION

The DS2714 supports four slot standalone chargers. It alternates charge to the four slots every two seconds, with

one half second available to each cell. Removal or insertion of a cell into the charger does not disturb the charge

timing or charge rates of the other cells. Charge pulses are fed alternately to each cell under the control of the CCx

pins so that the charge regimes occur in parallel. The duty cycle on the CCx pins are completely independent of

one another. Transitions from Pre-charge to Fast charge, Fast charge to Top-off and Top-off to Maintenance occur

independently for each cell. The configuration shown in Figure 3 is for charging four cells with a current limited

source of 2A. The effective average fast charge current for each cell is 2A x 0.25 x 15/16 = 0.469A. The 15/16 term

is due to the fact that every 16

current is delivered to the cell during that time. Mechanical design of the holders is required to prevent insertion of

more than one cell in each slot. The holder design should also prevent electrical contact with reverse polarity

insertion.

PERFORMANCE REQUIREMENTS OVER TEMPERATURE AND VOLTAGE

85

70

50

0

-20

-40

TEMPERATURE, DEGRES

CENTIGRADE

4.0

5.5

Full Performance

Valid NiMH

Charge Range

Low

Temperature

Range

High

Temperature

Range

Below Operating

Voltage Range

Abs. Max

Operating

Range

INTERNAL OSCILLATORS AND CLOCK GENERATION

An internal oscillator provides the main clock source used to generate timing signals for internal chip operation. The

pre-charge timer, hold-off timers, and duty factors for the charging operations are derived from this timebase. There

are two separate timers for the impedance test and Fast Charge/Topoff functions.