Rainbow Electronics DS2710 - Manual (Page 7)

the MAINTENANCE state. If at any time during FAST-
CHARGE the cell voltage exceeds V
MAX-CHARGE , the
DS2710 determines that the cell is either overcharged
or has been removed, and enters the FAULT state.
CELL TEST
CELL TEST is performed once every 31s during FAST-
CHARGE to determine if charging is complete. During
CELL TEST, the CS output is held high to prevent
charging. The cell’s voltage is measured and com-
pared against prior readings. The maximum cell volt-
age measurement during the charge is retained. If a
cell’s voltage falls more than 2mV (V
- ΔV ) from its peak
reading, the FAST-CHARGE terminates successfully
and moves to TOP-OFF. The DS2710 also moves to
TOP-OFF if the cell’s voltage reading does not exceed
the maximum over a 16min period (t
FLAT ). A hold-off
period for -
ΔV and flat voltage detection begins at the
start of fast charging and prevents false termination in
the first 4min of the charge cycle (t
THO ).
The impedance of the cell is also measured during
CELL TEST. The cell’s open-circuit voltage is compared
against the voltage of the cell under charge. The differ-
ence is compared against the impedance threshold set
by the CTEST pin. If the difference exceeds the thresh-
old set by CTEST, the cell’s impedance is considered
to be too high for charging and the DS2710 enters the
FAULT state. The DS2710 also enters FAULT state if
any voltage reading in CELL TEST exceeds the
V
MAX-OPEN threshold.
TOP-OFF
In the TOP-OFF state, the DS2710 charges at 25% the
rate of FAST-CHARGE. The voltage across the sense
resistor is regulated to 113mV with a 25% duty cycle.
The STATUS output is held high to indicate the cell
pack is being charged. The charge timer is reset and
restarted with a timeout period of one-half the
FAST-CHARGE duration. When the charge timer
expires or if the measured temperature exceeds
+50°C, the charger enters the MAINTENANCE state. If
the cell voltage is greater than V MAX-CHARGE during
the 25% of time when charge current is applied or
V
MAX-OPEN during the remaining time, TOP-OFF is exit-
ed early and the DS2710 goes to FAULT.
MAINTENANCE
The DS2710 enters the MAINTENANCE state whenever
the charge completes normally or if the measured cell
temperature exceeds +50°C during the charge. The
STATUS pin is driven low to indicate TOP-OFF has
completed. The cell’s state of charge is maintained
indefinitely by continuing a 1.56% duty-cycle charge of
the cell. The DS2710 remains in the MAINTENANCE
state until the cell is removed, the DS2710 is power
cycled, or the DS2710 is forced into SUSPEND state.
FAULT
The DS2710 can enter FAULT from any charge state if
the cell voltage exceeds V
MAX-CHARGE any time when
charge current is applied (CS low) or V
MAX-OPEN
at any
time when no charge current is flowing (CS high). In
addition, FAULT can be entered during PRECHARGE if
the cell’s temperature exceeds +50°C or the
PRECHARGE timer expires, or during FAST-CHARGE if
impedance threshold is exceeded. In the FAULT state,
CS is forced high to prevent charging and the STATUS
output toggles at a 4Hz rate to indicate that an error
has occurred. The DS2710 remains in FAULT until a
cell voltage greater than 1.75V (V
MAX-CHARGE ) is
detected, indicating that the cell has been removed.
The DS2710 then enters the PRESENCE state and waits
for the next cell insertion.
SUSPEND
Suspension of charge activity is possible by floating the
TMR pin (R
TMR > R SUSPEND ). The CS output is pulled
to V DD to disable the charge control FET to prevent cur-
rent flow to the cell. When the TMR connection is
restored, charging begins from the PRESENCE state
with all timers reset. The SUSPEND state is useful as a
means to stop charging by the application circuit, such
as with a microcontroller signal.
DS2710
Single-Cell NiMH Charger
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