Rainbow Electronics DS2715 - Manual (Page 7)

DS2715: NiMH Cell Pack Charge Controller
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DETAILED DESCRIPTION
Charge Cycle Overview
The DS2715 regulates the charge of up to 10 NiMH cells in a series configuration. With the mode select pin the
DS2715 can be configured to regulate either as an error amplifier in linear mode or as a comparator in switched
mode. A charge cycle begins in one of two ways: with the application of power to the DS2715 with cell pack already
inserted or with the detection of cell insertion after power-up.
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 each cell reaches 1V. The
algorithm proceeds to a fast-charge phase. Fast charging continues as long as the cell pack temperature is less
than 50
°C based on the THM voltage, the cell voltage as measured by the V
BATT
pin remains below 1.65V
indicating the cell pack is still present. Fast charging terminates by measuring the cell pack’s thermal rate of
change dT/dt. When the cell pack’s thermal rate of change exceeds 0.5°C per minute the DS2715 enters top-off.
The DS2715 has an internal charge timer as secondary overcharge protection if the charge is not terminated by the
dT/dt method. The charge termination timer duration is user selectable from 30 minutes up to 6 hours by an
external resistor on the R
T
pin.
The DS2715 remains in top-off for one-half of the period of the fast charge timer duration as selected by the
external resistor on R
T
. After the top-off charge timer expires, the done phase continues indefinitely until the cell
pack is removed from the charger or a load is attached. When a load attached to the cell pack, the DS2715
switches to discharge mode. All charge functions are disabled and the regulation FET is driven on to allow the cell
pack to discharge. Two LED indicators display the charge status to the user.
Undervoltage Lockout (Reset)
The UVLO circuit serves as a power-up and brownout detector by monitoring V
DD
to prevent charging until V
DD
rises above V
UVLO
, or when V
DD
drops below V
UVLO
- V
UVLO-HYS
. If UVLO is active, charging is prevented, the state
machine is forced to the RESET state, and all charge timers are reset. A 10 ms deglitch circuit provides noise
immunity. Once V
DD
reaches an acceptable operating voltage the DS2715 enters the PRESENCE state.
Presence
The DS2715 enters the PRESENCE state whenever V
DD
> V
UVLO
and V
BATT
> V
OPEN
indicating that the charge
source is present, but no cell is available to charge. The DS2715 will remain in the presence state until a cell is
inserted into the circuit causing the voltage on V
BATT
to fall below 1.55V (V
DET
) and the cell temperature is inside a
valid charging range between 0°C and 45°C (T
THM-MIN
and T
THM-MAX
). If both these conditions are met the DS2715
will enter pre-charge. If cells are inserted, but the temperature is outside the valid charging range the DS2715 will
remain in the PRESENCE state until the cell temperature falls within the valid charging range.
Pre-Charge
The DS2715 enters the PRE-CHARGE state when a valid cell voltage is applied to V
BATT
and the cell temperature
as measured by the DS2715 is within the valid charging range. Pre-charge has a 4 second filter to suppress noise
on V
BATT
caused by cell insertion. The DS2715 pre-charges the cell by regulating the voltage drop across the sense
resistor to -33mV (V
TO
) in linear mode or -29mV (V
TO
- 0.5 x V
HYS-TO
) in comparator mode. Pre-charge will last until
the cell voltage measured by V
BATT
exceeds 1.0V (V
BATT
> V
LB
) at which time the DS2715 will enter FAST CHARGE
state. If the cell voltage does not exceed V
LB
within 30 minutes or if the cell temperature exceeds 50°C at any time
during pre-charge, the DS2715 enters the FAULT state. If at any time during pre-charge the voltage on V
BATT
exceeds 1.65V (V
OPEN
), the DS2715 determines that the cell pack has been removed and returns to the
PRESENCE state.
Fast Charge
In fast charge mode, the DS2715 regulates the voltage across the sense resistor to -121mV (V
FC
) in linear mode or
-107mV (V
FC
- 0.5 x V
HYS-FC
) in comparator mode. LED1 indicates the cell pack is being charged. During fast
charge the DS2715 constantly measures the rate of change of the cell temperature (dT/dt). When the cell pack’s
dT/dt exceeds 0.5°C per minute (T
TERM
) the DS2715 enters the TOP-OFF state. The DS2715 ignores changes in
the cell temperature caused by charge initiation for the first 4.3 minutes (t
BLANK
). As secondary overcharge
protection, the DS2715 will terminate fast charge and enter top-off based on a time delay set by the external
resistor on the R
T
pin. This resistor value can set the secondary charge termination delay to anywhere from 30
minutes up to 6 hours. If the cell temperature exceeds 50°C at any time during fast charge, the DS2715 enters the