Rainbow Electronics DS2715 - Manual (Page 8)

DS2715: NiMH Cell Pack Charge Controller
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DONE state. If at any time during fast 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.
Top-Off
In top-off mode, the DS2715 regulates the voltage across the sense resistor to -33mV (V
TO
) in linear mode or -
29mV (V
TO
- 0.5 x V
HYS-TO
) in comparator mode. LED1 indicates the cell pack is being charged. The charge timer is
reset and restarted with a time-out 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 DONE state.
DONE/Maintenance
The DS2715 enters the DONE state whenever the charge completes normally or if the measured cell temperature
exceeds 50°C during the charge. While in the done state V
CH
is driven to high impedance to prevent further
charging of the cell pack and LED is driven on to indicate charge completion. A maintenance charge can be
applied to the cells by providing a resistive path from the cell pack to the charge source bypassing the regulator.
See the example circuit in Figure 3. The DS2715 remains in DONE until a cell voltage greater than 1.65V (V
OPEN
) is
detected on V
BATT
indicating the cell pack has been removed. The DS2715 then enters the PRESENCE state and
waits for the next cell insertion.
FAULT
The DS2715 enters FAULT if pre-charge is unable to charge the cell above 1.0V (V
LB
) before the 30 minute pre-
charge timeout of if the cell temperature exceeds 50°C during pre-charge. In the fault state V
CH
is driven to high
impedance and LED2 blinks to indicate the fault condition. The DS2715 remains in FAULT until a cell voltage
greater than 1.65V (V
OPEN
) is detected on V
BATT
indicating the cell pack has been removed. The DS2715 then
enters the PRESENCE state and waits for the next cell insertion.
Discharge Mode
When the DS2715 detects a discharge current voltage drop of V
DCHG-SET
or greater across the sense resistor,
charging is terminated and the DS2715 enters the DISCHARGE state. While in this mode, voltage sensing, thermal
sensing, and LED outputs are disabled. The V
CH
pin is driven low to allow the pack to be discharged. Current drain
of the DS2715 drops to I
DDD
. The DS2715 remains in discharge mode until a charge current of at least V
DCHG-RST
is
detected. When this occurs the DS2715 enters the presence detect state to begin a new charge cycle.
Application Circuit
Figure 3 shows a typical application circuit for charging a 3-cell stack. The mode pin is tied to V
SS
for linear
operation. A 75mΩ sense resistor (R10) sets the charge current at 1.6A. The DS2715 regulates the current through
Q1. 68kΩ on R
T
(R6) sets the fast charge timeout to 102 minutes. R5 and R8 form a 1:3 voltage divider to allow
V
BATT
to measure the voltage of a single cell. The charge source is isolated with diode CR1 to prevent discharge
through the source connection. R1 and CR2 form a maintenance charge path to supply the cell pack with ~40mA
constant current after the charge completes. Lastly VR1 and R4 create a current leakage path to bias the regulator
circuit and prevent an in-rush current spike when the cell stack is connected to the powered circuit.