Rainbow Electronics DS2715 - Manual (Page 10)

DS2715: NiMH Cell Pack Charge Controller
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CURRENT REGULATION
Three basic modes of charging operation are supported by the DS2715: Offline switching through an optocoupler,
linear regulation, and DC input switched mode. The DC Switched method requires a currrent-limited power source.
Mode of operation is selected through the Mode pin. Connecting the Mode pin to V
SS
configures the analog block
as a transconductance amplifier for linear mode of operation. Connecting the Mode pin to the Cbias pin configures
the DS2715 as a comparator for switched mode of operation.
Current-Sense Amplifier Mode
An error amplifier block provides several options to regulate the charge current. The 20mA open-drain output can
drive a PMOS or PNP pass element for linear regulation, or the output can drive an optocoupler for isolated
feedback to a primary-side PWM controller. The SNS- pin is a remote-sense return and should be connected to the
grounded side of the sense resistor using a separate, insulated conductor. During fast charge, an error signal
between the current-sense signal (across the sense resistor) and the internal reference is produced so the voltage
across the sense resistor is maintained at V
FC
in a closed-loop circuit. During top-off the voltage across the sense
resistor is maintained at V
TO
.
Current-Sense Comparator Mode
The comparator in the DS2715 switches between ON and OFF and is capable of driving a PNP bipolar or a PMOS
transistor, enabling the use of a switched-mode power stage. Hysteresis on the comparator input provides noise
rejection. In a closed-loop regulation circuit, the comparator regulates voltage across the sense resistor to a DC
average of:
V
RSNS
= V
FC
- 0.5 x V
HYS-FC
= -0.107V during fast charge
V
RSNS
= V
TO
- 0.5 x V
HYS-TO
= -0.029V during top-off
Charge Rate Selection
The charge rate is determined by an external sense resistor connected between the SNS+ and SNS- pins. The
DS2715 will regulate the charge current to maintain a voltage drop of V
FC
(V
FC
- 0.5 x V
HYS-FC
in comparator mode)
across the sense resistor during fast charge. The sense resistor can therefore be selected by:
Linear
Mode: R = V
FC
/ Desired Fast Charge Current
Comparator Mode: R = (V
FC
- 0.5 x V
HYS-FC
) / Desired Fast Charge Current
Charge Time and Top-Off Time Selection
Pre-charge has a fixed 30 minute limit generated by an internal oscillator. Charge time and top-off time are
controlled by an external resistor from the R
T
pin to V
SS
. Resistors can be selected to support fast-charge time-out
periods of 0.5 to 6 hours and top-off charge time-out periods of 0.25 to 3 hours. If the timer expires in fast-charge,
the timer count is reset and charging proceeds to the top-off charge phase. The top-off time-out period is half of the
fast charge time-out period. If the timer expires in top-off, the DS2715 enters the DONE state. The programmed
charge time approximately follows the equation:
t = 1.5 x R / 1000 (time in minutes)