Rainbow Electronics DS2710 - Manual (Page 8)

DS2710
Charge-Current Regulation
The DS2710 regulates charge current by maintaining a
constant average voltage across an external sense
resistor connected between the VN1 and VN0 pins.
VN1 and VN0 drive an internal comparator in the
DS2710 to switch the CS output ON and OFF to drive a
regulating pnp bipolar or a pMOS transistor. Hysteresis
on the comparator input provides noise rejection. The
DS2710 regulates the charge current during FAST-
CHARGE to maintain a voltage drop across the sense
resistor as follows:
V SENSE = V IREF - 0.5 x V HYS-COMP = 0.113V (typ)
Figure 3 shows the sense resistor voltage and CS pin
voltage of the regulating circuit during normal operation.
Charging with Load Applied
NiMH cells have a low, but finite, impedance. If load
current is flowing out of the battery, an internal voltage
drop appears at the battery terminals. This can interfere
with the CTEST and -
ΔV detection. If the load current is
variable, early termination is more likely than if the load
current is constant. If the load’s ground is connected to
the negative terminal of the cell (VN0), load current
flows through the current-sense resistor, resulting in less
charge current to the battery. The load-current return
path should be to charger ground to reduce the likeli-
hood of false termination or impedance-test errors.
Charging with load applied is not recommended.
Temperature Monitoring
Accurate temperature sensing is needed to detect tem-
perature FAULT conditions. Connecting an external
10k
Ω NTC thermistor between THM and V
SS
and a
10k Ω bias resistor between V
DD and THM allows the
DS2710 to sense temperature. To accurately monitor
the cell, the thermistor should make physical contact
either to the cell or cell tabs. Table 1 shows several rec-
ommended 10k
Ω thermistors.
MIN, MAX Temperature Compare
The voltage thresholds of the THM input (V THM-MIN ,
V THM-MAX ) are set to allow charging to start if the ther-
mistor temperature is between 0
°C and +45°C when
using the recommended 10k
Ω bias resistor and 10kΩ
thermistor circuit. If precharging is in progress and the
voltage on THM reaches V
THM-STOP , precharging
stops and a FAULT condition is generated. If the volt-
age on THM reaches V
THM-STOP during FAST-
CHARGE or TOP-OFF, charging stops and the DS2710
enters the MAINTENANCE state. FAST-CHARGE and
Single-Cell NiMH Charger
8 _______________________________________________________________________________________
TIME
V CS
V
OH1
V OL1
V
IREF
V SENSE (DC)
V IREF - V HYS-COMP
V SENSE
NOT DRAWN TO SCALE
Figure 3. Ideal Comparator Input and Charge Control Output
Waveforms
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0 10 20 30 40 50
DIVIDER OUTPUT
TEMPERATURE ( °C)
VOLTAGE RATIO
Figure 4. Ratio of THM Pin to V DD Pin Over Temperature
TEMPERATURE ( °C)
THM THRESHOLD RATIO OF V CBIAS
THERMISTOR
RESISTANCE
(k )
Semitec 103AT-2
Fenwal
197-103LAG-A01,
173-103LAF-301
MIN 0.73 27.04 0 +4
MAX 0.33 4.925 +45 +42
STOP 0.29 4.085 +50 +47
Table 1. THM Thresholds