LTC4010 Datasheet, PDF (17/20 Pages) Linear Integrated Systems – High Efficiency Standalone Nickel Battery Charger

LTC4010
APPLICATIO S I FOR ATIO
and is 500mV above BAT. The READY output is asserted
when the LTC4010 completes charge qualification.
When the LTC4010 determines charging should begin, it
starts a precharge cycle because V_CELL is less than 900mV.
As long as the temperature remains within prescribed
limits, the LTC4010 charges (TGATE switching), applying
limited current to the battery with the PWM in order to
bring the average cell voltage to 900mV.
When the precharge state timer expires, the LTC4010 be-
gins fast charge if V_CELL is greater than 900mV. The PWM,
charge timer and internal termination control are sus-
pended if pause is asserted (V_CELL < 200mV), but all status
outputs continue to indicate charging is in progress. The
fast charge state continues until the selected voltage or
temperature termination criteria are met. Figure 9 sug-
gests termination based on ∆T/∆t, which for NiMH would
be an increase of 1°C per minute.
Because NiMH charging terminated due to ∆T/∆t and the
fast charge cycle had lasted more than t_MAX/12 minutes,
the LTC4010 begins a top-off charge with a current of
I_PROG/10. Top-off is an internally timed charge of t_MAX/3
minutes with the CHRG output continuously asserted.
Finally, the LTC4010 enters the automatic recharge state
where the CHRG output is deasserted. The PWM is dis-
abled but V_CDIV remains asserted to monitor V_CELL. The
charge timer will be reset and fast charging will resume if
V_CELL drops below 1.325V. The LTC4010 enters shutdown
when the DC adapter is removed, minimizing current draw
from the battery in the absence of an input power source.
While not a part of the sample waveforms of Figure 9,
temperature qualification is an ongoing part of the charg-
ing process, if an external thermistor network is detected
by the LTC4010. Should prescribed temperature limits be
exceeded during any particular charging state, charging
would be suspended until the sensed temperature re-
turned to an acceptable range.
Battery-Controlled Charging
Because of the programming arrangement of the LTC4010,
it may be possible to configure it for battery-controlled
charging. In this case, the battery pack is designed to
provide customized information to an LTC4010-based
charger, allowing a single design to service a wide range
of application batteries. Assume the charger is designed to
provide a maximum charge current of 800mA (R_SENSE =
125mΩ). Figure 10 shows a 5V NiCd battery pack for
which 800mA represents a 0.75C rate. When connected to
the charger, this pack would provide battery temperature
information and correctly configure both fast charge ter-
mination parameters and time limits for the internal NiCd
cells.
A second possibility is to configure an LTC4010-based
charger to accept battery packs with varying numbers of
cells. By including R2 of the average cell voltage divider
network shown in Figure 3, battery-based programming
of the number of series-stacked cells could be realized
without defeating LTC4010 detection of battery insertion
or removal. Figure 11 shows a 2.5V NiMH battery pack that
programs the correct number of series cells when it is
connected to the charger, along with indicating chemistry
and providing temperature information.
Any of these battery pack charge control concepts could
be combined in a variety of ways to service custom
application needs.
TIMER CHEM ^VTEMP
8
3
5
BATTERY
PACK
+
NC
66.5k
10k
1200mAhr
NTC
NiCd CELLS
–
4010 F10
Figure 10. NiCd Battery Pack with Time Limit Control
CHEM ^VTEMP
3
5
V_CELL
6
BATTERY
PACK
+
10k
R2
1500mAhr
NTC
NiMH CELLS
–
4010 F11
Figure 11. NiMH Battery Pack Indicating Number of Cells
4010p
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