LTC4010_15 Datasheet, PDF (20/24 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4010
Applications Information
When the precharge state timer expires, the LTC4010
begins fast charge if V_CELL is greater than 900mV. The
PWM, charge timer and internal termination control are
suspended if pause is asserted (V_TEMP < 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 greater than 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 disabled
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 returned
to an acceptable range.
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-cell 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. Charging parallel cells is not recommended.
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
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 4-cell NiCd battery pack for
which 800mA represents a 0.75C rate. When connected
to the charger, this pack would provide battery tempera-
ture information and correctly configure both fast charge
termination parameters and time limits for the internal
NiCd cells.
20
–
4010 F11
Figure 11. NiMH Battery Pack Indicating Number of Cells
PCB Layout Considerations
To prevent magnetic and electrical field radiation and
high frequency resonant problems, proper layout of the
components connected to the LTC4010 is essential. Refer
to Figure 12. For maximum efficiency, the switch node
rise and fall times should be minimized. The following
PCB design priority list will help ensure proper topology.
Layout the PCB using this specific order.
4010fb