LTC4011_15 Datasheet, PDF (21/26 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4011
Applications Information
Unlike all of the other applications discussed so far, the
battery continues to power the system during charging.
The MCU could be powered directly from the battery or
from any type of post regulator operating from the battery.
In this configuration, the LTC4011 relies expressly on the
ability of the host MCU to know when load transients will
be encountered. The MCU should then pause charging
(and thus –∆V processing) during those events to avoid
premature fast charge termination. If the MPU cannot reli-
ably perform this function, full PowerPath control should
be implemented. In most applications, there should not be
an external load on the battery during charge. Excessive
battery load current variations, such as those generated
by a post-regulating PWM, can generate sufficient voltage
noise to cause the LTC4011 to prematurely terminate a
charge cycle and/or prematurely restart a fast charge. In
this case, it may be necessary to inhibit the LTC4011 after
charging is complete until external gas gauge circuitry
indicates that recharging is necessary. Shutdown power
is applied to the LTC4011 through the body diode of Q2
in this application.
Waveforms
Sample waveforms for a standalone application during
a typical charge cycle are shown in Figure 10. Note that
these waveforms are not to scale and do not represent the
complete range of possible activity. The figure is simply
intended to allow better conceptual understanding and to
highlight the relative behavior of certain signals generated
by the LTC4011 during a typical charge cycle.
Initially, the LTC4011 is in low power shutdown as the
system operates from a heavily discharged battery. A DC
adapter is then connected such that V_CC rises above 4.25V
and is 500mV above BAT. The READY output is asserted
when the LTC4011 completes charge qualification.
When the LTC4011 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 LTC4011 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 LTC4011
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 10 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 LTC4011 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 and TOC outputs continuously
asserted.
Finally, the LTC4011 enters the automatic recharge state
where the CHRG and TOC outputs are 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 LTC4011 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 10,
temperature qualification is an ongoing part of the charg-
ing process, if an external thermistor network is detected
by the LTC4011. Should prescribed temperature limits be
exceeded during any particular charging state, charging
would be suspended until the sensed temperature returned
to an acceptable range.
Battery-Controlled Charging
Because of the programming arrangement of the LTC4011,
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 LTC4011-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 11 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.
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