LTC4010_15 Datasheet, PDF (19/24 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4010
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 bat-
tery. In this configuration, the LTC4010 relies expressly
on the ability of the host MCU to know when load tran-
sients 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 reliably perform this function, the battery should
be disconnected from the load with a rectifier or switch
when charging. 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 LTC4010 to prematurely terminate a
charge cycle and/or prematurely restart a fast charge. In
this case, it may be necessary to inhibit the LTC4010 after
charging is complete until external gas gauge circuitry
indicates that recharging is necessary. Shutdown power
is applied to the LTC4010 through the body diode of the
P-channel MOSFET in this application.
Waveforms
Sample waveforms for a standalone application during
a typical charge cycle are shown in Figure 9. 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 LTC4010 during a typical charge cycle.
Initially, the LTC4010 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 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.
SHDN
V_CC
PRECHARGE
V_CC ≥ BAT + 500mV
FAST CHARGE
TOP-OFF
AUTO
RECHARGE
SHDN
V_CC < BAT + 25mV
READY
V_CDIV
TGATE
0.9V
V_CELL
V_TEMP
(PAUSE)
200mV
CHRG
EXTERNAL
PAUSE
4010 F09
Figure 9. Charging Waveforms Example
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