LTC4011_15 Datasheet, PDF (12/26 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4011
Operation (Refer to Figure 1)
Shutdown State
The LTC4011 remains in micropower shutdown until DCIN
(Pin 1) is driven above V_CC (Pin 18). In shutdown all status
and PWM outputs and internally generated terminations
or supply voltages are inactive. Current consumption from
V_CC and BAT is reduced to a very low level.
Charge Qualification State
Once DCIN is greater than V_CC, the LTC4011 exits
micropower shutdown, enables its own internal supplies,
provides V_RT voltage for temperature sensing, and switches
V_CDIV to GND to allow measurement of the average single-
cell voltage. The IC also verifies that V_CC is at or above 4.2V,
V_CC is 510mV above BAT and V_CELL is between 350mV and
1.95V. If V_CELL is below 350mV, no charging will occur, and
if V_CELL is above 1.95V, the fault state is entered, which
is described in more detail below. Once adequate voltage
conditions exist for charging, READY is asserted.
If the voltage between V_TEMP and GND is below 200mV, the
LTC4011 is paused. If V_TEMP is above 200mV but below
2.85V, the LTC4011 verifies that the sensed temperature is
between 5°C and 45°C. If these temperature limits are not
met or if its own die temperature is too high, the LTC4011
will indicate a fault and not allow charging to begin. If V_TEMP
is greater than 2.85V, battery temperature related charge
qualification, monitoring and termination are disabled.
Once charging is fully qualified, precharge begins (unless
the LTC4011 is paused). In that case, the V_TEMP pin is
monitored for further control. The charge status indicators
and PWM outputs remain inactive until charging begins.
Charge Monitoring
The LTC4011 continues to monitor important voltage and
temperature parameters during all charging states. If the
DC input is removed, charging stops and the shutdown
state is entered. If V_CC drops below 4.25V or V_CELL drops
below 350mV, charging stops and the LTC4011 returns
to the charge qualification state. If V_CELL exceeds 1.95V,
charging stops and the IC enters the fault state. If an external
thermistor indicates sensed temperature is beyond a range
of 5°C to 60°C, or the internal die temperature exceeds an
internal thermal limit, charging is suspended, the charge
timer is paused and the LTC4011 indicates a fault condition.
12
Normal charging resumes from the previous state when
the sensed temperature returns to a satisfactory range. In
addition, other battery faults are detected during specific
charging states as described below.
Precharge State
If the initial voltage on V_CELL is below 900mV, the LTC4011
enters the precharge state and enables the PWM current
source to trickle charge using one-fifth the programmed
charge current. The CHRG status output is active during
precharge. The precharge state duration is limited to
t_MAX/12 minutes, where t_MAX is the maximum fast charge
period programmed with the TIMER pin. If sufficient V_CELL
voltage cannot be developed in this length of time, the fault
state is entered, otherwise fast charge begins.
Fast Charge State
If adequate average single-cell voltage exists, the LTC4011
enters the fast charge state and begins charging at the
programmed current set by the external current sense
resistor connected between the SENSE and BAT pins.
The CHRG status output is active during fast charge. If
V_CELL is initially above 1.325V, voltage-based termination
processing begins immediately. Otherwise –∆V termination
is disabled for a stabilization period of t_MAX/12. In that
case, the LTC4011 makes another fault check at t_MAX/12,
requiring the average cell voltage to be above 1.22V. This
ensures the battery pack is accepting a fast charge. If
V_CELL is not above this voltage threshold, the fault state is
entered. Fast charge state duration is limited to t_MAX and
the fault state is entered if this limit is exceeded.
Charge Termination
Fast charge termination parameters are dependent upon
the battery chemistry selected with the CHEM pin. Voltage-
based termination (–∆V) is always active after the initial
voltage stabilization period. If an external thermistor network
is present, chemistry-specific limits for ∆T/∆t (rate of tem-
perature rise) are also used in the termination algorithm.
Temperature-based termination, if enabled, becomes active
as soon as the fast charge state is entered. Successful
charge termination requires a charge rate between C/2 and
2C. Lower rates may not produce the battery voltage and
temperature profile required for charge termination.
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