LTC4010 Datasheet, PDF (9/20 Pages) Linear Integrated Systems – High Efficiency Standalone Nickel Battery Charger

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OPERATIO (Refer to Figure 1)
beyond a range of 5°C to 60°C, or the internal die tempera-
ture exceeds a resonable value, charging is suspended,
the charge timer is paused and the LTC4010 indicates a
fault condition. Normal charging resumes from the previ-
ous state when the sensed temperature returns to a satis-
factory 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 LTC4010
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 LTC4010
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, cell voltage processing begins
immediately. Otherwise –∆V termination is disabled for a
stabilization period of t_MAX/12. In that case, the LTC4010
makes another fault check at t_MAX/12, requiring the aver-
age 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. Volt-
age-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 temperature rise) are also used in the termination
algorithm. Temperature-based termination, if enabled,
becomes active as soon as the fast charge state is entered.
LTC4010
Top-Off Charge State
If NiMH fast charge termination occurs because the ∆T/∆t
limit is exceeded after an initial period of t_MAX/12 has ex-
pired, the LTC4010 enters the top-off charge state. Top-off
charge is implemented by sourcing one-tenth the pro-
grammed charge current for t_MAX/3 minutes to ensure that
100% charge has been delivered to the battery. The CHRG
status output is active during the top-off state. If NiCd cells
have been selected with the CHEM pin, the LTC4010 never
enters the top-off state.
Automatic Recharge State
Once charging is complete, the automatic recharge state
is entered to address the self-discharge characteristics of
nickel chemistry cells. The charge status output is inactive
during automatic recharge, but V_CDIV remains switched to
GND to monitor the average cell voltage. If the V_CELL
voltage drops below 1.325V without falling below 350mV,
the charge timer is reset and a new fast charge cycle is
initiated.
The internal termination algorithms of the LTC4010 are
adjusted when a fast charge cycle is initiated from auto-
matic recharge, because the battery should be almost fully
charged. Voltage-based termination is enabled immedi-
ately and the NiMH ∆T/∆t limit is fixed at a battery
temperature rise of 1°C/minute.
Fault State
As discussed previously, the LTC4010 enters the fault
state based on detection of invalid battery voltages during
various charging phases. The IC also monitors the regu-
lation of the PWM control loop and will enter the fault state
if this is not within acceptable limits. Once in the fault state,
the battery must be removed or DC input power must be
cycled in order to initiate further charging. In the fault
state, the FAULT output is active, the READY output is
inactive, charging stops and the charge indicator output is
inactive. The V_CDIV output remains connected to GND to
allow detection of battery removal.
Note that the LTC4010 also uses the FAULT output to
indicate that charging is suspended due to invalid battery
or internal die temperatures. However, the IC does not
enter the fault state in these cases and normal operation
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