Datasheet

12
LTC4011
4011fa
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 LTC4011 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
and TOC status outputs are active during the top-off state.
If NiCd cells have been selected with the CHEM pin, the
LTC4011 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 outputs are
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 LTC4011 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 LTC4011 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 outputs
are inactive. The V
CDIV
output remains connected to GND
to allow detection of battery removal.
Note that the LTC4011 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
will resume when all temperatures return to acceptable
levels. Refer to the Status Outputs section for more detail.
Insertion and Removal of Batteries
The LTC4011 automatically senses the insertion or re-
moval of a battery by monitoring the V
CELL
pin voltage.
Should this voltage fall below 350mV, the IC considers the
battery to be absent. Removing and then inserting a
battery causes the LTC4011 to initiate a completely new
charge cycle beginning with charge qualification.
External Pause Control
After charging is initiated, the V
TEMP
pin may be used to
pause operation at any time. When the voltage between
V
TEMP
and GND drops below 200mV, the charge timer
pauses, fast charge termination algorithms are inhibited
and the PWM outputs are disabled. The status and V
CDIV
outputs all remain active. Normal function is fully restored
from the previous state when pause ends.
Status Outputs
The LTC4011 open-drain status outputs provide valuable
information about the IC’s operating state and can be
used for a variety of purposes in applications. Table 1
summarizes the state of the four status outputs and the
V
CDIV
pin as a function of LTC4011 operation. The status
outputs can directly drive current-limited LEDs termi-
nated to the DC input. The V
CDIV
column in Table 1is
strictly informational. V
CDIV
should only be used for the
V
CELL
resistor divider, as previously discussed.
OPERATIO
U
Table 1. LTC4011 Status Pins
READY FAULT CHRG TOC V
CDIV
CHARGER STATE
Off Off Off Off Off Off
On Off Off Off On Ready to Charge
(V
TEMP
Held Low)
or Automatic Recharge
On Off On Off On Precharge or Fast Charge
(May be Paused)
On Off On On On NiMH Top-Off Charge
(May be Paused)
On On On or Off On or Off On Temperature Limits
Exceeded
Off On Off Off On Fault State (Latched)