Datasheet

LTC4011
13
4011fb
operaTion
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 expired, the LTC4011 enters the top-off charge state.
Top-off charge is implemented by sourcing one-tenth the
programmed 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 immediately
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 vari
-
ous charging
phases. The IC also monitors the regulation
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
indi-
cate 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 removal
of a battery by monitoring the V
CELL
pin voltage. Should
this voltage fall below 350mV, the IC considers the bat-
tery
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 terminated
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.
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 o
r Off On or Off On Temperature Limits
Exceeded
Off On Off Off On Fault
State (Latched)