LTC4011_15 Datasheet, PDF (13/26 Pages) Linear Technology – High Efficiency Standalone Nickel Battery Charger

LTC4011
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
Off
Off Off
Off
On
Off Off
Off
V_CDIV
Off
On
On
Off On
Off On
On
Off On
On On
On
On On or Off On or Off On
Off
On Off
Off On
CHARGER STATE
Off
Ready to Charge
(V_TEMP Held Low)
or Automatic Recharge
Precharge or Fast Charge
(May be Paused)
NiMH Top-Off Charge
(May be Paused)
Temperature Limits
Exceeded
Fault State (Latched)
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