LTC4015_15 Datasheet, PDF (36/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Operation
and charge current has dropped below the C/x threshold,
or the battery absorb charge voltage is reached and the
maximum absorb phase time has passed. If neither of
the criteria for exiting the absorb charge phase is met
then CC-CV charging with V_ABSORB charge voltage can
continue indefinitely. Upon exiting the absorb phase the
charge voltage absorb delta is set to zero and CC-CV float
charging continues indefinitely.
Upon exiting the absorb phase an equalization charge can
be performed upon user request.
Lead Acid Equalization Charge
The equalization charge voltage, V_EQUALIZE, can be signifi-
cantly higher than the absorption voltage. This aggressive
overcharging of the battery can equalize acid concentrations
throughout the battery and remove electrode sulfation that
may have formed during low charge conditions. Equal-
ization can restore battery capacity, but it can also result
in battery heating, overcharging of some or all cells and
the loss of electrolyte which can lead to battery damage.
Equalization is typically not performed with sealed batter-
ies because they are usually not re-wettable in the event
of electrolyte loss. Due to its aggressive nature, specific
equalization frequency, voltage and time duration should
be obtained from the battery manufacturer.
Battery Detection
The LTC4015 begins a charging cycle by performing a
two to four second battery detection test, during which
a 1mA load is drawn from the battery. If the battery volt-
age remains stable during the battery detection test, the
LTC4015 proceeds with battery charger soft-start. If the
battery voltage does not remain stable, the LTC4015
proceeds with a battery open/short test. The battery is
charged at minimum charge current for one to two seconds.
If the battery voltage as a result of this brief charging is
within a reasonable range the LTC4015 will proceed with
a battery charger soft-start. A battery open fault will also
occur if the NTC resistor is open or has a very high value.
A programmable interrupt can be set to alert the system
if a battery detection fault has occurred.
Battery Charger Soft-Start
The LTC4015 softs starts by ramping ICHARGE_DAC from
minimum to ICHARGE_TARGET at a nominal rate of 400µS
per ICHARGE_TARGET LSB. This results in a maximum
soft-start time of 31 • 400µS = 12.4mS.
Low Battery
If the BATSENS pin voltage is lower than 2.4V, the switching
charger operates in constant peak current mode, where the
peak current is 7mV/R_SNSB. Exact average current value
depends on a number of factors including input voltage,
battery voltage and inductance value. When the BATSENS
pin voltage is higher than 2.6V, normal charging proceeds.
Absorb Charge
The LTC4015 begins lead-acid battery charging with an
absorb charge phase, and reports absorb_charge = 1
via the serial port. During absorb charging, the LTC4015
charges at a constant-current I_CHG, with a target set by
ICHARGE_TARGET and R_SNSB, unless one of the following
conditions occurs:
a) The battery voltage reaches the absorb target
voltage (V_ABSORB)(determined by VCHARGE_
SETTING+VABSORB_DELTA, limited to a maximum
of 2.6V/cell), indicated by constant_voltage=1
b) Input current limit (IIN_LIMIT_SETTING) is reached,
indicated by iin_limit_active=1
c) The UVCLFB pin voltage falls to the undervoltage current
limit (VIN_UVCL_SETTING), indicated by vin_uvcl_ac-
tive=1
If either the input current limit or the input undervoltage
current limit is activated, the LTC4015 will limit charge
current and will attempt to regulate at the input current
limit or input undervoltage limit, as permitted by the input
source and system load.
Absorb phase ends after the battery voltage reaches V_ABSORB
and either ABSORB_TIMER reaches MAX_ABSORB_TIME
or IBAT falls below C_OVER_X_THRESHOLD. When the
absorb phase ends, the LTC4015 proceeds to constant-
current/constant-voltage (CC-CV) charge phase.
4015f
36
For more information www.linear.com/LTC4015