LTC4015_15 Datasheet, PDF (31/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Operation
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
(determined by VCHARGE_SETTING + VABSORB_
DELTA, limited to a maximum of 3.8V/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 undervoltage current
limit is activated, the LTC4015 will limit charge current
and will attempt to regulate at the input current or input
voltage limit level, as permitted by the input source and
system load.
LiFePO₄ Absorb Phase Ends After Either:
a) MAX_ABSORB_TIME has passed
b) The battery voltage reaches absorb target voltage (as
determined by VCHARGE_SETTING+VABSORB_DELTA,
limited to a maximum of 3.8V/cell), indicated by
constant_voltage=1, and IBAT falls below C_OVER_X_
THRESHOLD. When the absorb phase ends, the LTC4015
proceeds to the CC-CV charging phase.
Constant-Current/Constant-Voltage (CC-CV) Charging
Constant-Current
When the battery voltage is above 2.9V per cell, the
charger will attempt to deliver the programmed charge
current I_CHG, as set by ICHARGE_TARGET and R_SNSB,
in constant-current mode. Depending on available input
power and external load conditions, the battery charger
may not be able to charge at the full programmed rate. If
input current limit is reached, the system load will always
be prioritized over the battery charge current. Likewise, the
input undervoltage control loop will always be observed
and may limit the power available to charge the battery.
When system loads permit, battery charge current will
be maximized.
The upper limit of charge current I_CHG is programmed
by the combination of the current sense resistor (R_SNSB)
from CSP to CSN and a servo voltage of 32mV or a value
programmed via the serial port (see register descriptions
for ICHARGE_TARGET and icharge_jeita_n). The voltage
across R_SNSB divided by its value determines the maximum
possible charge current. The maximum servo voltage that
can be programmed is 32mV. A 4mΩ resistor, for example,
would have an upper limit charge current of 8A.
Independent of whether or not the charge current loop is
controlling the switching charger, the voltage across the
R_SNSB will represent the actual charge current delivered to
the battery. The voltage across R_SNSB is measured by the
LTC4015’s onboard measurement system and is available
via the serial port in register IBAT.
R_SNSB should be set based on the maximum charge
current of the battery without regard to source or load
limitations from any other control loop. The multiple
control loop architecture of the LTC4015 will correct for
any discrepancies, always sorting out the optimal transfer
of power to the battery.
Battery Series Resistance (BSR) Measurement
The LTC4015 can optionally measure the series resistance
of the battery. If run_bsr is set to 1 the LTC4015 momentarily
suspends the battery charger and calculates the battery
series resistance by dividing the change (charging vs
charger suspended) in battery voltage by the change in
charge current (ICHARGE_BSR). The per cell resistance
value is reported in the BSR register and the change in
charge current is reported in the ICHARGE_BSR register
via the serial port. The LTC4015 resets run_bsr to 0 after
the BSR measurement is complete. The battery series
resistance value is proportional to the charge current
For more information www.linear.com/LTC4015
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