LTC4015_15 Datasheet, PDF (40/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Operation
Coulomb Counter Applications
The following examples demonstrate simple applications of
the LTC4015 Coulomb counter function. For more advanced
applications, including calibration of state-of-charge based
on temperature, please contact Linear Technology Applica-
tions Engineering for more information.
EXAMPLE: Setting QCOUNT_PRESCALE_FACTOR and
initializing and calibrating QCOUNT: Charging two Li-Ion
3.5Ah batteries in parallel at 6.4A total
Total capacity = 7.0 Ah = 7.0A • 3600sec = 25200C
Constant-current charging at 6.4A requires R_SNSB =
32mV/6.4A = 5mΩ
The maximum value of the QCOUNT register is 65535.
To prevent over ranging QCOUNT, the highest allowed
value of q_LSB is 25200C/65535 = 0.385C
Using:
q_LSB
=
QCOUNT_PRESCALE_FACTOR
K_QC •R_SNSB
results in q_LSB = 0.024 • QCOUNT_PRESCALE_FACTOR.
In order to achieve a q_LSB of at least 0.385C, QCOUNT_
PRESCALE_FACTOR must be at least q_LSB • (K_QC •
R_SNSB) = 0.385 • (8333.33 • 0.005). Doubling (to allow
for margin) and rounding to the nearest integer gives
QCOUNT_PRESCALE_FACTOR = 32.
The initial state of charge (SOC) of the battery is known
to be approximately 25%. Given that the QCOUNT_
PRESCALE_FACTOR = 32 has now been set for ap-
proximately twice the capacity of the battery, the desired
valid range of QCOUNT is from 16384 (to represent 0%
SOC) to 49152 (to represent 100% SOC). The value of
QCOUNT is thus initialized to (32768 • 0.25) + 16384
= 24576 to represent the approximate 25% initial SOC.
In this case, the SOC of the battery at any time can be
calculated as 100% • (QCOUNT–16384)/32768.
Upon the first termination (c_over_x_term=1 or
timer_term=1), the battery is known to be very near
100% SOC, so the value of QCOUNT can be calibrated
accordingly. For example, if the initial state of charge
was actually 23% instead of the expected 25%, termina-
tion will occur when QCOUNT is approximately 49806.
Upon the first termination, the value of QCOUNT is over
written to 49152 to calibrate QCOUNT to the true SOC
of the battery.
EXAMPLE: Coulomb Counter Charge Termination
The Coulomb counter alert limits can be used to imple-
ment a maximum state of charge termination algorithm.
The following example demonstrates such a procedure:
1) Set the Coulomb counter register (QCOUNT) to the
battery's initial state of charge, and set QCOUNT_
PRESCALE_FACTOR based on the battery capacity, as
indicated in the example above.
2) Set QCOUNT_HI_ALERT_LIMIT to the desired maximum
state of charge of the battery (e.g. QCOUNT_HI_ALERT_
LIMIT = 49152)
3) Set en_qcount_hi_alert=1
4) If the battery charges enough that QCOUNT exceeds
QCOUNT_HI_ALERT_LIMIT, the LTC4015 will issue
an SMBALERT and set qcount_hi_alert=1. After
completing the alert response algorithm and verifying
that the LTC4015 is issuing qcount_hi_alert=1, set
suspend_charger=1 to stop the battery from charging.
At this time, write QCOUNT_LO_ALERT_LIMIT to a
lower level corresponding to a recharge threshold
(e.g. QCOUNT_LO_ALERT_LIMIT = 0.95 • QCOUNT_
HI_ALERT_LIMIT) and set en_qcount_lo_alert=1 and
en_qcount_hi_alert=0.
5) When the battery discharges enough that QCOUNT falls
below QCOUNT_LO_ALERT_LIMIT, the LTC4015 will
issue an SMBALERT and set qcount_lo_alert=1. After
completing the alert response algorithm and verifying
that the LTC4015 is issuing qcount_lo_alert=1, set
en_qcount_lo_alert=0, en_qcount_hi_alert=1, and
suspend_charger=0 to allow the battery to charge again.
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