LTC4015_15 Datasheet, PDF (39/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Operation
The LTC4015 does not directly sense battery current, but
instead senses V_CSP–CSN developed across R_SNSB. This
voltage is divided by the value of R_SNSB to calculate cur-
rent. The Coulomb counter integrates V_CSP–CSN to infer
charge. The differential voltage across R_SNSB is applied to
a voltage to the frequency converter (V-to-F). When the
integrator portion of the V-to-F output ramps to the CCREFP
or CCREFM level, switches S1, S2, S3 and S4 toggle to
reverse the ramp direction. By observing the state of the
switches and the ramp direction, polarity is determined.
The frequency of this ticking is directly proportional to
V_CSP-CSN. The coulomb counter V-to-F transfer function is:
f_TICK = K_QC • V_{CSP – CSN}
Where K_QC = 8333.33Hz/V
f_TICK is then divided by a prescaler (QCOUNT_PRESCALE_
FACTOR) that is programmable from 1 to 65535 (default of
512). The prescaler effectively increases integration time
by a factor equal to QCOUNT_PRESCALE_FACTOR. At
each under or overflow of the prescaler, the accumulated
voltage register (QCOUNT) is incremented or decremented
one count. The value of accumulated voltage is read via
the I²C interface.
To achieve the specified precision of the Coulomb counter
the differential voltage V_CSP–CSN must stay within ±50mV.
For differential input signals up to ±300mV the Coulomb
counter will remain functional but the precision of the
Coulomb counter is not guaranteed.
The value of external sense resistor, R_SNSB, influences
the gain of the Coulomb counter. A larger sense resistor
gives a larger differential voltage for the same current
which results in more precise Coulomb counting. Thus
the amount of charge represented by the least significant
bit (q_LSB) of the accumulated charge is given by:
q_LSB
=
QCOUNT_PRESCALE_FACTOR
K_QC •R_SNSB
q_LSBunits are in coulombs (Amp-seconds)
where:
K_QC• 8333.33Hz / V
QCOUNT_PRESCALE_FACTOR = value of the
prescaler
R_SNSB is in Ω
If the value of QCOUNT reaches 0 or 65535, the value
saturates. QCOUNT does not wrap.
As long as input power is applied and the battery charger
is enabled, the LTC4015 will allow lead-acid batteries to
remain in a CV state indefinitely. As the battery slowly
discharges itself internally, the LTC4015 replenishes the
lost charge, and this charge is accumulated by the Coulomb
counter. After a long period of time, the Coulomb counter
accumulated charge due to battery self discharge current
while the charger is in CV mode could cause significant
errors relative to the actual SOC of the battery.
CHARGER
CSP
S1
S2
R_SNSB
S3
CSN
S4
I_BAT
+
BATTERY
GND
CCREFP +
–
–
+
+
CCREFM –
CONTROL
LOGIC
POLARITY
DETECTION
PRESCALAR
M
TO
LTC4015
LOGIC/
SERIAL PORT
4015 F10
Figure 10. Coulomb Counter Section of the LTC4015
For more information www.linear.com/LTC4015
4015f
39