Datasheet

Inputs
bq78PL114
SLUS850B – SEPTEMBER 2008 – REVISED APRIL 2009 ................................................................................................................................................ www.ti.com
Current Measurement
Current is monitored by four separate ADCs. All use the same very low-value sense resistor, typically 10, 5, 3, or
1 milliohms in series with the pack negative connection. CCBAT and CCPACK connections to the sense resistor
use an R/C filter for noise reduction. (CSBAT and CSPACK are direct connections used for secondary safety).
The bq78PL114 requires a 5-milliohm sense resistor. The bq78PL114S12 can be configured to use 10-, 3-, or
1-milliohm sense resistors. When configured to use a 1-milliohm sense resistor, the maximum available pack
capacity increases to 655 Ah from 65 Ah.
A 14-bit delta-sigma ADC is used to measure current flow accurately in both directions. The measurements are
taken simultaneously and synchronously with all the cell voltage measurements, even those cells measured by
bq76PL102 dual-cell battery monitors.
Coulomb Counting
A dedicated coulomb counter is used to measure charge flow with 18-bit precision in both directions by a
calibrated, integrating delta-sigma ADC. This allows the bq78PL114 to keep very accurate state-of-charge (SOC)
information and battery statistics. A small deadband is applied to further reduce noise effects. The coulomb
counter is unique in that it continues to accumulate (integrate) current flow in either direction even as the rest of
the internal microcontroller is placed in a very low power state, further lowering power consumption without
compromising system accuracy.
Safety Current
Two additional ADCs are used to directly monitor for overcurrent or short-circuit current conditions, independently
of the internal function. This provides a direct and rapid response to insure pack integrity and safe operation by
opening the appropriate MOSFETs. These functions are implemented in hardware, and do not require firmware
for functionality.
Voltage Measurement
Voltage measurement is performed by four independent delta-sigma ADCs which operate simultaneously and
are triggered synchronously so that all voltages are read at precisely the same moment. The bq78PL114
coordinates the attached bq76PL102 dual-cell battery monitors so they also perform their cell voltage
measurements in sync with the bq78PL114 voltage and current measurements. Voltage measurements are
converted with better than 1 mV of resolution, providing superior accuracy. One-ADC-per-cell technology means
that voltage is also measured simultaneously with current, permitting accurate, real-time cell impedance
calculation during all operating conditions. This technique also provides greatly enhanced noise immunity and
filtering of the input signal without signal loss.
Temperature Measurement
XT1 – XT4 are dedicated temperature-sensor inputs. Each external sensor consists of a low-cost silicon diode
(dual diode in one package is recommended) and capacitor combination. Temperatures for cells 5 and above are
measured by up to four external bq76PL102(s), and the temperature data is received by the bq78PL114 or
bq78PL114S12 over the PowerLAN link for processing. The bq78PL114 or bq78PL114S12 can report all of
these temperatures individually. The bq78PL114 allows multiplexed configuration of up to twelve temperature
sensors. This permits the pack designer to measure temperatures of all cells and other items such as the charge
and discharge MOSFETs. The bq78PL114S12 firmware uses the internal temperature sensor of the device for
board temperature measurements.
EFCIx
The external MOSFET control inputs are for user control of MOSFETs based on external circuitry and conditions.
The polarity of the input signal is user-programmable. These pins can be used to force the protection MOSFETs
to an OFF state.
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