Outputs

Inputs

bq78PL114

Charge Control

The CHG and PRE outputs are ordinarily used to drive MOSFET transistors controlling charge to the cell stack.

Charge or precharge mode is selected based on the present cell voltage compared to the user-definable cell

precharge, undervoltage, and temperature thresholds. When below these limits, the PRE signal is active and the

CHG signal is inactive. This turns on the precharge MOSFET and is used to charge a depleted system through a

current-limiting series resistor. When all cell voltages are above the limit and the temperature is above the charge

temperature minimum, then the CHG output also becomes active and enables the charge MOSFET to turn on,

providing a high-current path between charger and battery cells.

The CHG and PRE MOSFET control outputs are both disabled (low) when any cell reaches the safety cutoff limit

or temperature threshold. During active charging modes (and above cell voltage thresholds), the discharge

MOSFET is also enabled to avoid excessive heating of the body diode. Similarly, the charge MOSFET is active

during discharge, provided current flow is in the correct direction and no safety violations are present.

The CHG and PRE outputs are intended to drive buffer transistors acting as inverting level shifters.

Discharge Control

The DSG output operates similarly to control-system discharging. It is enabled (high) by default. If a cell voltage

falls below a programmable threshold, or excessive current or other safety related fault is sensed, the DSG

output is disabled (low) to prevent damage to the cells.

All facets of safely charging and discharging the cell stack are controlled by user-definable parameters which

provide precise control over MOSFET states. Both system and cell over- and undervoltage limits are provided, as

well as programmable hysteresis to prevent oscillation. Temperature and current thresholds are also provided,

each with independent timers to prevent nuisance activations.

LEDEN

LEDEN is a dual-function pin. One function is to provide output current to the LED display array. It also serves as

an input that monitors for closure of a state-of-charge indicator (SOCi) push-button switch.

LED SOCi Outputs

LED1–LED5 are current-sinking outputs designed to drive low-current LEDs. The LEDs can be activated by the

LEDEN pin via a pushbutton switch. They can be configured (using SBS parameters) to operate in bar or dot

mode and to use three to five LEDs to represent state-of-charge information.

Current Measurement

Current is monitored by four separate ADCs. All use the same very low-value sense resistor, typically either 5 or

10 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.) It is possible to use even lower values for the sense resistor in very high-current designs by employing

external circuitry. Contact Texas Instruments directly for details.

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.

Copyright © 2008, Texas Instruments Incorporated

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