SLUAAT5 December   2023 BQ25756

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2LiFePO4 Charging Profile
  6. 3BQ25756 Settings For LiFePO4
    1. 3.1 Set Charging Voltages
      1. 3.1.1 Regulation Voltage
      2. 3.1.2 Recharge Voltage
      3. 3.1.3 Battery Low Voltage
    2. 3.2 Set Charging Current
      1. 3.2.1 Charging Current
        1. 3.2.1.1 ICHG Pin Pull Down Resistor
        2. 3.2.1.2 ICHG_REG Register Bits
      2. 3.2.2 Pre-Charge Current
      3. 3.2.3 Termination Current
    3. 3.3 TS Pin Setting
      1. 3.3.1 JEITA Enabled
      2. 3.3.2 JEITA Disabled
      3. 3.3.3 NTC Monitoring Disabled
  7. 4Supercapacitor Charging Profile
  8. 5SBQ25756 Settings For Supercapacitor
    1. 5.1 Charge Current Setting
    2. 5.2 Charge Voltage Setting
    3. 5.3 TS Pin Setting
  9. 6Test Results From EVM
    1. 6.1 Charging LiFePO4 Battery
    2. 6.2 Charging Supercapacitor
  10. 7Summary
  11. 8References

6.1 Charging LiFePO4 Battery

On the EVM original RBOT of 13.7 kΩ is replaced with 30.1 kΩ which translates to VBATREG being 14.25 V. This means we can charge 14.25 V/3.6 V or about 4-cell battery. To set VBATREG more accurately which is 4x3.6 V=14.4 V, VFB_REG register is set to 1.552 V using bqStudio. The charging current is set to 2 A, pre-charge current is set to 500 mA and trickle charge current is set to 250 mA. VBAT_LOWV is set to 55% × VFB_REG and VRECHG is set to 93% × VFB_REG. The input voltage to the charger is at 31 V. With a heavily depleted battery, the charger starts charging with a trickle charge when charging is enabled by setting EN_CHG. When the battery voltage reaches pre-charge threshold, the charger employs pre-charge current. When the battery voltage is close to 7.9 V, the battery charges with full fast charge current. When the battery reaches 14.4 V the charge current is tapered down to termination current and the charging cycle is complete. Charge status register is shown as ‘charge done’. During the charging the STAT1 pin is ON and STAT2 pin is OFF indicating charge in progress. When charging is done STAT1 pin is OFF and STAT2 pin is ON.

GUID-20231205-SS0I-5FKG-QF8K-KRPDSLTHLHWB-low.svg Figure 6-1 LiFePO4 Battery Charging Cycle on an EVM