SLUS756D September   2007  – June 2026 BQ24751A

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
    1. 4.1 Terminal Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 Recommended Operating Conditions
    3. 5.3 Package Thermal Data
    4. 5.4 Electrical Characteristics
    5. 5.5 Typical Charateristics
  7. 6 Detailed Description
    1. 6.1  Functional Block Diagram
    2. 6.2  Battery Voltage Regulation
    3. 6.3  Battery Current Regulation
    4. 6.4  Input Adapter Current Regulation
    5. 6.5  Adapter Detect and Power Up
    6. 6.6  Enable and Disable Charging
    7. 6.7  System Power Selector
    8. 6.8  Battery Learn Cycles
    9. 6.9  Automatic Internal Soft-Start Charger Current
    10. 6.10 Converter Operation
    11. 6.11 Synchronous and Non-Synchronous Operation
    12. 6.12 High Accuracy IADAPT Using Current Sense Amplifier (CSA)
    13. 6.13 Input Overvoltage Protection (ACOV)
    14. 6.14 Input Undervoltage Lock Out (UVLO)
    15. 6.15 Battery Overvoltage Protection
    16. 6.16 Battery Shorted (Battery Undervoltage) Protection
    17. 6.17 Charge Overcurrent Protection
    18. 6.18 Thermal Shutdown Protection
    19. 6.19 Adapter Detected Status Register ( ACGOOD Pin)
    20. 6.20 Input Over-Power Protection (ACOP)
      1. 6.20.1 Conditions for ACOP Latch Off:
  8. 7 Application Information
    1. 7.1 Input Capacitance Calculation
    2. 7.2 PCB Layout Design Guideline
  9. 8 Device and Documentation Support
    1. 8.1 Receiving Notification of Documentation Updates
    2. 8.2 Support Resources
    3. 8.3 Trademarks
    4. 8.4 Electrostatic Discharge Caution
    5. 8.5 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

6.7 System Power Selector

The bq24751A automatically switches between connecting the adapter or battery power to the system load. By default, the battery is connected to the system during power up or when a valid adapter is not present. When the adapter is detected, the battery is first disconnected from the system, then the adapter is connected. An automatic break-before-make algorithm prevents shoot-through currents when the selector transistors switch.

The ACDRV signal drives a pair of back-to-back p-channel power MOSFETs (with sources connected together and to PVCC) connected between the adapter and ACP. The FET connected to the adapter prevents reverse discharge from the battery to the adapter when it is turned off. The p-channel FET with the drain connected to the adapter input provides reverse battery discharge protection when off; and also minimizes system power dissipation, with its low Rdson, compared to a Schottky diode. The other p-channel FET connected to ACP separates the battery from the adapter, and provides both ACOC current limit and ACOP power limit to the system. The BATDRV signal controls a p-channel power MOSFET placed between BAT and the system.

When the adapter is not detected, the ACDRV output is pulled to PVCC to turn off the ACFET, disconnecting the adapter from system. BATDRV stays at ACN – 6 V to connect the battery to system.

At 700 ms after adapter is detected, the system begins to switch from the battery to the adapter. The PVCC voltage must be 185 mV above BAT to enable the switching. The break-before-make logic turns off both ACFET and BATFET for 10μs before ACFET turns on. This isolates the battery from shoot-through current or any large discharging current. The BATDRV output is pulled up to ACN and the ACDRV pin is set to PVCC – 6 V by an internal regulator to turn on the p-channel ACFET, connecting the adapter to the system.

When the adapter is removed, the system waits till ACN drops back to within 285 mV above BAT to switch from the adapter back to the battery. The break-before-make logic ensures a 10-μs dead time. The ACDRV output is pulled up to PVCC and the BATDRV pin is set to ACN – 6 V by an internal regulator to turn on the p-channel BATFET, connecting the battery to the system.

Asymmetrical gate drive for the ACDRV and BATDRV drivers provides fast turn-off and slow turn-on of the ACFET and BATFET to help the break-before-make logic and to allow a soft-start at turn-on of either FET. The soft-start time can be further increased, by putting a capacitor from gate to source of the p-channel power MOSFETs.