ZHCS229C September 2011 – January 2020 BQ24725A
PRODUCTION DATA.
- 1 特性
- 2 应用
- 3 说明
- 4 修订历史记录
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Parameter Measurement Information
-
8 Detailed Description
- 8.1 Overview
- 8.2 Functional Block Diagram
- 8.3 Feature Description
- 8.4
Device Functional Modes
- 8.4.1 Adapter Detect and ACOK Output
- 8.4.2 Adapter Over Voltage (ACOVP)
- 8.4.3 System Power Selection
- 8.4.4 Battery LEARN Cycle
- 8.4.5 Enable and Disable Charging
- 8.4.6 Automatic Internal Soft-Start Charger Current
- 8.4.7 High Accuracy Current Sense Amplifier
- 8.4.8 Charge Timeout
- 8.4.9 Converter Operation
- 8.4.10 Continuous Conduction Mode (CCM)
- 8.4.11 Discontinuous Conduction Mode (DCM)
- 8.4.12 Input Over Current Protection (ACOC)
- 8.4.13 Charge Over Current Protection (CHGOCP)
- 8.4.14 Battery Over Voltage Protection (BATOVP)
- 8.4.15 Battery Shorted to Ground (BATLOWV)
- 8.4.16 Thermal Shutdown Protection (TSHUT)
- 8.4.17 EMI Switching Frequency Adjust
- 8.4.18 Inductor Short, MOSFET Short Protection
- 8.5 Register Maps
-
9 Application and Implementation
- 9.1 Application Information
- 9.2
Typical Applications
- 9.2.1
Typical System with Two NMOS Selector
- 9.2.1.1 Design Requirements
- 9.2.1.2
Detailed Design Procedure
- 9.2.1.2.1 Negative Output Voltage Protection
- 9.2.1.2.2 Reverse Input Voltage Protection
- 9.2.1.2.3 Reduce Battery Quiescent Current
- 9.2.1.2.4 Inductor Selection
- 9.2.1.2.5 Input Capacitor
- 9.2.1.2.6 Output Capacitor
- 9.2.1.2.7 Power MOSFETs Selection
- 9.2.1.2.8 Input Filter Design
- 9.2.1.2.9 BQ24725A Design Guideline
- 9.2.1.3 Application Curves
- 9.2.2 Simplified System without Power Path
- 9.2.1
Typical System with Two NMOS Selector
- 9.3 System Examples
- 10Power Supply Recommendations
- 11Layout
- 12器件和文档支持
- 13机械、封装和可订购信息
9.2.1.2.4 Inductor Selection
The BQ24725A has three selectable fixed switching frequency. Higher switching frequency allows the use of smaller inductor and capacitor values. Inductor saturation current should be higher than the charging current (ICHG) plus half the ripple current (IRIPPLE):
The inductor ripple current depends on input voltage (VIN), duty cycle (D = VOUT/VIN), switching frequency (fS) and inductance (L):
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging voltage range is from 9V to 12.6V for 3-cell battery pack. For 20V adapter voltage, 10V battery voltage gives the maximum inductor ripple current. Another example is 4-cell battery, the battery voltage range is from 12V to 16.8V, and 12V battery voltage gives the maximum inductor ripple current.
Usually inductor ripple is designed in the range of (20-40%) maximum charging current as a trade-off between inductor size and efficiency for a practical design.
The BQ24725A has charge under current protection (UCP) by monitoring charging current sensing resistor cycle-by-cycle. The typical cycle-by-cycle UCP threshold is 5mV falling edge corresponding to 0.5A falling edge for a 10mΩ charging current sensing resistor. When the average charging current is less than 125mA for a 10mΩ charging current sensing resistor, the low side MOSFET is off until BTST capacitor voltage needs to refresh the charge. As a result, the converter relies on low side MOSFET body diode for the inductor freewheeling current.