MP2659 Reference Design: 12V Lead-Acid Battery Charging Solution

REFERENCE DESIGN

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• Input Current Limit Regulation
• Minimum Input Voltage Regulation
• Charge Operation Indicator
• Dead Battery Pack Recovery
• Battery Over-Voltage Protection (OVP)
• Configurable Safety Timer
• Battery NTC Thermal Monitor

1.3 Applications

• Industrial Medical Equipment
• Power Tools
• Robot and Portable Vacuum Cleaners
• Wireless Speakers

2 Reference Design

2.1 Block Diagram

Figure 1 shows a block diagram for a highly integrated switching charger for lead-acid batteries. This application has a 40W output capability and an input voltage up to 36V. To adjust the regulation voltage of the lead-acid batteries, adjust the resistance of the voltage dividers.

Figure 1: Block Diagram

2.2 Related Solutions

This reference design is based on the following MPS solution:

Table 1: System Specifications

2.3 System Specifications

Table 2: System Specifications

3 Design

3.1 Design Method

Figure 2 shows an application circuit to charge lead-acid batteries with OR-selection power path management. The circuit’s power stage uses one inductor (L₁) and three capacitors (C_IN, C_PMID, and C_BATT). With the addition of external components, the complete charging function with power path management can be implemented.

Figure 2: Application Circuit

OR-selection power path management can be realized with two P-channel MOSFETs and other components (e.g. ZD1, ZD2, D1, and resistors). When there is no input source, Q_BATT turns on and transfers energy from the battery to the system. When an input source is present, Q_BATT turns off, and the system’s power is supplied by the input source from Q_IN.

The MP2659 is designed for 3-cell to 6-cell series Li-ion and Li-polymer batteries. Each cells has a regulated battery voltage (3.6V, 4.15V, 4.2V, or 4.35V). To charge a lead-acid battery, there is a specific regulated battery voltage that can be set using resistor dividers (R₁ and R₂). R1 and R₂ can be calculated with Equation (1):

Where V_{BATT_REG} = the number of cells multiplied by V_{BATT_CELL} (set by the CELL and VB pins), and V_{BATT_TERM} is the lead-acid battery’s termination voltage. R₁ should range between 2kΩ and 5kΩ.

3.2 Schematics

Figure 3: MP2659 Solution Schematic

Figure 3 shows MP2659 solution schematic. To create this schematic, follow the guidelines below:

1. This circuit can work safely under applications where V_IN < 20V.
2. For applications where VIN exceeds 20V, place a ≥47µF electrolytic capacitor between V_IN and GND. Add a Schottky diode with a higher current capacity (e.g. B240A) between V_IN and PMID. Use a TVS diode to clamp the V_IN voltage if its voltage spike reaches 45V.
3. Consider the voltage spike on PMID during battery insertion. Add an extra TVS diode to clamp the PMID voltage if its voltage spike reaches 45V.
4. The inductor on this evaluation board can only be used in applications where f_SW = 680kHz or I_CC < 2.2A. For applications where f_SW = 350kHz and I_CC > 2.2A, select an inductor with a higher inductance or higher saturation current.
5. For more component selection information, refer to the MP2659 datasheet.

Table 4 lists recommended components for applications where V_IN exceeds 20V.

Table 4: Component Selections