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

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1 Overview

1.1 Description

Lead-acid batteries are widely used in battery-powered devices due to their advantages, such as a stable voltage, low price, simple maintenance, and high reliability. However, there are few chips on the market that are designed specifically for applications that charge lead-acid batteries.

This reference design showcases a lead-acid battery charging solution. The solution uses the MP2659, a highly integrated switching charger designed for portable devices with 3-cell to 6-cell series Li-ion or Li-polymer battery packs.

1.2 Features

  • Up to 36V Operating Input Voltage
  • 45V Maximum Sustainable Voltage When Not Switching
  • Up to 3A Charge Current
  • 1-Cell, 12V Lead Acid Battery
  • 0.5% Reference Voltage Accuracy
  • 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:

MPS Integrated Circuit Description
MP2659 36V, standalone switching charger with integrated MOSFETs, 3-cell to 6-cell series battery pack

Table 1: System Specifications

2.3 System Specifications

Parameter Specification
Input voltage range 4.5V to 36V
Output voltage Up to 14.4V
Maximum output current 3A
Switching frequency 680kHz or 350kHz (under nominal conditions)
Efficiency >92%

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 (CIN, CPMID, and CBATT). 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, QBATT turns on and transfers energy from the battery to the system. When an input source is present, QBATT turns off, and the system’s power is supplied by the input source from QIN.

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):

$$\frac {R_{2}} {R_{1} + R_{2}} = \frac {V_{BATT\_REG}} {V_{BATT\_TERM}}$$

Where VBATT_REG = the number of cells multiplied by VBATT_CELL (set by the CELL and VB pins), and VBATT_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 VIN < 20V.
  2. For applications where VIN exceeds 20V, place a ≥47µF electrolytic capacitor between VIN and GND. Add a Schottky diode with a higher current capacity (e.g. B240A) between VIN and PMID. Use a TVS diode to clamp the VIN 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 fSW = 680kHz or ICC < 2.2A. For applications where fSW = 350kHz and ICC > 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 VIN exceeds 20V.

Pin Condition Recommendations
IN ≤20V input Add a 1µF/50V ceramic capacitor to the IN pin for adaptor applications. Add a ≥47µF capacitor for solar applications. 
>20V input Add a 47µF/50V electrolytic capacitor to the IN pin. A TVS diode is required if the IN voltage exceeds the pin’s maximum voltage rating during the VIN hot-insertion test. 
BATT 3-cell or 4-cell Add a 10µF/50V ceramic capacitor to the BATT pin.
5-cell or 6-cell Add a TVS diode or ≥47µF electrolytic capacitor to the BATT pin. 
PMID - Add a 2.2µF/50V ceramic capacitor (1206 size preferred) to the PMID pin. Add a 2A/40V Schottky diode from IN to PMID. A TVS diode is required if the PMID voltage exceeds the pin’s maximum voltage rating during the VBATT hot-insertion test.

Table 4: Component Selections