An Implementation to Adjust I_CC with a Digital PWM Signal

REFERENCE DESIGN

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1 Overview

1.1 Description

Much of today’s electric equipment – such as industrial medical equipment, robot vacuums, drones, and high power speakers – require multiple batteries for power. Some of these devices require the ability to adjust the charging current in real time to fine-tune the device’s performance under different operation modes. For example, many battery manufacturers require specific charging current levels at different temperatures to guarantee battery safety. Although the MP2759 has JEITA to adjust the charging current, a microcontroller (MCU) can enhance user configuration.

This reference design showcases a method to adjust the charging current in real time with a PWM signal from an MCU. This design is based on the MP2759, a highly integrated switching charger designed for applications with 1-cell to 6-cell series Li-ion or Li-polymer battery packs.

1.2 Features

Up to 36V Operation Input Voltage
Up to 3A Charge Current
1-Cell to 6-Cell Series with 3.6V, 4.0V, 4.1V, 4.15V, 4.2V, 4.35V, or 4.4V Battery Regulation Voltage for Each Cell
Input Current Limit Regulation
Input Minimum Voltage Regulation
Supports OR Selection Power Path Management
0.5% Battery Regulation Voltage Accuracy
Charge Operation Indicator
Input Status Indicator
Battery Over-Voltage Protection (OVP)
Charging Safety Timer
Battery Thermal Monitoring and Protection with JEITA Profile

1.3 Applications

Industrial Medical Equipment
Power Tools
Robots and Portable Vacuum Cleaners
Wireless Speakers

2 Reference Design

2.1 Block Diagram

Figure 1 shows a highly integrated switching charger used for applications with 1-cell to 6-cell series Li-ion or Li-polymer battery packs. This solution offers a 40W output capability, a maximum 36V input voltage, and a charging current that can be adjusted in real time with an external PWM signal.

Figure 1: Block Diagram

2.2 Related Solutions

This reference design is based on the following MPS solutions:

MPS Integrated Circuit	Description
MP2759	36V switching charger with power path management for 1-cell to 6-cell batteries

Table 1: System Specifications

2.3 System Specifications

Parameter	Specification
Input voltage range	4V to 36V
Output voltage	Up to 26.4V
Maximum output current	3A
Switching frequency	700kHz or 450kHz (under nominal conditions)
Efficiency	>92%

Table 2: System Specifications

3 Design

3.1 Design Process

Figure 2 shows the application circuit that can charge multiple batteries. This circuit provides the ability to adjust the charging current in real time with power path management. This circuit’s power stage uses one inductor (L₁) and 3 capacitors (C_IN, C_PMID, and C_BATT). With the addition of external components, the complete charging function with power path management can be realized.

Figure 2: Application Circuit

The MP2759 provides a feature that allows the charging current to be regulated. This is accomplished by connecting a resistor (R_ISET) between the ISET pin and AGND. The voltage on the ISET pin is fixed at about 1.2V. The relationship between R_ISET and charging current can be calculated with Equation (1):

$$I_{CHG}= \frac {96(k\Omega)}{R_{ISET}(k\Omega)}$$

Figure 3 shows the equivalent R_ISET circuit. It is possible to change the equivalent R_ISET by modifying the duty cycle of the PWM signal from an MCU. This means that the charging current can be adjusted in real time.