An Implementation to Adjust ICC with a Digital PWM Signal


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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 (CIN, CPMID, and CBATT). 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 (RISET) between the ISET pin and AGND. The voltage on the ISET pin is fixed at about 1.2V. The relationship between RISET and charging current can be calculated with Equation (1):

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

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

Figure 3: Equivalent RISET Circuit

3.4 PCB Layout

The PCB layout in Figure 5, Figure 6, Figure 7, and Figure 8 refers to the standard MP2759 evaluation board.

Figure 5: Top Layer

Figure 7: Middle Layer 2

Figure 6: Middle Layer 1

Figure 8: Bottom Layer

4 Test Results

4.1 Efficiency

L = 10µH/35mΩ, fSW = 700kHz, RSNS = 20mΩ, and TA = 25°C.

Constant voltage mode

Figure 10: Efficiency vs. Charge Current

4.3 Thermal Measurements

L = 10µH/35mΩ, RSNS = 20mΩ, fSW = 700kHz, TA = 25°C, and burns in 20 minutes. Board information: 63.5mmx63.5mm, 4-layer, 1oz/layer.

VIN = 36V, VBATT = 24V, ICC = 2A,

TRISE = 67.3°C to 25°C = 42.3°C

Figure 27: Thermal Image

VIN = 16V, VBATT = 12V, ICC = 3A,

TRISE = 70.8°C to 25°C = 45.8°C

Figure 28: Thermal Image