AN155 - Using a Microcontroller to Adjust the MP2759's Charging Current

Although the charging current can be adjusted with R_ISET, this method is unreliable for real-time regulation. By using the PWM of an MCU, the charging current can be adjusted in real time with greater reliability. Figure 1 shows the standard schematic for the MP2759.

Figure 1: MP2759 Application Schematic

METHOD DETAILS

The MP2759 regulates the charging current with different resistor values read by the ISET pin. The ISET pin maintains a constant voltage of 1.2V, meaning the charging current can be adjusted by changing the equivalent resistor between the ISET pin and AGND. This change can be initiated by modifying the MCU’s PWM duty cycle.

Figure 2: Equivalent Circuit of the ISET Pin

Method Overview

Figure 3 shows how to build a design when the equivalent resistor is connected to the MP2759’s ISET pin.

Figure 3: Equivalent Resistor Design Circuit

The PWM signal has a controllable duty cycle. This signal is generated by an MCU. The RC filter (consisting of R₂, R₃, and C_ISET) filters the PWM signal to the DC signal. The equivalent resistance (R_EQ) between the ISET pin and AGND can be calculated with Equation (2):

$$ R_{\text{EQ}} = \frac{1.2 R_1 G_{123}}{1.2 G_{123} - \left( \frac{\text{DUTY} \cdot V_{M\_PWM}}{R_2} + \frac{1.2}{R_1} \right)} $$

Where DUTY is the PWM duty cycle, and V_{M_PWM} is the PWM amplitude (the same as the MCU supply voltage, which is about 3.3V). G₁₂₃ can be estimated with Equation (3):

$$ G_{123} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} $$

For Equations 1, 2, and 3, R_EQ must exceed 0Ω. When R_EQ falls below 0Ω, I_CHG = 0A. Table 1 lists the charging current based on the PWM input.

Table 1: Charging Current vs. PWM Input

PWM Input	PWM Duty	Charging Current
GND	0%	Max charging current
Logic high	100%	0A
PWM duty cycle	0% to MAX_DUTY(1)	Linearly adjust charging current
Float	N/A	96(kΩ)/R1+R3

Note:

1) MAX_DUTY is the maximum PWM duty when the charging current falls to 0A. It is recommended to be about 80%.

Parameters-Related Design Guide

Based on the analysis above, the parameters (R₁, R₂, R₃, C_ISET) can be designed with the following guidelines:

The equivalent resistor that corresponds to the max charging current is R_{MAX_ICHG}, which can be calculated with Equation (4):

$$ R_{\text{MAX_ICHG}} = R_1 + R_2 \parallel R_3 $$

Choose an appropriate R₁, estimated with Equation (5):

$$ R_1 = 0.5 \cdot R_{\text{MAX_ICHG}} $$

Solve the equations, then calculate R₂ and R₃ with Equation (6):

$$ \left\{ \begin{array}{l} R_2 \parallel R_3 = R_{\text{MAX_ICHG}} - R_1 \\ \displaystyle \frac{\text{MAX_DUTY} \cdot V_{M\_PWM} - 1.2}{R_2} = \frac{1.2}{R_3} \end{array} \right. $$

Choose an appropriate C_ISET to filter the PWM signal to DC signal, estimated with Equation (7):

$$ \frac{1}{2\pi (R_2 \parallel R_3) C_{\text{ISET}}} \ll f_{\text{PWM}} $$

Where f_PWM is the PWM frequency.

DESIGN EXAMPLE

This section demonstrates how to successfully design the parameters to adjust the charging current in real time when MAX_ICHG =2.7A, MAX_DUTY =80%, V_{M_PWM}=3.3V, and f_PWM=2kHz.

Design Process

1. Calculate R_{MAX_ICHG} using Equation (8):

$$ R_{\text{MAX_ICHG}} = \frac{96\,\text{k}\Omega}{\text{MAX_ICHG}} = 35\,\text{k}\Omega $$

2. Choose R₁=20kΩ.

3. Use Equation(6) to calculate R₂=33kΩ, and R₃=27kΩ.

4. Set C_ISET=1µF to filter the PWM signal to the DC signal.

Design Results

Build a demo board with the parameters determined above to verify the effectiveness of the method proposed in this application note. Figure 4 shows the demo board schematic.

Figure 4: MP2759 Charging Current Adjustment Application Demo Board Schematic

According to the results obtained after testing the demo board (under the test conditions: V_IN=16V, V_BATT=12V, V_{M_PWM}=3.3V, and f_PWM=2kHz), there is a linear relationship between the charging current and the PWM duty cycle, and the duty cycle offers a wide range of 0% to 82%. (see Figure 5).

Figure 5: Charging Current vs. PWM Duty Cycle

The MP2759 does not experience overshoot or undershoot when the charging current changes (see Figure 6). In addition, the device demonstrates normal start-up and shutdown (see Figure 7).

All the test results offer evidence supporting the effectiveness of the design method introduced in this application note.

Figure 6: Charging Current Transition between 0.5A and 2.4A

Figure 7: Waveforms of MP2759 On/Off Application (Duty = 50%)

CONCLUSION

Charging performance can be optimized with an adjustable charging current. The application note proposed a method to adjust the MP2759’s charging current in real time with an MCU. A design example was provided, as well as test results to prove the validity of this current-charging method.

ADDITIONAL READING

For more information about MPS battery management products, please visit this page.

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