AN193 - Selecting the MP2760’s BATFET

Application Note

By Tessie Zhan

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Abstract

Portable devices today increasingly require a longer usage time, shorter charging time, and limited system voltage (V_SYS). A shorter charging time requires a higher charging current (I_CHG) within the heat loss tolerance. To achieve a limited V_SYS and accurate I_CHG, many manufacturers use a narrow-voltage DC (NVDC) architecture for battery-charging applications.

The MP2760 is an I²C-controlled, 1-cell to 4-cell buck-boost charger with NVDC power path management. A battery FET (BATFET) driver is included to control an external N-channel MOSFET, which regulates the system’s minimum voltage (V_{SYS_REG_MIN}) and provides a battery supplement function. Different BATFET characteristics may affect I_CHG.

This application note discusses the features of the MP2760, as well as the relationship between I_CHG and the BATFET’s characteristics. It also provides a design example for selecting the BATFET.

Introduction

The MP2760 provides a BATFET driver (via the BGATE pin) to support NVDC power path management. The device can regulate the system voltage at V_{SYS_REG_MIN}, even when the battery is depleted.

Figure 1 shows the NVDC power path management structure.

Figure 1: NVDC Power Path Management Structure

The MP2760 has a configurable V_{SYS_MIN}. When V_BATT < V_{SYS_REG_MIN} (where V_{SYS_REG_MIN} = V_{SYS_MIN} + V_TRACK), the BATFET driver controls the charging current loop (trickle charge, pre-charge, and linear constant-current (CC) charge). This ensures that V_SYS is regulated at V_{SYS_REG_MIN}.

When V_BATT > V_{SYS_REG_MIN}, the BATFET fully turns on, and the charging current loop (switching CC charge) and battery voltage loop (constant voltage (CV) charge) are implemented via the buck-boost converter’s pulse-width modulation (PWM) control. V_SYS always tracks the actual battery voltage, which is limited above V_BATTby V_TRACK.

Figure 2 shows the system voltage regulation, where R_{SYS_BAT} is the total resistance between the system and battery.

Figure 2: System Voltage Regulation

Figure 3 show the MP2760’s charging profile.

Figure 3: Complete Charging Cycle Profile

Method Details

Linear Charging Current

When V_BATT is below V_{SYS_REG_MIN}, BATFET works linearly. In 1-cell applications, the charge pump is powered by V_CC, which is the internal LDO output that can provide a 3.6V voltage for the internal circuit. The charge pump’s input determines the maximum charge pump output (V_CP). V_CP is then used to drive the external BATFET via a current source.

The current source’s output impedance (Z_ISRC) should be several hundred kΩ, and the charge pump’s output impedance (Z_{O_CP}) should be between several kΩ and several tens of kΩ. If the selected BATFET’s gate-to-source threshold voltage (V_{GS_TH}) results in an insufficient DC voltage bias (about 2V) for the current source, then Z_ISRC drops to several tens of kΩ, and I_CHG drops below its configured value (I_{CHG_CONF}) due to inadequate loop gain. Therefore, a proper BATFET must be selected to meet the required minimum charging current (I_{CHG_REQ}), especially for 1-cell applications.

Figure 4 shows the simplified block diagram of the BATFET’s linear on loop in a 1-cell application.

Figure 4: Simplified Block Diagram of the BATFET’s Linear On Loop in 1-Cell Applications

Figure 5 shows the relationship between the MP2760’s I_CHG and the BATFET’s gate-to-source voltage (V_GS) when I_{CHG_CONF} is set to 1A, which reflects the driving ability under 1-cell applications. In 2-cell to 4-cell applications, the charge pump’s input is sufficiently high, and the current source that drives BATFET does not become saturated. Due to this, this application note only provides guidance on selecting BATFET in 1-cell applications.

Figure 5: I_CHG vs. V_GS of the MP2760’s BATFET (I_{CHG_CONF} = 1A)

Figure 6 shows the transfer characteristics of three different N-channel MOSFETs. These characteristic curves are paired with the curves from Figure 5 on page 4.

Figure 6: Estimated I_CHG When the MP2750's BATFET Works Linearly with Different MOFSET Types (I_{CHG_CONF} = 1A)

Based on Figure 6, it is possible to estimate the BGATE pin voltage (V_BGATE) and I_CHG values for different MOSFET types. Table 1 lists the estimated V_GS and I_CHG values to select a MOSFET with the optimal transfer characteristics.

Table 1: Estimated V_GS and I_CHG Values^{(1) (2) (3)}

V_BGATE Leakage (μA)	V_TH Low Sample		V_TH Typical Sample		V_TH High Sample
V_BGATE Leakage (μA)	V_GS (V)	I_CHG (A)	V_GS (V)	I_CHG (A)	V_GS (V)	I_CHG (A)
0	1.86	0.95	2.29	0.8	2.74	0.55
3	1.86	0.95	2.29	0.8	2.73	0.52
20	1.85	0.85	2.26	0.6	2.69	0.32

Notes:

These estimated valuesresult from a 1A I_{CHG_CONF}. When I_{CHG_CONF} increases by 500mA, the actual I_CHG also increases by about 500mA.
This application note only discusses 1-cell applications.
If there is any external leakage current (I_LKG) pulled from BGATE, such as the BATFET’s gate-to-source discharge resistance (R_GS), the results are different.

If the selected MOSFET is not sufficient to make I_CHG reach I_{CHG_REQ}, there are two methods to ensure I_CHG reaches I_{CHG_REG}, described below:

Increase the BATFET’s drain-to-source voltage (V_DS) by increasing V_TRACK to shift its transfer characteristics curve left. This results in a higher I_CHG. Note that V_TRACK cannot be set too high because a higher V_DS causes additional losses on the BATFET.
Set I_{CHG_CONF} higher to meet the minimum I_CHG requirement.

Switching Charging Current

When V_BATT exceeds V_{SYS_REG_MIN}, the BATFET fully turnson,and its linear on loop does not impact I_CHG. Instead, I_CHG is limited by V_TRACK, as calculated with Equation (1):

$$ I_{\text{CHG}} = \frac{V_{\text{TRACK}}}{R_{\text{SNS}} + R_{\text{DS(ON)}} + R_{\text{PCB}}} $$

Where R_SNS is the I_CHG sense resistance, R_DS(ON) is the on resistance of the BATFET when it is fully on, and R_PCB is the parasitic resistance of the PCB.

Based on Equation (1), R_DS(ON) must satisfy the constraints estimated with Equation (2):

$$ R_{\text{DS(ON)}} < \frac{V_{\text{TRACK}}}{I_{\text{CHG_REQ}}} - R_{\text{SNS}} - R_{\text{PCB}} $$

If the BATFET R_DS(ON) is not low enough, V_TRACK can be set higher via the I²C to ensure that I_{CHG_REQ} is met.

Design Example

This section demonstrates how to successfully select a proper BATFET in a 1-cell application to meet the I_CHG requirement for both linear charge mode and switching charge mode.

Consider the following design parameters as an example. I_{CHG_REQ} is set to 0.8A in linear charge mode and 4A in switching charge mode. Based on the following conditions, in linear charge mode, I_{CHG_CONF} is 1A, the minimum drain-to-source voltage (V_{DS_MIN}) is 100mV, and R_GS is 1MΩ. In switching charge mode, I_{CHG_CONF} is 4A, V_TRACK is 100mV, and R_SNS is 10mΩ.

Figure 7 shows the MP2760’s typical application circuit.

Figure 7: Typical Application Circuit of the MP2760

Linear Charge Mode

To meet the I_CHG requirement for linear charge mode, follow the steps below:

Based on Figure 6 on page 5, when I_{CHG_REQ} is set to 0.8A, VGS is between 2V and 2.5V.
The leakage current (I_LKG) pulled from the external BGATE can be calculated with Equation (3):
Based on Table 1 on page 5, when I_{CHG_REQ} is set to 0.8A, V_GS is about 2.29V.
Select an N-channel MOSFET with a proper transfer characteristics curve at V_DS = 0.1V,where the drain-to-source current (I_DS) exceeds 0.8A when V_GS is 2.29V.

Switching Charge Mode

Ignoring the parasitic impedance on the PCB, the BATFET R_DS(ON) can be calculated with Equation (4):

$$ R_{\text{DS(ON)}} < \frac{100\,\text{mV}}{4\,\text{A}} - 10\,\text{m}\Omega = 15\,\text{m}\Omega $$

Conclusion

The NVDC architecture has been increasingly implemented in battery charger applications. The MP2760 is a battery charger that provides NVDC power path management using an external BATFET. The BATFET selection impacts NVDC performance, especially in 1-cell applications.

This application note proposed a method for selecting a proper BATFET with suitable characteristics, ensuring the I_CHG requirements are met in both linear charge mode and switching charge mode.