Configuring Standalone Linear Power Path Chargers

In this application report, proper configuration of standalone power path linear chargers will be discussed as well as the different mode these chargers have. Power Path chargers have an advantage over Non Power Path chargers due to the extra FET between the SYS rail and the BAT pin. This gives priority to the System which allows for system startup with a depleted battery and proper battery termination.

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1 Introduction

The BQ2407X family of devices is a popular Power Path standalone linear charger used in applications that require 1.5A or less charging current. Since these devices are standalone, most of the important parameters like input current limit, charge current and charge timers can be configured using external resistors. The family also has different options depending on the application.

2 Configuring 1.2A Fast Charge

To configure the BQ2407x, a few calculations will need to be done to get the resistors that would be used to program the desired values. Parameters that need to be programmed include the input current limit R_ILIM, the fast charge current R_ISET, the fast charge timer R_TMR, and in some devices like the BQ24074 where the termination current can be programmed, R_ITERM. Since the termination current depends on the fast charge current in the BQ24074, R_ITERM should be changed when R_ISET is changed to have the same termination current.

Figure 1. 1.2A Fast Charge Configuration

ILIM set to 1.5A with R_{ILIM =} KILIM / ILIM = 1610 / 1.5A = 1.07kΩ
ICHG set to 1.2A with R_ISET = KISET / ICHG = 890 / 1.2 = 750Ω
ITERM set to 0.05A with R_ITERM = (ITERM * RISET) / KITERM = (0.05 * 750) / 0.03 = 1.25kΩ
Charge timer set to 6.65 hours; R_TMR = (Time * 3600)/ (10 * K_TMR) = (6.65 * 3600) / (10 * 48) = 49.9kΩ
EN2 set High and EN1 set Low to set input current through ILIM
KILIM, KISET, KITERM and K_TMR are obtained from the data sheet

3 Configuring USB100 and USB500 Mode

To set the device in USB500 mode, the EN1 pin should be pulled high and the EN2 pin should be pulled low as shown in Figure 2. In this mode, the ILIM setting is ignored with the input current and the charge current limited to 500 mA. In USB100 mode, the input current limit is set to 100 mA. To set the device in USB100 mode, set both EN1 and EN2 low. In USB mode, the input voltage is monitored with the input current being dynamically reduced when the input voltage drops below the VIN-DPM threshold to prevent the input voltage from crashing below the threshold voltage. ITERM can be set with a resistor or left floating to set the termination current to the default of 10% ICHARGE.

Figure 2. USB500 Mode